NCHurricane2009's Blog

2012 Atlantic Hurricane Season Birdseye Discussion #93

By: NCHurricane2009, 11:46 AM GMT on August 31, 2012

...AUGUST 31 2012...7:50 AM EDT...
Isaac finally weakens to a remnant low while pushing northward into Arkansas this morning. Technically because he is no longer expected to be a tropical cyclone...I do not have a special feature section for Isaac on this blog...but this does not mean the remnants are incapable of significant impacts. See paragraph P2 for updated assessment of Isaac remnant on this blog.

Tropical Storm Kirk strengthens quickly into a category 2 hurricane over the open Atlantic in last 24 hours...but remains a marine threat is he curves northward and eventually northeastward in the open Atlantic. See Kirk special feature section for details.

Tropical wave...Invest 98-L...midway between the Cape Verde Islands and Lesser Antilles has strengthened into Tropical Depression 12 and then into Tropical Storm Leslie in the last 24 hours...and continues to steadily intensify. Expect Leslie to become a hurricane this afternoon. Although currently expected to pass NE of the Lesser Antilles...interests in this area should monitor her progress in case of a westward shift in track. Interests in Bermuda should monitor Leslie in case her track takes the more westerly scenario. Regardless...Leslie is expected to become a strong hurricane over the western Atlantic...and if large enough could spread some surf and rip currents to the northern Caribbean Islands...Bahamas...Bermuda...and the east US shore by day 5. See Leslie special feature section for further details.

The formation of Leslie makes this the second earliest formation of an Atlantic season's twelvth tropical storm...behind Luis of 1995. This now means Lee of 2005 is thrid place for this record. Despite the hyperactive pace reminiscent of the 1995 and 2005 seasons...none of this seasons's storm have yet to become a major hurricane (115 mph+ max sustained winds...category 3 or higher). Perhaps Kirk may change that if he briefly becomes a major hurricane later today.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0124Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...HURRICANE KIRK...
Concerning Kirk...I previously had a slight rightward bias tracked in my previous discussion...thinking he would pass over 50W-30N for a few reasons posted in that discussion. Instead he continues to have a slight westward component in track that allowed him to pass west of that location. Even as recently as 5 AM...the NHC recorded track of Kirk (seen in Figure 1 below) has slight westward lean. Moreover...the way the 00Z GFS presents the low-level ridge weakness Kirk is within...I still would prefer a straight north track between now and 5 AM Sat. So now I have a slight leftward-bias in track for all these reasons...as seen in Figure 1 below. As I showed previously...I still think Kirk will be transitioning to an extratropical (non-tropical) remnant low by 5 AM Sunday as he merges with cold front currently mentioned in paragraph P1.


Figure 1: My forecast for Hurricane Kirk this morning

While I finished my track forecast at 4 AM EDT this morning...I waited till 5 AM EDT to finish my graphic in Figure 1...wanting to see if Kirk would be upgraded to a major hurricane (115 mph + max winds). He is certainly very impressive on this morning's infrared...a compact hurricane whose small eye is shrinking to pinhole size. I forecast him to be a major hurricane briefly today as seen in Figure 1...slightly above what the new 5 AM NHC forecast shows. Some unfavorable factors surround Kirk...including paragraph P5 dry air to the south...and southwesterly vertical shear and cooler sea-surface temperatures to his north (the shear generated by southwesterly upper wind jet ahead of paragraph P1 and P3 upper troughs). I surmise that by Saturday morning...his track will take him into the southwesterly shear and cooler waters...and the southwesterly shear will allow this dry air to the south to entrain into his circulation. And given that small-sized storms like Kirk can weaken quickly...I forecast more rapid weakening than what NHC shows as of their 5 AM advisory.

Impact swath in Figure 1 is initialized based on a the very small tropical storm wind radius shown at 5 AM NHC advisory and the presumption that Kirk should be a compact tropical cyclone thru the forecast period. Although he should get southwesterly shear by Saturday...the NE track by that time and beyond still allows the impact swath to be symmetric about the storm track.

...SPECIAL FEATURE...TROPICAL STORM LESLIE...
Vigorous tropical wave Invest 98-L midway between the Cape Verde Islands and Lesser Antilles is now a steadily-intensifying Tropical Storm Leslie. My forecast versus the NHC's is shown in Figure 2.


Figure 2: My forecast for Tropical Storm Leslie this morning

There remains a break in low-level ridging to Leslie's north...located between the paragraph P5 ridge to the NE and paragraph P3 eastern US low-level ridge to the NW. We can now assume Leslie is strong/tall enough to feel upper-level weakensses as well...but with the paragraph P6 upper anticyclonic center located due north...there are no such upper-level weaknesses to speak of at this time. Therefore the more northward bend in track recently recorded at 5 AM EDT by the NHC I believe is associated with Leslie getting influenced by the aformentioned low-level weakness. I had made my forecast track before the 5 AM advisory...using the 11 PM recorded NHC track (seen in Figure 2) to justify having a more southward slant in my forecast track from the get-go. Because of the more northerly position of Leslie as of 5 AM...it looks like she is dead-nuts following the NHC track in Figure 1 as opposed to mine. Then again as Kirk lifts northward...the low-level ridge weakness causing Leslie's northward bend should close...so perhaps a bend back toward my Leslie track forecast is possible. We shall see.

Longer term...towards 48 hrs and beyond...models show another low-level ridge weakness developing thanks to paragraph P1 front...but there is a strong low-level ridge building to the north near Atlantic Canada by that time as well (also mentioned in paragraph P1)...so its not a significant low-level ridge weakness....but a weakness where Leslie is juxtaposed between the Atlc Canada ridge to the north and paragraph P5 ridge to the NE. Beyond 48 hrs...I model Leslie tracking NW then more NNW as she rounds the west side of paragraph P5 surface ridge and NE half of a cut-off upper vortex (still progged to originate from paragraph P1 upper trough) that Leslie is strong/tall enough to feel. The westerly component in my track thru 48 to 120 hrs is on the philosophy of Leslie being on the NE side of the upper vortex rather than on the east side of the upper vortex (in which case there would be a straight north track)...as models push this upper vortex southward into the Bahamas. The slow down in track by 120 hrs is due to blocking effect of aforementioned Atlantic Canada low-level ridge building to the north. My track forecast is more threatening to Bermuda...and the NHC's is less so...and the good news is that so far Leslie shows signs of following the NHC track as mentioned in the previous paragraph.

Its cloud pattern is still slightly signatory of easterly vertical shear on the south side of paragraph P6 upper anticyclonic cell...but GFS shows this cell becoming directly centered over Leslie to support healthy upper outflow over the storm. Moreover...an upper vortex in paragraph P4 is progged to dive southward about the east side of the upper anticyclone...enhancing the eastern outflow of Leslie. Even better for Leslie...another upper vortex over the Bahamas (mentioned in paragraph above) would enhance her western outflow...so I see no reason why to not make Leslie a major hurricane (115 mph +)...so my forecast is similar but more aggressive than NHC's 5 AM forecast. In case I am wrong for some reason to suggest a major hurricane....I stop intensifying Leslie past 120 mph max winds. With the slow-down in track by 120 hrs...I think Leslie is at risk of upwelling cooler waters below the ocean surface...so I weaken her a bit by that time. Also...computer models suggest possible outflow blockage to the north...or northerly shear...due to northerly flow on back side of paragraph P1 upper trough passing by to the north. This potential northerly shear is another reason I weaken Leslie by 120 hrs.

Impact swath in Figure 2 is based on 5 AM NHC tropical storm wind radius...which I inflate to represent a strengthening storm. Notice by 120 hrs...the swath is biased a bit SE with respect to my forecast track...to reflect possible northerly shear by that time...

...MID-LATITUDES DISCUSSION...
P1...Shortwave upper trough with low amplitude...and surface frontal system...continue to rapidly eject eastward and is passing north of Isaac's remnant low while located over the northern US and southern Canada. By 48 hrs...it is expected to leave behind a cut-off upper low over the W Atlantic to help turn Leslie northward...and transition Kirk to an extratropical system over the north Atlantic. By that time...its upper convergent west side will be building a low-level ridge pushing into Atlantic Canada...and this low-level ridge's west side will steer Isaac northward then northeastward...and the low-level ridge's south side will slow the northward progress of Leslie. For now...beginnings of this low-level ridge are 1017 and 1015 mb centers in upper-left of atmo chart...and warm warm air advection ahead of the surface front is supporting an upper ridge over the southern US and spreading into the W Atlantic. This upper ridge now contains the upper anticyclone once generated by Isaac's latent heat release

P2...Remnant surface low of Isaac was centerd in west-central Arkansas as of 5 AM EDT juging by radar loops...which means so far he has followed the NHC forecast track better than mine presented in previous discussion #92. Its remnant upper anticyclonic outflow has merged with upper ridge in paragraph P1...the remnant upper anticyclone now located east of Isaac. In the last 24 hrs...an upper vortex spun up in relatively lower pressures between Isaac's old anticyclone and paragraph P1 upper ridge...and this upper vortex is now directly above Isaac...which effectively makes him a deep-layered remnant low at this hour. Animating the last days worth of above atmo charts also tells me some of this upper vortex is a cut-off fragment of paragraph P3 upper trough mentioned below. Heaviest rains with flooding potential are in Arkansas and should spread into Missouri in next 24 hrs as he continues northward. Low-level ridge that will steer Isaac is mentioned in paragraph P1. Whether he turns more directly eastward into that ridge as GFS showed yesterday...or rounds the ridge like I showed in discussion #92...determines whether he follows the forecast I presented in previous discussion #92...or whether he follows the last forecasts shown by NHC.

P3...Upper trough in the mid-latitude westerlies has entered the NW Atlantic from the eastern US and eastern Canada. Upper divergence east of this upper trough continues supports a surface front that has moved into the western Atlantic that curls into a vigorous cyclone moving past the south tip of Greenland (marked with 990 and 987 mb centers in above atmo chart). Upper convergence behind this upper trough supports a 1020 to 1023 mb surface ridge centered over the eastern US.

P4...Upper trough in the Atlantic high seas has moved into Europe...leaving behind cut-off upper vortex over the Azores in last 48 hrs. 1017 mb frontal low moving into the Azores in previous discussion has weakened to 1021 mb in last 24 hrs...under a non-divergent environment below the Azores cut-off upper vortex. Upper vortex is progged to dive southward about the Leslie-Kirk upper anticyclonic cell mentioned in paragraph P6...merging with inverted upper trough near the Cape Verde Islands also mentioned in paragraph P6.

P5...Atlantic surface ridge has been eroded out of the western Atlantic thanks to cold front pushing in from the NW...associated with paragraph P3 system. Easterly flow on the south side of this surface ridge (in conjunction with easterly flow on south side of paragraph P6 upper ridge) is helping to waft Africa desert dry air westward across the Atlantic tropics. 1028 mb center of this ridge toward Europe is supported by upper convergence on back side of upper trough mentioned in paragraph P4.

...TROPICAL BELT DISCUSSION...
P6...Upper ridge across the tropical Atlantic persists. The Isaac anticyclonic center of this ridging has merged with paragraph P1 upper ridge...leaving sprawling upper vorticity in relatively lower pressures south of the paragraph P1 upper ridge and located across the Caribbean. Hurricane Kirk and Tropical Storm Leslie t-storm latent heat release has locally inflated the upper ridge into an anticyclonic center in the central Atlantic...with relatively lower pressures west of this anticyclone still supporting adjacent upper trough near Kirk...but moving away from Kirk and into the afoermentioned Caribbean upper vorticity. Relatively lower pressures east of this upper anticyclone continue supporting embedded upper vortex near the Cape Verde Islands that has become a large inverted upper trough.

P7...As marked in lower-right corner of above atmo chart...I have been using Meteosat-9 satellite animation to track what I believe is a tropical wave that has recently emerged Africa. I estimate the wave to now be SSW of the Cape Verde Islands. It is currently suppressed by inverted upper trough near the Cape Verde Islands mentioned in paragraph P6.

Updated: 11:18 AM GMT on September 01, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #92

By: NCHurricane2009, 11:07 AM GMT on August 30, 2012

...AUGUST 30 2012...7:10 AM EDT...
Isaac still pinwheeling over Louisiana...but has weakened due to landfall. Unfortunately before weakening...storm surge toppled over the levee system in Plaquemines Parish Louisiana during the last 24 hours...creating a severe flooding event necessitating rescues. In the next 24 hours...storm surge from Isaac should dissipate. See Isaac special feature section below for additional details.

Tropical Storm Kirk strengthening faster than expected...but remains a marine threat is he curves northward and eventually northeastward in the open Atlantic. See Kirk special feature section for details.

Strong tropical SW of the Cape Verde Islands in the previous discussion...Invest 98-L...is now midway between the Cape Verdes and Lesser Antilles this morning and has become very well organized. I expect a 100% chance this will become our next Atlantic tropical cyclone sometime today. Based on the expected track...interests in the northern Lesser Antilles...Virgin Islands...and Puerto Rico should monitor the progress of this system carefully. See Invest 98-L special feature section below for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0124Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM ISAAC...
My latest forecast versus the NHC's is shown in Figure 1 below.

Track-wise for Isaac...it is important to highlight that I made my forecast in Figure 1 with information available around 3 AM EDT (2 AM CDT). I used a straight-line projection of the NW track shown by NHC since landfall at the time...which pointed me toward the NW corner of Louisiana. The track has bent more northward toward north-central Louisiana since that time...so realistically I believe Isaac and its remnants will track a little to the right of what I showed in Figure 1.

With that said...I still prefer to have a leftward-biased and slower track inland than what the NHC shows...based on examining this morning's 00Z GFS model run. However my track forecast is a little faster than the previous...accounting for the fact that Isaac has moved inland a little faster than what I previously showed. Still confident that the paragraph P1 frontal system and its shortwave upper trough will pass too far to the north to recurve Isaac northeastward...and yet another blocking low-level ridge quickly should quickly build beneath the upper convergent back side of the passing shortwave upper trough. I do show a faster northward speed than I did previously between 4 AM Fri and 4 AM Sat...as 00Z GFS shows this ridge already to the NE and a bit out of the way by 4 AM Sat. After 4 AM Sat...GFS still pushes Isaac rightwards directly into the blocking low-level ridge to the NE...which doesn't make sense to me. I do see the 00Z GFS has zonal upper westerly flow NE of Isaac by that time...so perhaps it sees Isaac as an extratropical (non-tropical) low supported by and attracted to surface pressure falls of an upper westerly jet to the NE (maybe that's why 00Z GFS bends it rightwards directly into the low-level ridge). Instead at this time...I prefer moving Isaac into northern Missouri by 4 AM Sun as he rounds the west side of the low-level ridge...then I bend him rightward across the Chicago...Illinois area and then southern Michigan ahead of yet another frontal system/upper trough swinging in from the NW and passing to the north of Isaac. Isaac's remnant low may get absorbed by the south side of this next frontal system by 120 hrs.


Figure 1: My forecast for Tropical Storm Isaac this morning

Intensity-wise...my previous landfall weakening forecast has been doing very well...so I maintain that forecast. This means by tomorrow morning...Isaac should be a remnant low.

Impact swath in Figure 1 begins with the shape/size of the 2 AM CDT NHC tropical storm wind radius blended with the heavy spiral rain bands seen in radar to more emphasize the heavy rain and flood threat. Although no impact swath is shown beyond 4 AM Sunday, enhanced rainfall can be expected along the remnant's track, but not as severely as areas within the blue-dashed impact swath. Realistically...the impact swath can be shifted a little to the right given that Isaac's center is tracking more toward north-central Louisiana instead of NW Louisana.

...SPECIAL FEATURE...TROPICAL STORM KIRK...
As expected 24 hrs ago...Kirk is turning northward in the vicinity of 50W longitude as he falls into a break of the paragraph P4 surface ridge caused by the cold front from the 980 mb vortex in paragraph P2. Later he should curve NE and accelerate in fast deep-layered southwesterly flow ahead of the frontal system and upper trough mentioned in paragraph P1. After studying satellite loops this morning...looking at the trajectory of Kirk recorded by NHC...and keeping in mind that he should continue curving more northward throughout the day...I think the center will pass over 50W-30N...which is why I have a slight rightward bias in my track by 5 AM Fri. The way the 00Z GFS presents the low-level ridge weakness...I would prefer a straight north track between 5 AM Fri and 5 AM Sat...but I use this as a chance to re-align with the NHC suggested track and hence bend it more NNE. I agree with the NHC track after 5 AM Sat. As I showed previously...I still think Kirk will be transitioning to an extratropical (non-tropical) remnant low by 5 AM Sunday as he merges with cold front currently mentioned in paragraph P1.


Figure 2: My forecast for Tropical Storm Kirk this morning

While I finished my track forecast at 3 AM EDT this morning...I waited till 5 AM EDT to finish my graphic in Figure 2...wanting to see what the NHC shows for the intensity forecast at 5 AM. His surface center has turned northward...distancing himself from the adjacent upper vortex (paragraph P5) that was previously shearing him. This means the center has slid underneath his favorable t-storm canopy whose latent heat release is providing good upper outflow. I had previously been below NHC intensity guidance from concern that paragraph P4 dry air to his south would get pulled into the t-storm canopy as his low-level center marched northward...but this is not the case. In fact...he looks like an impressive and small tropical cyclone with a banding-type eye at this hour...and we know small tropical cyclones are more subject to brisk intensification than larger ones. Based on his satellite appearance...I made my intensity forecast a bit more elevated than the 5 AM NHC intensity guidance on Kirk. Intensity is elevated thru 72 hrs...because despite southwesterly shear and increasingly cooler waters...he should get supportive upper divergence from accelerating southwesterly flow above the cold front he is merging with.

Impact swath in Figure 2 is initialized based on a the very small tropical storm wind radius shown at 5 AM NHC advisory and the presumption that Kirk should be a compact tropical cyclone thru the forecast period. As he becomes a hurricane...I do slightly inflate the size of the impact swath. Although he may get southwesterly shear as he gets non-tropical...the NE track by that time still allows the impact swath to be symmetric about the storm track.

...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE INVEST 98-L...
Vigorous tropical wave Invest 98-L is now midway between the Cape Verde Islands and Lesser Antilles. Its cloud pattern is still signatory of easterly vertical shear on the south side of paragraph P5 upper ridge. However...its t-storm latent heat release has locally inflating this the upper ridge into an upper anticyclonic center nearby that enhances the outflow and reduces the shear. Given the impressive organization this morning...I am assuming a 100% chance it will eventually become a tropical cyclone today.

As it becomes a tropical cyclone...the paragraph P4 surface ridge should steer it westward. The break in this ridge is too small and too far north to allow this system to turn northward behind Kirk. Despite this...models show a track bending to the north even though the paragraph P4 surface ridge is intact to the north...so what is causing the northward turn must be an upper-level feature that the models presume this system will be strong/tall enough to feel. Indeed by 72 hrs...00Z GFS dumps in a west Atlantic cut-off upper vortex originating from what is now the paragraph P1 mid-latitude system. I currently agree with a bend to the north...because I expect based on its current impressive satellite appearance that 98-L will be strong and deep-layered enough by that time to feel the upper vortex. As it curves northward...this system will pass near enough the northern Lesser Antilles to produce some impact. If this system delays its northward turn for whatever reason...perhaps the Virgin Islands and Puerto Rico could get clipped.

...MID-LATITUDES DISCUSSION...
P1...Next shortwave upper trough and surface frontal system is entering the picture from the upper-left of the above birdseye charts...and is expected to rapidly eject eastward to the north of Isaac in the next 24 hrs. By 72 hrs...it is expected to leave behind a cut-off upper low over the W Atlantic to help turn Invest 98-L northward...and transition Kirk to an extratropical system over the north Atlantic. For now...the surface front is over NW US and central Canada pushing quickly east...with warm air advection ahead of the surface front supporting a shortwave upper ridge over the central US.

P2...Upper trough in the mid-latitude westerlies is entering the NW Atlantic from the eastern US and eastern Canada. Upper divergence east of this upper trough continues supports a surface front that has moved into the western Atlantic that curls into a vigorous cyclone moving past the south tip of Greenland (marked with 980 mb center in above atmo chart). Upper convergence behind this upper trough supports a 1019 mb surface ridge centered over the Great Lakes.

P3...Main of upper trough persists in the Atlantic high seas southeast of Greenland...leaving behind a shortwave that has moved from the NW Atlantic to the Azores last 24 hrs...and has left behind a new cut-off upper vortex over the Azores in last 24 hrs. All surface frontal cyclones supported by divergence from this upper trough has moved into Europe from the British Isles. 1010 mb frontal low moving into the Azores has weakened to 1017 mb...under a less divergent environment below the new Azores cut-off upper vortex.

P4...Atlantic surface ridge with 1019 to 1024 mb centers persists. Easterly flow on the south side of this ridge (in conjunction with easterly flow on south side of paragraph P5 upper ridge) is helping to waft Africa desert dry air westward across the Atlantic tropics. Surface ridge currently has a break in it where Tropical Storm Kirk is centered...and this break is enhanced by the surface front mentioned in paragraph P2. 1024 mb center toward Europe is supported by upper convergence on back side of main upper trough mentioned in paragraph P3.

...TROPICAL BELT DISCUSSION...
P5...Upper ridge across the tropical Atlantic persists. T-storm latent heat release from Isaac has caused this upper ridge to concentrate into an anticyclonic center over the storm and across the Gulf of Mexico...SE US...and west Caribbean. A sprawling upper vortex persists in relatively lower pressures southeast of Isaac's upper anticyclone...currently located in the western Caribbean...and whose western split flow divergence with the respect to the upper anticyclone's flow supports t-storms across the western Caribbean and SE Mexico. Tropical Storm Kirk and Invest 98-L t-storm latent heat release has locally inflated the upper ridge into an anticyclonic center in the eastern Atlantic...with relatively lower pressures west of this anticyclone still supporting adjacent upper vortex near Kirk (albeit this vortex has weakened to inverted upper trough in last 24 hrs). Relatively lower pressures east of this anticyclone continue supporting embedded upper vortex N of the Cape Verde Islands noted in yesterday's discussion.

P6...As marked in lower-right corner of above atmo chart...yet another tropical wave has emerged from the west coast of Africa on the basis of satellite imagery. I estimate the wave to be SE of the Cape Verde Islands. Satellite appearance of the t-storms is signatory of easterly vertical shear on south side of paragraph P4 upper ridge. I expect that the upper vortex N of the Cape Verdes (paragraph P5) will intensify its definition in relatively lower pressures east of 98-L's outflow...so either this wave will get suppressed by the upper vortex...or receive northerly shear from the eastern outflow of 98-L. That is why I am reluctant to make this a special feature on this blog.

Updated: 7:37 AM GMT on August 31, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #91

By: NCHurricane2009, 11:21 AM GMT on August 29, 2012

...AUGUST 29 2012...7:10 AM EDT...
In the last 24 hours...Isaac finally strengthened into a category 1 hurricane. Unforutnately...he has stalled as he makes landfall in southeastern Lousiana this morning. The large-sized wind field and slow motion of Isaac near the shoreline for the next several hours prolongs the storm surge flooding threat. For all US Gulf coast residents...mandatory evacuation orders should have been obeyed by now in storm surge prone areas. Latest watches/warnings can be found on www.nhc.noaa.gov. See Isaac special feature section below for additional details.

Open Atlantic tropical wave Invest 97-L has become Tropical Depression Eleven...then Tropical Storm Kirk...in last 24 hours...but is expected to recurve northeastward into the open Atlantic. See Kirk special feature section for details.

Strong tropical wave that emerged from Africa in previous discussion is already SW of the Cape Verde Islands this morning while becoming better organized. As a result...it has been upgraded to Invest 98-L...and I expect a 100% chance this will become our next Atlantic tropical cyclone. See Invest 98-L special feature section below for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1930Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...
>
This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...HURRICANE ISAAC...
My latest forecast versus the NHC's is shown in Figure 1 below.

Track-wise for Isaac...the NW track caused him to make landfall over the Missisippi River delta near Buras, Louisana as we have been expecting. This NW track was caused by surface ridge weakness of surface front mentioned in paragraph P1. Since that time...he has actually moved a little more left than expected under the influence of blocking low-level ridge that has built into the central US (cause of central US ridge mentioned in paragraph P1). This means my track and the NHC's in Figure 1 are adjusted leftwards from the previous. The blocking ridge has also slowed his track to a nearly-stalled to perfectly-stalled track for the time being...but the next surface frontal system and mid-latitude shortwave upper trough should pass to the north and weaken this ridge for the NW track to resume. That is why for the short-term I agree on a slow NW track into Lousiana in alignment with NHC's track.

Longer-term...it looks like the picture is more complicated than I previously thought...causing me to have a leftward-biased and much slower track inland than what the NHC shows. After examining the 00Z GFS model this morning...currently I am confident that the next shortwave upper trough and surface frontal system will pass too far to the north to recurve Isaac northeastward...unlike what we were previously promoting. In fact yet another blocking low-level ridge quickly builds behind the upper convergent back side of the passing shortwave upper trough...this second blocking low-level ridge causing me to show a slow NW track and fall behind the NHC's forward pace thru the first 72 hrs (thru 4 AM Fri).

By 72 hrs and beyond...I decide to keep my track further left than 00Z GFS...as the 00Z GFS by that time pushes Isaac rightwards directly into the second blocking low-level ridge to the NE...which doesn't make sense to me. I do see the 00Z GFS has zonal upper westerly flow over Isaac by that time...so perhaps it sees Isaac as an extratropical (non-tropical) low supported by and attracted to surface pressure falls of an upper westerly jet (maybe that's why 00Z GFS bends it rightwards directly into the low-level ridge). Instead at this time...I prefer moving Isaac NE into Iowa as it rounds the west side of the low-level ridge and recurves ahead of yet another frontal system/upper trough swinging in from the NW by 96 and 120 hrs.

Warnings remain spread across across US Gulf coast from Louisiana to ...parts of Florida panhandle...including all of the Alabama and Mississippi coastline in between. Mandatory evacuation orders in all storm surge prone areas along the US Gulf coast should have been obeyed by now. Latest watches/warnings can be found on www.nhc.noaa.gov Please pay attention to local and national media for any mandatory evacuation orders.


Figure 1: My forecast for Hurricane Isaac this morning

Intensity-wise...Isaac became a hurricane finally...after mixing out the dry air mentioned in the Isaac section of previous discussion #90. Now as he moves into Lousiana...its all about forecasting the weakening rate. My intensity forecast for weakening due to landfall is upped from my previous due to him strengthening 5 mph above what I thought he would reach...and due to the delay in him moving inland. In short...I now have him a remnant low by 4 AM Fri instead of 4 PM Thu like I showed previously.

Impact swath in Figure 1 begins with the shape/size of the tropical storm wind radius shown at 10 PM CDT NHC advisory...later declining in size to represent weakening due to landfall. The latter part of the swath has more to do with heavy rain rather than gusty winds...as Isaac should be a tropical depression or remnant low later on. Some highlights to note about the impact statements of Figure 1. Impact statement (c) highlights the storm surge threat...which is going to be more significant than usual for a minimal hurricane....as the creation of the large tropical storm wind radius stirs up a large amount of water...so all mandatory evacuation orders along the US Gulf coast should continue to be obeyed. Impact statement (b) makes a point about an emerging flood risk when the storm slows and associated heavy rains persist over the same area. Impact statement (b) also emphasizes that heavy rains can be expected in squalls outside of the shown impact swath. I continue to expect these far reaching squalls to be biased to the east...thanks to frontal system moving into the eastern US in paragraph P1 spreading the moisture. Based on radar and satellite....these rains have already started and ended in last 24 hrs...as this batch of moisture is already racing into the Atlantic offshore of South Carolina and paralleling the NC coast on radar.

...SPECIAL FEATURE...TROPICAL STORM KIRK...
Tropical wave midway between the Cape Verdes and Lesser Antilles...in the open tropical Atlantic waters...has strengthened into TD 11 then TS Kirk in last 24 hrs. It strengthened in a southerly shear environment on the east side of paragraph P4 upper vortex located adjacent to this feature. Satellite image in Figure 3 shows when TD 11 became upgraded to Kirk...which shows the surface center is on the south edge of the storm canopy rather than at eye feature in the storm canopy that was visible at the time. Therefore this eye feature is believed to be the mid-level center.

Atlantic surface ridge north of Kirk (paragraph P3) is steering this system westward...while the adjacent upper vortex is currently just an agent of southerly vertical shear. As I generated my track forecast in Figure 2...I had quiet a rightward bias relative to the NHC 11 PM EDT forecast from last night. The new 5 AM EDT NHC track forecast has come much closer into agreement with mine...but I still have a tad of a rightward bias. This gives me more confidence in my track forecast...which was genereated before the 5 AM NHC forecast was released. The following three reasons are why I have a rightward-biased track forecast

(1) The 1021 mb ridge center to the west of Kirk seen in the above atmo chart would slow his westward progress.

(2) In as little as 24 hrs out...00Z GFS from this morning shows the cold front from the 988 mb vortex (paragraph P1) creating a break in the paragraph P3 subtropical ridge...pulling Kirk northward.

(3) The surface center may regenerate northward into the storm canopy.

By 96 hrs in my track forecast...I expect Kirk to become non-tropical as it merges with a cold front of the next system to shoot into the Atlantic right behind the system in paragraph P1.


Figure 2: Forecast for Tropical Storm Kirk this morning


Figure 3: Shortwave Infrared Early morning satellite image of when TD 11 became TS Kirk. Blue clouds are higher in the atmosphere...and black clouds are lower in the atmosphere. The low cloud motions are used to deduce where the storm is centered at the surface.

For intensity...I chose to stay below the 5 AM EDT NHC forecast which predicts a 75 mph hurricane (my suggested max intensity is a 60 mph tropical storm). I presume the NHC and models expect that Kirk will distance himself from the shearing upper vortex as he turns northward...and hence allow the surface center to slide into his favorable t-storm canopy whose latent heat release is providing good upper outflow as seen in Figure 3. However...I am more shy with my intensity forecast because of the above thermo chart...which shows paragraph P3 dry air covering the south half of the storm...which is inherently more stable. My thinking is that the surface center could pull this dry air into the t-storm canopy and weaken it...and in fact the t-storm canopy has already been steadily weakening since the time of Figure 3 and now. The reason max intensity is held thru 96 hrs is that he would get supportive upper divergence from accelerating southwesterly flow above the cold front he is merging with.

Impact swath in Figure 2 is initialized based on a combo of the tropical storm wind radius shown at 5 AM NHC advisory and the orange shaded t-storm clouds in the AVN infrared satellite color scheme. It becomes more symmetric about the storm track as Kirk turns more northward into his t-storm canopy...and alothough he may get southwesterly shear as he gets non-tropical...the NE track by that time still allows the impact swath to be symmetric about the storm track.

...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE INVEST 98-L...
Vigorous tropical wave is emerging from Africa in previous discussion is now SW of the Cape Verde Islands. Its cloud pattern is still signatory of easterly vertical shear on the south side of paragraph P4 upper ridge. However...its t-storm latent heat release is locally inflating this the upper ridge into an upper anticyclonic center that enhances the outflow and reduces the shear. Given the recent string of tropical wave development and computer model support this wave has...I am assuming now a 100% chance it will eventually become a tropical cyclone. For the shorter-term...computer model runs suggest that this system will not get swept up in the paragraph P3 ridge weakness that Kirk will...and therefore show a more westerly track with this system than Kirk.

...MID-LATITUDES DISCUSSION...
P1...Upper trough in the mid-latitude westerlies has entered the eastern US and eastern Canada. Upper divergence east of this upper trough continues supports a surface frontal that has moved into the eastern US that curls into a vigorous cyclone moving into NE Canada (marked with 988 mb center in above atmo chart). Upper convergence behind this upper trough supports a surface ridge building into the central US and central Canada...with multiple centers above 1020 mb becoming marked in the area.

P2...Main of upper trough persists in the Atlantic high seas southeast of Greenland...leaving behind a shortwave along the NE US shore that has pushed into the NW Atlantic in last 24 hrs. Surface frontal cyclone supported by divergence from this upper trough has moved into Europe from the British Isles...with a second frontal cyclone seen on satellite trailing behind and also about to head into the British Isles. 1020 mb frontal low...supported by divergence from the NE US-NW Atlantic shortwave mentioned above....has passed north of Bermuda. Actually...this frontal low is no longer shown in HPC or TAFB maps as if it dissipated...but there is a band of clouds NE of Bermuda that is a continuity of this system. North-central Atlantic 1018 mb low mentioned in paragraph P2 of previous discussion is now 1010 mb and moving into the Azores...its intensification supported by divergence east of the Atlantic high seas upper trough.

P3...Atlantic surface ridge with 1021 to 1022 mb centers persists. Easterly flow on the south side of this ridge (in conjunction with easterly flow on south side of paragraph P5 upper ridge) is helping to waft Africa desert dry air westward across the Atlantic tropics. Surface ridge is supported by northwesterly flow (on back sides of paragraph P2 upper troughs) converging with westerly flow (on north side of paragraph P4 upper ridge).

...TROPICAL BELT DISCUSSION...
P4...Upper ridge across the tropical Atlantic persists. T-storm latent heat release from Isaac has caused this upper ridge to concentrate into an anticyclonic center over the storm and across the Gulf of Mexico/west Caribbean. The remainder of the upper ridge is in east half of the Atlantic. A sprawling upper vortex persists in relatively lower pressures east of Isaac's upper anticyclone...currently located in the central Caribbean. Tropical wave Invest 97-L has strengthened into Tropical Storm Kirk...resulting in local inflation of the upper ridge which in turn is resulting in a persistent upper vortex SW of and adjacent to Kirk. Another embedded upper vortex in this upper ridge appears N of the Cape Verde Islands...perhaps remnants of upper vortex near the Canary Islands mentioned in paragraph P3 of previous discussion #90.

P5...Upper vortex in the Bay of Campeche is dissipating near the Yucatan as seen in the above atmo birdseye chart...getting squahsed out by the sprawling upper anticyclone of Hurricane Isaac.

Updated: 3:34 PM GMT on August 29, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #90

By: NCHurricane2009, 9:04 AM GMT on August 28, 2012

...AUGUST 28 2012...5:00 AM EDT...
Thankfully as Tropical Storm Isaac approaches the US Gulf coast...he did not rapidly intensify...nor intensify into a hurricane. Despite this...the large-sized wind field of Isaac is capable of stirring a large amount of water...and therefore the storm surge threat still exists. For all US Gulf coast residents...mandatory evacuation orders should have been obeyed in storm surge prone areas. Latest watches/warnings can be found on www.nhc.noaa.gov

Tropical wave currently in the eastern tropical Atlantic...Invest 97-L... has stopped organizing due to unfavorable upper winds. See second special feature section below for details.

Yet another tropical wave has emerged from western Africa...and continues to be a special feature on this blog. See 3rd special feature section below for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1924Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM ISAAC...
My latest forecast versus the NHC's is shown in Figure 1 below.

Track-wise for Isaac...my previous forecast was done at 5 PM EDT Aug 26 (full discussion #89)....in which I leaned toward the westward-biased GFS model due to some stated observations and theories...and also due to GFS model accurately forecasting the atmosphere. To make that judgement... basically I looked at how one of the GFS runs that day was predicting the surface and upper-level states of the atmosphere for 00Z that evening...and indeed it proved correct as we hit 00Z that evening. Even right now..that same GFS run from yesterday (in Figure 2 below) is still proving correct with near-perfection...for instance compare the 00Z Aug 28 frame of surface pressures with tonight's actual 00Z Aug 28 HPC surface map shown in that same figure. Meanwhile...the Euro model (which had a rightward bias in track) under-predicted the intensity of the central US blocking surface ridge currently building (see paragraph P1 for what is causing this surface ridge). To see this...you can compare the Figure 2 HPC map below (showing the actual isobar contours of that ridge) with yesterday's 00Z Aug 28 Euro output in Figure 3 of discussion #89.

As stated in special update #89A...Isaac has been tracking NW midway between my leftward biased track in discussion #89 and the NHC's rightward biased track at the time of discussion #89. The NHC has adjusted leftward to that current NW track...and I suppose it is appropriate for me to meet in the middle and join the current NHC forecast and current NW track. Therefore in the short-term...I have no difference to the NHC forecast track as seen in Figure 1 below. The current NW track is caused by surface ridge weakness of surface front mentioned in paragraph P1. On this track...we remain highly confident of a landfall over or very near Buras, Louisiana later this afternoon.

Longer-term...the NHC is last 24 hrs has adjusted their inland track more leftwards...basically placing it where I placed it in my previous track forecast (Figure 1 discussion #89). Therefore I also do not disagree with the longer-term inland track that the NHC currently shows...as seen in Figure 1 below. The details of the longer term track forecast make sense. We see a slow down later this afternoon and Wednesday consistent with the building of the central US blocking ridge shown in the accurate GFS run of Figure 2 below (cause of central US ridge in paragraph P1). Then later we see a northward then northeastward acceleration as the blocking ridge gets knocked out ahead of next frontal system diving southeast in the mid-latitudes. This next frontal system is shown by the last (lower-right) frame of the accurate GFS model output in Figure 2 below...via the blue shading in the upper-left of that frame representing the low surface pressures of that frontal system. This next frontal system is not yet in the scope of the above birdseye charts...nor mentioned in the mid-latitude discussion below.

Warnings remain spread across across US Gulf coast from Louisiana to ...parts of Florida panhandle...including all of the Alabama and Mississippi coastline in between. Mandatory evacuation orders in all storm surge prone areas along the US Gulf coast should have been obeyed at this point in time. Latest watches/warnings can be found on www.nhc.noaa.gov Please pay attention to local and national media for any mandatory evacuation orders.


Figure 1: My forecast for Tropical Storm Isaac this morning



Figure 2: GFS model output from Aug 26 2012 afternoon. Top four panes are sea-level pressure in mb. The bottom image is an HPC map from 00Z Aug 28 2012...which proves the near perfect accuracy of the above Aug 26 2012 GFS model output.

Intensity-wise...Isaac has not strengthened that much in the last 36 hours. Part of my reasoning for the bullish intensity forecast in my previous discussion #89 (which now obviously has big error) was that I believed Isaac's current northwesterly track would have distanced himself from the unfavorable paragraph P6 upper vortex that induced southerly shear...and was a source of adjacent dry air. Indeed the southerly shear has relaxed as he distanced himself...but some of this dry air is still entraining into the circulation (see above thermo chart...where a slot of dry air starts in the southwest Gulf near the upper vortex...curling into a narrow band reaching into the north half of Isaac's core). I think this dry air band is why Isaac's satellite presentation is impressive on his south half...but not so much the case in his north half. What is more puzzling is that on several occasions...it seemed Isaac was forming a banding-type eye and tight core as if he was about to rapidly intensify...but I think this dry air allowed the core to collapse several times such that he never really re-obtained a tight core since he left Haiti and Cuba days ago. Without a tight core...the t-storm latent heat release is not centrally concentrated...but rather spread out throughout the storm. In turn...this means Isaac's upper anticyclone is inflated by latent heat release over a wide area...and when that upper anticyclone ventilates over a wide area...surface pressures fall over a wide area rather than just at the center...creating a more lax pressure gradient (and hence less wind) for the same central pressure value. This might be why despite a low central pressure of 977 mb (as of 4 AM CDT) characteristic of a hurricane...the max sustained winds remain only at 70 mph (just under hurricane force). This wide area of surface pressure falls means Isaac's wind field (although weaker than normal for that central pressure) has a large diameter. With these thoughts...I only forecast a meager intensification to 75 mph max winds (minimal hurricane) by landfall later this afternoon...then dissipation to an inland remnant low by Thursday afternoon. This is less aggressive than what the NHC shows (NHC 10 PM CDT forecast showed a peak of 90 mph max winds before landfall...and shows a peak of 80 mph max winds per hot-off-the-press 4 AM CDT forecast).

Impact swath in Figure 1 begins with the shape/size of the tropical storm wind radius shown at 10 PM CDT NHC advisory...later declining in size to represent weakening due to landfall. Some highlights to note about the impact statements of Figure 1. Impact statement (c) highlights the storm surge threat...which is going to be more significant than usual for a tropical storm/minimal hurricane....as the creation of the large tropical storm wind radius stirs up a large amount of water...so all mandatory evacuation orders along the US Gulf coast should have been obeyed despite the less-than-expected intensification of the storm. Impact statement (b) makes a point about an emerging flood risk when the storm slows and associated heavy rains persist over the same area. Impact statement (b) also emphasizes that heavy rains can be expected in squalls outside of the shown impact swath. For example...Palm Beach on the east Florida coast saw a persistent squall of heavy rain for several hours that created unexpected flooding...despite having been outside of my drawn impact swaths for the last couple of forecasts. I expect these far reaching squalls...in part due to the large size of Isaac...to be biased to the east...thanks to the 1020 mb frontal low ejecting from SE Virginia in paragraph P2...and thanks to the incoming central US frontal system in paragraph P1 progged to spread the moisture eastward as the front itself sweeps eastward. According to the latest HPC forecasts...the worst of the rains in these eastern squalls should gradually lift northward from Florida...and into Georgia and South Carolina (and maybe into southern North Carolina).

...SPECIAL FEATURE...ATLANTIC TROPICAL WAVE INVEST 97-L...
Tropical wave west of the Cape Verde Islands is now midway between the Cape Verde Islands and Lesser Antilles. In the last 24 hrs...its cloud pattern has stopped becoming better organized about its surface low pressure spin...due to southerly shear on the east side of paragraph P5 upper vortex located adjacent to this feature. However...with the paragraph P5 upper ridging surrounding the area (which would enhance the upper outflow and reduce the shear if the adjacent upper vortex dissipated)...I do not yet want to discount potential for tropical cyclone formation.

The combination of the adjacent upper vortex west of the disturbance (paragraph P5) and Atlantic ridge north of the disturbance (paragraph P4) appear to be steering this system on a general WNW heading...albeit a turn to the west is possible if it rounds the north side of the adjacent upper vortex. I still agree that a large-scale ridge weakenss from the west will recurve this system northward then northeast into the open Atlantic. With the paragraph P2 system passing by to the north...it now appears this weakness would be a combination of paragraph P1 upper troughs/surface features shooting eastward into and across the Atlantic.

...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE EMERGING FROM AFRICA...
Yet another vigorous tropical wave is emerging from Africa into the Atlantic tropics based on satellite imagery. Its cloud pattern is signatory of easterly vertical shear on the south side of paragraph P5 upper ridge. However...as seen with the recent developments of Isaac...Joyce...and Invest 97-L...t-storm latent heat release from a tropical wave can locally inflate the upper ridge into an upper anticyclonic center that enhances the outflow and reduces the shear. Given the recent string of tropical wave development and computer model support this wave has...I am already upgrading it to a special feature on this blog...on the presumption it will become yet another Atlantic tropical cyclone. Computer model runs suggest that this system will not get swept up in the ridge weakness that 97-L will...and therefore show a more westerly track with this system than 97-L.

...MID-LATITUDES DISCUSSION...
P1...Upper trough in the mid-latitude westerlies has entered the central US and central Canada. Upper divergence east of this upper trough continues supports a surface frontal system across the central US that curls into a vigorous cyclone moving into NE Canada (marked with 988 mb center in above atmo chart). Upper convergence behind this upper trough supports a surface ridge building into the central US...with multiple centers above 1020 mb becoming marked in the area.

P2...Main portion of upper trough over the eastern US/Canada has entered the Atlantic high seas southeast of Greenland in last 72 hours...leaving behind a NW Atlantic shortwave...and leaving behind upper trough over Carolinas that is now along the NE US shore. Surface frontal cyclone supported by divergence from this upper trough...now located SE of Greenland...is heading toward Europe...with a second frontal cyclone seen on satellite trailing behind. 1020 mb frontal low over SE Virginia...supported by divergence from the NE US shore upper trough....has moved into the NW Atlantic NW of Bermuda. Aforementioned NW Atlantic shortwave has merged with parent high seas upper trough...with the associated 1020 mb surface low deepening to 1018 mb while moving into the north-central Atlantic. Warm air advection ahead of this surface frontal activity still supports north Atlantic upper ridge....but this upper ridge has merged with tropical upper ridging in paragraph P5.

P3...Cut-off upper vortex west of the Canary Islands has dissipated.

P4...Atlantic surface ridge with 1022 to 1025 mb centers persists. Easterly flow on the south side of this ridge (in conjunction with easterly flow on south side of paragraph P5 upper ridge) is helping to waft Africa desert dry air westward across the Atlantic tropics. Surface ridge is supported by northwesterly flow (on back side of paragraph P2 Atlc high seas upper trough) converging with westerly flow (on north side of paragraph P5 upper ridge).

...TROPICAL BELT DISCUSSION...
P5...Upper ridge across the tropical Atlantic persists. T-storm latent heat release from Isaac has caused this upper ridge to concentrate into an anticyclonic center over the storm and across the Gulf of Mexico/west Caribbean. The remainder of the upper ridge is in east half of the Atlantic. A sprawling upper vortex persists in relatively lower pressures east of Isaac's upper anticyclone...currently located in the eastern Caribbean. Tropical wave Invest 97-L continues to locally inflate the upper ridge with its t-storm latent heat release...resulting in a large scale inverted upper trough west of the disturbance. In last 24 hrs...this inverted upper trough has shrunken into an upper vortex adjacent to 97-L.

P6...Upper vortex in the western Caribbean Sea has been pushed into the Bay of Campeche by the sprawling upper anticyclone over Tropical Storm Isaac.

Updated: 9:15 AM GMT on August 28, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #89A (Special Update)

By: NCHurricane2009, 3:56 PM GMT on August 27, 2012

...AUGUST 27 2012...11:50 AM EDT...
This special update is written concerning Tropical Storm Isaac. At 11 AM EDT...Isaac was churning straight NW across the eastern Gulf of Mexico...located at 25.7N-84.7W. Maximum sustained winds at the center are 65 mph...as Isaac refuses to strengthen into a hurricane (75+ mph max winds).

The northwest track lies perfectly in the middle between my track and NHC's track generated at 5 PM EDT yesterday (see Figure 1 at this link for yesterday's 5 PM EDT forecast graphic). The northwest track also lies precisely on the most recent 11 AM EDT NHC graphic forecast track. The northwest track is Isaac repsonding to a low-level ridge weakness associated with central US surface front (see discussion #89...paragraph P1 of mid-latitude discussion for info on this feature). With this information...we are highly certain that the center of Isaac should make landfall near Buras, Louisiana (in the SE corner of the state) sometime Tuesday afternoon...with rainfall and gusty winds arriving well-in-advance of that time due to the large size of the storm. Storm surge impacts should already be in progress across the Gulf (worse closer to the storm center)...and will be getting worse over time near the expected landfall area. Any mandatory evacuations on the US Gulf coast should be obeyed for your safety...due to storm surge potential from Isaac's large wind field that is capable of stirring up a large amount of water. As we saw during Hurricane Ike of September 2008...it does not take an intense tropical cyclone to make big storm surge...but rather a large sized wind field also works in doing this.

Intensity-wise...it appears that Isaac should have been modeled as a large and broad-cored tropical cyclone incapable of quick intensification...also similar to what we saw with Ike of 2008 when he entered the Gulf. Like Ike....Isaac has traveled over quiet a bit of land (for example over Haiti) before entering the Gulf...and to begin with Isaac had a hard time establishing a tight core before Haiti. So perhaps it is no suprise then he is haivng a hard time establishing a tight core after Haiti. With this information...I no longer expect Isaac to become a major hurricane (115+ mph max winds). In fact...he may not ever get stronger than category 1 (75 to 95 mph max winds).

On an interesting note...despite the unusually high number of Atlantic storms so far this year...none have succeeded in become a major hurricane (115+ mph max winds).

Return to full discussion #89 for my info on the rest of the Atlantic tropics.

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2012 Atlantic Hurricane Season Birdseye Discussion #89

By: NCHurricane2009, 11:41 PM GMT on August 26, 2012

...AUGUST 26 2012...7:40 PM EDT...
This is the most important birdseye discussion I have released all season due to the ominous potential that Isaac brings to the US Gulf shores later this week. Tropical Storm Isaac has been lashing south Florida and the Florida Keys...and weather in these areas will begin improving on Monday. In the next couple of days....I believe Isaac has a high chance of becoming a major hurricane...perhaps one of super intensity. The forecast models and NHC track have shifted westward into the Mississippi-Louisana area...and I agree with an even more westerly solution headed specifically for Louisiana. For all US Gulf coast residents...please obey mandatory evacuation orders in all storm surge prone areas that may be issued in the coming hours or days. In storm surge prone New Orleans...please be mindful that you may need to evacuate in the next day or day-and-half...so begin preparations for evacuation in the event the worse case scenario starts to materialize. Latest watches/warnings can be found on www.nhc.noaa.gov Please pay attention to local and national media for any mandatory evacuation orders.

Remnants of Joyce dissipate at the surface levels...and therefore is no longer expected to regenerate into a tropical cyclone. Info on remnant of Joyce in paragraph P7.

Tropical wave currently in the eastern tropical Atlantic...Invest 97-L... has slowed its organization...but still expected to become the next Atlantic tropical cyclone. See second special feature section below for details.

Yet another tropical wave is emerging from western Africa...and is already upgraded to a special feature on this blog. See 3rd special feature section below for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1327Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM ISAAC...
My latest forecast versus the NHC's is shown in Figure 1 below.

Track-wise for Isaac...computer models are split in two camps. Models like the Euro (Figure 3) suggest that the surface ridge that builds behind the paragraph P1 upper trough will be too weak to push Isaac westward toward Louisiana...while the opposite is true of the GFS (Figure 2). For instance compare the expanse of the sea-level pressure orange shading across the central US in the 00Z Aug 28 points of both models.

Unfortunately due to objective evidence I see...I am going to lean toward the GFS solution...which is more threatening to Louisiana...including storm-surge-prone New Orleans. This means quiet a westward shift from my previous forecast (the NHC has also been shifting their track westward thru the last 24 hrs). Evidence for the longer-term westerly solution is that Isaac has been tracking more south and west compared to previous short-term track solutions by both the NHC and mine. The more southerly position means that Isaac is going to get pulled less-poleward by the surface ridge weakness ahead of paragraph P1 upper trough. The more westerly position means he is less likely to get trapped in the arpmit between the paragraph P4 ridge to the east and blocking ridge that build behind the paragraph P1 upper trough. This means he is more likely to be steered west by the blocking ridge. I also think this blocking surface ridge is going to be as strong (or stronger) than what GFS shows...especially if Isaac becomes a strong hurricane whose upper anticyclone adds resistance to the northwesterly flow behind the paragraph P1 upper trough. The added resistance would increase the upper convergence...hence strengthening the blocking surface ridge. Lastly...the GFS seems to have a good handle on the paragraph P1 upper trough timing & evolution...as what it forecasts for 00Z tonight (00Z Aug 27) is matching current 200 mb wind barbs...which shows fragment of the upper trough becoming cut-off near E New Mexico-W Texas...while the remainder of the fragment is moving into SW Hudson Bay.

After 5 PM Sunday (i.e. anytime now)...the upsteam shortwave upper ridge (between the paragraph P1 and P2 upper troughs) is still shown by GFS passing to the north of Isaac...and with the paragraph P2 surface ridging firmly in place to the north...I lean my track more leftwards (relative to NHC) between 5 PM Sun and 5 AM Mon. After 5 AM Monday is when the low-level ridge weakness from the paragraph P1 upper trough begins passing to the north of Isaac (even in the case of the westward-biased GFS)...so I bend my track gradually NW after that time.

Around Tuesday is when I expect the blocking low-level ridge beneath the back side of the paragraph P1 upper trough to slow and influence the track of Isaac. I bend the track further west from the NHC by early Wednesday as I think the low-level ridge will strengthen as Isaac and its upper anticyclone reach significant intensity (see two paragraphs ago for theory on how the strong upper anticyclone does this). By late Wednesday and beyond...I expect the next frontal system will dive southeast as shown by the models...which would knock out the blocking low-level ridge to the north and coax Isaac into a more north and eventually more northeasterly track well inland.

Warnings remain spread across across the Florida Keys...parts of south Florida and the Florida panhandle. Advisories are also spread across the Missisippi...Alabama...and Lousiana coasts...including the New Orleans Louisiana area which is most vulnerable to storm surge effects of a strong hurricane. Please obey mandatory evacuation orders in all storm surge prone areas along the US Gulf coast. In storm surge prone New Orleans...please be mindful that you may need to evacuate in the next day or day-and-half...so begin preparations for evacuation in the event the worse case scenario starts to materialize. Latest watches/warnings can be found on www.nhc.noaa.gov Please pay attention to local and national media for any mandatory evacuation orders.


Figure 1: My forecast for Tropical Storm Isaac this afternoon



Figure 2: GFS model output this afternoon. Top four panes are sea-level pressure in mb...and bottom two panes are 300 mb (upper-level) vorticity and wind direction.


Figure 3: ECMWF (Euro) model output this afternoon. Four panes are sea-level pressure in mb.

Intensity-wise...Isaac has been sputtering a bit due to southerly shear and dry air ingestion from paragraph P6 upper vortex just to his southwest. However...I urge this maybe about to change...as Isaac has been close to developing a tight core and eye on radar images from the Florida Keys. As we approach sunset...visible satellite imagery shows an increase of t-storms over the center and ominous hot-tower bursts near the banding-type eye location. His upper anticyclonic outflow appears to be energized from the latent heat release of this t-storm activity...which is squashing out the unfavorable paragraph P6 upper vortex. Given the warm Gulf waters of 30 deg C and improving upper outflow...I expect a high chance of a rapid intensification episode...so my intensity forecast in Figure 1 for Monday (which makes him a high-end category 1 hurricane at 5 PM EDT) maybe too conservative. By 5 AM Tuesday...I have him a major hurricane (115+ mph max winds). The reason I show an incredible intensification to 145 mph max winds (category 4) by 11 AM Tuesday is because I believe Isaac's northwesterly track would have distanced himself from the unfavorable paragraph P6 upper vortex...making conditions even more favorable for Isaac. As he enters less oceanic heat content (below 30 deg C) in the northern Gulf...and as he receives more land interaction...I expect Isaac to begin weakening for a category 3 landfall...which is what I show in Figure 1. After that...I show a generic weakening trend as he tracks further inland Thursday and Friday.

Impact swath in Figure 1 begins with the shape/size of the tropical storm wind radius shown at 11 PM EDT NHC advisory....growth during the forecast rapid intensification...then declination in size to represent landfall and weakening.

...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE INVEST 97-L...
Eastern Atlantic tropical wave is now west of the Cape Verde Islands. In the last 24 hrs...its cloud pattern has stopped becoming better organized about its surface low pressure spin...perhaps due to southeasterly shear on the south side of the paragraph P5 upper ridge and east side of inverted upper trough to the west of the disturbance (inverted upper trough also mentioned in paragraph P5). However...I still expect a 100% chance of this becoming a tropical cyclone (depression or storm) by sometime on Monday August 27...if not Tuesday August 28.

The combination of the inverted upper trough west of the disturbance (paragraph P5) and Atlantic deep-layered ridge north of the disturbance (paragraph P4) appear to be steering this system on a general WNW heading. Model consensus on www.wunderground.com/tropical still suggests that the large scale ridge weakness to the west of this system should recurve the track northward then NE into the open seas. Large scale ridge weakness is a combination of paragraph P1 and P2 upper troughs/surface features shooting eastward into and across the Atlantic.

...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE EMERGING FROM AFRICA...
Yet another vigorous tropical wave is emerging from Africa into the Atlantic tropics based on satellite imagery. Its cloud pattern is signatory of easterly vertical shear on the south side of paragraph P5 upper ridge. However...as seen with the recent developments of Isaac...Joyce...and Invest 97-L...t-storm latent heat release from a tropical wave can locally inflate the upper ridge into an upper anticyclonic center that enhances the outflow and reduces the shear. Given the recent string of tropical wave development and computer model support this wave has...I am already upgrading it to a special feature on this blog...on the presumption it will become yet another Atlantic tropical cyclone. Computer model runs suggest that this system will not get swept up in the ridge weakness that 97-L will...and therefore show a more westerly track with this system than 97-L.

...MID-LATITUDES DISCUSSION...
P1...Next upper trough in the mid-latitude westerlies has entered the picture from the upper-left of the above charts. Upper divergence east of this upper trough continues supports a surface frontal system across the central US that curls into a vigorous cyclone over central Canada. Upper convergence behind this upper trough supports a surface ridge entering the upper-left corner of the above atmo chart with a 1020 mb center.

P2...Main portion of upper trough over the eastern US/Canada has entered the Atlantic high seas southeast of Greenland in last 48 hours...leaving behind a NW Atlantic shortwave...and leaving behind upper vorticity from the Carolinas to the eastern Gulf that is merged with paragraph P6 upper vortex. Surface frontal cyclone supported by divergence from this upper trough...now located SE of Greenland...is intensifying on its way to Europe. 1017 mb frontal low over SE Virginia...supported by divergence from the upper vorticity over the Carolinas....has weakened to 1020 mb. Aforementioned NW Atlantic shortwave's divergence supports a frontal low north of Bermuda that weakened to 1020 mb. Warm air advection ahead of this surface frontal activity still supports north Atlantic upper ridge. Meanwhile...upper convergence on the back side of this upper troughing has been supporting a 1020 mb surface ridge across the eastern US.

P3...Cut-off upper vortex west of the Canary Islands persists.

P4...Atlantic surface ridge with 1023 to 1025 mb centers persists. The north Atlantic upper ridge (paragraph P2) has become stacked directly above....creating a deep-layered ridge axis. Deep-layered easterly flow on the south side is helping to waft Africa desert dry air westward across the Atlantic tropics. Surface ridge supported by northwesterly flow (on back side of paragraph P2 Atlc high seas upper trough) converging with westerly flow (on north side of upper ridge axis).

...TROPICAL BELT DISCUSSION...
P5...Upper ridge across the tropical Atlantic persists. T-storm latent heat release from Isaac has caused this upper ridge to concentrate into an anticyclonic center over the storm. The remainder of the upper ridge is toward west Africa. A sprawling upper vortex persists in relatively lower pressures east of Isaac's upper anticyclone. Tropical wave Invest 97-L continues to locally inflate the upper ridge with its t-storm latent heat release...resulting in a large scale inverted upper trough west of the disturbance.

P6...Upper vortex in the western Caribbean Sea persists...and is merging with fragment of paragraph P2 upper trough (extending across the eastern Gulf of Mexico to Carolinas).

P7...Remnant surface trough of Joyce has dissipated SE of Bermuda. Split flow divergence between the north Atlantic upper ridge (paragraph P2) and adjacent upper vortex (paragraph P5) supports t-storm activity in the area.

Updated: 3:37 AM GMT on August 27, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #88

By: NCHurricane2009, 5:24 AM GMT on August 26, 2012

...AUGUST 26 2012...1:30 AM EDT...
Tropical Storm Isaac en route to south Florida and the Florida Keys at this hour. Later this week....in addition to wind damage...Isaac could cause a significant rainfall and flood event in NW Florida...W Georgia...and Alabama if it meanders as forecast. There are also computer models that suggest an intense hurricane...and also a more westerly track threatening Mississippi and SE Louisiana. Although I do not agree with an intense hurricane scenario...nor the Mississippi/Lousiana scenario...residents in these areas should also keep abreast of Isaac. See Isaac special feature section for details.

Interests in Bermuda should continue monitoring the remnants of Joyce as the upper winds have become a bit more favorable for her to regenerate. Due to her potential for regeneration...I am keeping the remnants as a special feature on this blog. See Joyce special feature section for details.

Tropical wave currently in the eastern tropical Atlantic...Invest 97-L... continues to become better organized. See third special feature section below for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1925Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM ISAAC...
My latest forecast versus the NHC's is shown in Figure 1 below.

Track-wise for Isaac...initially he followed my more northward track shown in discussion #87...then he veered westward and hugged the north coast of Cuba such that the NHC track was the more accurate. I attribute this to attraction towards 1004 mb Cayman Islands low mentioned in paragraph P6. I may have also overplayed the interation with Carolinas vorticity and 1017 mb low mentioned in paragraphs P2 and P6. Because he has hugged the north coast of Cuba...my forecast track in Figure 1 is an overall leftward shift of my previous...but in the very short term is still a hair to the right of the NHC's due to the way his track is peeling off the north coast of Cuba.

After 5 PM Sunday...the upsteam shortwave upper ridge (between the paragraph P1 and P2 upper troughs) is still shown by GFS passing to the north of Isaac...and with the paragraph P2 surface ridging firmly in place to the north...I bend my track more leftwards between 5 PM Sun and 5 AM Mon. Immediately after 5 AM Monday is when the low-level ridge weakness from the paragraph P1 upper trough is to the NW of Isaac...so I bend my track back NW after that time. The more northward bend on Tuesday is due to the west lobe of paragraph P4 surface ridge influencing the steering.

In the longer term...the paragraph P1 upper trough is expected to pass north of Isaac...whose western upper convergence creates a blocking low-level ridge that slows him quiet a bit as he approaches the east US Gulf coast. I believe this blocking low-level ridge to the north...paragraph P4 low-level ridge to the east...and east-west frontal surface trough separating the two ridges...are all going to create conflicting steering for a very slow Isaac track by day 4. I lean the track rightwards on day 4 thinking the frontal surface trough might try to drag Isaac thru the narrow gap between the low-level ridges. By day 5...GFS shows the blocking ridge shifting eastward ahead of the next frontal system...and hence stacking with the paragraph P4 surface ridge. The combined ridge makes a southeasterly flow that I think would push Isaac NW by day 5.

Warnings remain spread across across much of Cuba...the Bahamas...the Florida Keys...and Florida. Latest watches/warnings can be found on www.nhc.noaa.gov


Figure 1: My forecast for Tropical Storm Isaac this morning

Thru the forecast period...Isaac should maintain his favorable upper anticyclonic outflow...so its a matter of how much exposure to water and the condition of his core in determining his future intensity. Due to the earlier land interactions with Haiti and Cuba..his core initially looked poor on infrared satellite...but is now improving with a recent t-storm burst over and NE of the center.

I show Isaac strengthening into a modest category 1 hurricane before reaching the Florida Keys tomorrow afternoon...at a similar rate shown in my previous forecast since that forecast has done quiet well in the last 24 hours (and given that his core is improving as noted above). My Gulf of Mexico intensity forecast is more bullish than previous to account for less land interaction with south Florida than I previously thought...but matches the NHC's peak of 105 mph max winds. I begin weakening him on Tuesday as I think his slower track will begin upwelling cooler waters in the NE corner of the Gulf. By 5 days...he should be inland...so I made a guess of a generic weakening rate for days 4 and 5.

Impact swath in Figure 1 begins with the shape/size of the tropical storm wind radius shown at 11 PM EDT NHC advisory. Towards the end of 5 days...I make my impact swath more representative of what I think the "heavy rain radius" will be (the wind radius by that time should be much smaller and less important due to Isaac's weakening from landfall). I did this to emphasize the potential of a flood problem looming for north Florida...east Alabama...and Georgia if this storm meanders as shown.

I would like to thank fellow Wunderground bloggers for posting several computer model runs and other Isaac info throughout the day on Dr. Jeff Master's Blog. I have added a tornado statement to my graphic in Figure 1 due to bloggers referring to tornado forecasts issued from The Weather Channel's Dr. Greg Forbes. There were also a few computer models posted that suggested a much stronger hurricane in addition to more westerly tracks than shown here that threaten Misissippi and SE Louisiana. Although I do not agree with these scenarios...residents along the central Gulf coast should also keep abreast of Isaac given the westward shift in many of the models.

...SPECIAL FEATURE...REMNANTS OF JOYCE...
This section makes reference to the previous Joyce forecast in Figure 2 of discussion #86. Click on this link to view that forecast.

Joyce remnant has weakened further into a surface trough. Remnant is beginning to escape unfavorable southerly vertical shear from east side of upper vortex. This upper vortex persists due to relatively lower pressures east of Isaac's upper outflow (see paragraph P5). As stated in the Joyce section of previous discussion #87...after 5 AM Sunday (i.e. very shortly)...I predicted she would be under more favorable upper winds beneath the north Atlantic upper ridge (paragraph P2). During 1800Z...split flow divergence between the north Atlantic upper ridge and the upper vortex supported a transient t-storm burst the north side of the remnant Joyce trough. Later on...northerly shear from Isaac's outflow...and-or a shearing upper westerly jet delivered by paragraph P1 upper trough...could limit potential for regeneration.

Her remnant remains generally on-track with what was shown in discussion #86...and therefore Bermuda should continue monitoring the remnants of this system in case she regenerates.

...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE INVEST 97-L...
Eastern Atlantic tropical wave is now west of the Cape Verde Islands. Its cloud pattern continues getting better organized about its surface low pressure spin...but still a little suggestive of easterly shear on the south side of the paragraph P5 upper ridge. However we are seeing that its t-storm latent heat release is locally inflating this upper ridge into an upper anticyclone overhead that makes the upper outflow more symmetric...which in turn is reducing the easterly vertical shear. Given the rate of organization taking place...I currently expect a 100% chance this will be a tropical cyclone (depression or storm) by sometime on Monday August 27.

The combination of the inverted upper trough west of the disturbance (paragraph P5) and Atlantic deep-layered ridge north of the disturbance (paragraph P4) appear to be steering this system on a general WNW heading. Model consensus on www.wunderground.com/tropical suggests that the large scale ridge weakness to the west of this system should recurve the track northward then NE into the open seas. Large scale ridge weakness is a combination of Isaac and Joyce both tracking more poleward...in conjunction with paragraph P1 and P2 upper troughs/surface features shooting eastward into and across the Atlantic.

...MID-LATITUDES DISCUSSION...
P1...The next upper trough in the mid-latitude westerlies has entered the picture from the upper-left of the above charts. Upper divergence east of this upper trough continues supports a diffuse surface frontal system across the central US that curls into a vigorous cyclone over central Canada. Upper convergence behind this upper trough supports a surface ridge entering the upper-left corner of the above atmo chart with a 1016 mb center.

P2...Main portion of upper trough over the eastern US/Canada has entered the Atlantic high seas south of Greenland...leaving behind a NW Atlantic shortwave...and leaving behind upper vorticity from the Carolinas to the eastern Gulf that is merged with paragraph P6 upper vortex. Surface frontal cyclone supported by divergence from this upper trough...now located SE of Greenland...remains a diffuse 1006 mb area. 1017 mb frontal low has moved northward along North Carolina coast and into SE Virginia in last 24 hrs...supported by divergence from the upper vorticity over the Carolinas. Aformentioned NW Atlantic shortwave's divergence supports a new 1019 mb frontal low north of Bermuda. Warm air advection ahead of this surface frontal activity still supports north Atlantic upper ridge. Meanwhile...upper convergence on the back side of this upper troughing has been supporting surface ridging...with multiple 1027 to 1020 mb centers across the eastern US and SE Canada.

P3...Upper trough over Atlantic high seas has exited the picture. It leaves behind a cut-off upper vortex west of the Canary Islands...and a surface frontal zone still extending into the Azores.

P4...Atlantic surface ridge with 1022 to 1023 mb centers persists. The north Atlantic upper ridge (paragraph P2) has become stacked directly above....creating a deep-layered ridge axis. Deep-layered easterly flow on the south side is helping to waft Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridge across the tropical Atlantic persists. T-storm latent heat release from Isaac has caused this upper ridge to concentrate into an anticyclonic center over the storm. The remainder of the upper ridge is toward west Africa. A sprawling upper vortex persists in relatively lower pressures east of Isaac's upper anticyclone. Tropical wave Invest 97-L continues to locally inflate the upper ridge with its t-storm latent heat release...resulting in a large scale inverted upper trough west of the disturbance.

P6...Upper vortex in the western Caribbean Sea now extends to SE Mexico and northern Central America...and is merging with fragment of paragraph P2 upper trough (extending across the eastern Gulf of Mexico to Carolinas). Split flow upper divergence between Isaac's upper anticyclone (paragraph P5) and this upper vorticity has resulted in the formation of a 1004 mb surface low near the Cayman Islands. It is possible that a tropical wave that had been west of Isaac (paragraph P9 disc #85...paragraph P8 disc #86) contributed to this 1004 mb low.

Updated: 7:49 PM GMT on August 26, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #87

By: NCHurricane2009, 10:19 AM GMT on August 25, 2012

...AUGUST 25 2012...6:20 AM EDT...
Tropical Storm Isaac has taken longer to reach Hispaniola (Haiti and the Dominican Republic) due to a southward center reformation in last 24 hrs...but is now finally crossing the island through Haiti at this hour. It is also tracking at a more northward angle than previously thought...adding to the threat in the Bahamas and Florida...and lessening (but not eliminating) the threat to parts of Cuba. Later this week....Isaac could cause a significant rainfall and flood event in northern Florida...Georgia...and eastern Alabama if it meanders as forecast. See Isaac special feature section for details.

As I forecasted explicitly in the previous discussion...Joyce has dissipated. However...interests in Bermuda should continue monitoring the remnants as the upper winds will be a bit more favorable for her to regenerate just after 24 hours from now. Due to her potential for regeneration...I am keeping the remnants as a special feature on this blog. See Joyce special feature section for details.

Shortly after the previous discussion was written...the next tropical wave currently in the eastern Atlantic was upgraded to disturbance Invest 97-L. It continues to become better organized. See third special feature section below for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1919Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM ISAAC...
In the last 24 hrs...Isaac has pulled three tricks up his sleeve that has caused all of us to once again adjust our forecasts. First...his broad center regenerated southward...which seemed to threaten Jamaica and the Cayman Islands where watches were raised. Second...the broad center finally tightened up...finally allowing him to strengthen briskly while taking advantage of the favorable environment that has been around him. Third...he has tracked more poleward (northward) than expected track...reducing the threat to Jamaica and the Caymans...but increasing the threat for Florida and the Bahamas.

Track-wise for Isaac...after studying infrared satellite loops repeatedly during this writing...it appears that he is tracking a bit to the right of the NHC forecast track shown in Figure 1...which is why my forecast track in Figure 1 begins with a rightward bias immediately. Hot-of-the-press center fix from NHC at 5 AM EDT confirms where I thought the center was. This rightward bias means that he is tracking even more poleward (northward)...but why? As mentioned in previous discussion #86 (Isaac section)...paragraph P6 upper vortex is in the process of merging with paragraph P2 upper trough...creating upper vorticity to the west of Isaac that he is close enough/tall enough to feel a poleward tug from. I also think Isaac's impressive upper anticyclonic outflow has amplified the definition of this upper vorticity...making the winds on the east side of the upper vorticity less zonal/more meridional (the more meridional flow pulling Isaac more northward). Or alternatively...the more amplified upper vorticity increases the upper divergence on its east side...which in turn is supporting a 1017 mb low off the Carolinas (see paragraph P2). I am not sure how influential the 1017 mb low's low-level ridge weakness is in pulling Isaac northward...but perhaps there is a mid-level weakness right above the 1017 mb low that is more influential.

With that said...based on GFS model animation of 200 mb (upper winds)...the paragraph P2/P6 upper vorticity is too amplified for my liking (especially over the Carolinas) thru 5 PM Sunday (36 hrs) to follow the NHC track...so I have a growing rightward bias in my track forecast thru that time. It isn't until after 36 hrs that the GFS has the Carolinas upper vorticity zoom eastward and de-amplify from Isaac's anticyclone. In the GFS...the upsteam shortwave upper ridge (between the paragraph P1 and P2 upper troughs) is also shown passing to the north of Isaac by 36 to 48 hrs...and with the paragraph P2 surface ridging firmly in place to the north...I bend my track more leftwards between 5 PM Sun and 5 AM Mon. Immediately after 48 hrs is when the large low-level ridge weakness from the paragraph P1 upper trough is to the NW of Isaac...so I bend my track back NW after 5 AM Mon.

In the longer term...the paragraph P1 upper trough is expected to pass north of Isaac...whose western upper convergence creates a blocking low-level ridge that slows him quiet a bit as he makes landfall in the Florida panhandle. I believe this blocking low-level ridge to the north...paragraph P4 low-level ridge to the east...and east-west frontal surface trough separating the two ridges...are all going to create conflicting steering for a very slow Isaac track by day 5. I lean the track rightwards on day 5 thinking the frontal surface trough might try to drag Isaac thru the narrow gap between the low-level ridges.

Warnings remain spread across across Hispaniola (Haiti and DR) and much of Cuba...and have completely encompassed the Bahamas in the last 24 hrs. Outside of some sea swells...Jamaica and parts of the Caymans should not expect much impact from Isaac thanks to the more poleward than expected track. Advisories across Florida and the Florida keys are beginning. Latest watches/warnings can be found on www.nhc.noaa.gov.


Figure 1: My forecast for Tropical Storm Isaac this morning

Intensity-wise...Isaac's core has finally tightened up...allowing him to strengthen more than I expected before landfall in Hispaniola (Haiti). His southward reformation described in the opening statement of this special feature section gave him more time before landfall to strengthen with his newly-tightened core...becoming a strong tropical storm of 70 mph max winds just before the landfall this morning. As I was writing this...the landfall has weakened him to 60 mph...so I suppose my 60 mph forecast point for this afternoon (5 PM Sat) was a good guess. Thru the forecast period...Isaac should maintain his favorable upper anticyclonic outflow...so its a matter of how much exposure to water and the condition of his core in determining his future intensity.

I show Isaac strengthening into a modest category 1 hurricane before a south Florida coast landfall late Sunday. He could get stronger than shown if his core turns out to be in better shape than I thought...or alternatively he could stay weaker than shown if the core turns out to be more battered from the Haiti landfall (and the interaction with the east tip of Cuba to occur later today). I view my forecast strengthening en-route to south Florida as compromise between these two scenarios.

After south Florida...I re-strengthen Isaac into a category 1 hurricane in the east Gulf of Mexico...then begin weakening him after 5 AM Tuesday as I think his slower track will begin upwelling cooler waters in the NE corner of the Gulf. By 5 days...he should be inland...so I made a guess of a minimal tropical storm (40 mph max winds) to represent weakening from landfall. I suppose this was a good guess...as the 5 AM EDT NHC advisory (created and released while I was writing this) also shows the same thing.

Impact swath in Figure 1 begins with the shape/size of the tropical storm wind radius shown at 5 AM NHC advisory. Towards the end of 5 days...I make my impact swath more representative of what I think the "heavy rain radius" will be (the wind radius by that time should be much smaller and less important due to Isaac's weakening from landfall). I did this to emphasize the potential of a flood problem looming for north Florida...east Alabama...and Georgia if this storm meanders as shown.

...SPECIAL FEATURE...REMNANTS OF JOYCE...
This section frequently refers to the previous Joyce forecast in Figure 2 of discussion #86. Click on this link to view that forecast.

Joyce has collapsed into a remnant low as I previously predicted due to southerly vertical shear from east side of upper vortex. This upper vortex persists due to relatively lower pressures east of Isaac's upper outflow (see paragraph P5). My previous forecast suggests potential for regeneration by 5 AM Monday...although after 5 AM Sunday (after 24 hrs) she is already north of the shearing upper vortex and in more favorable upper winds beneath the north Atlantic upper ridge (paragraph P2). However I expect her to be so beaten by the shear that it will take her a while before she has potential to regenerate beneath the more favorable north Atlantic upper ridge. Because she will be sharing this upper ridge with Isaac's tremendous outflow...some northerly shear is possible from Isaac...somewhat limiting her regeneration potential. A shearing upper westerly jet delivered by paragraph P1 upper trough by 4 to 5 days from now may also limit her regeneration potential.

Her remnant remains almost perfectly on-track with what was shown in discussion #86...and therefore Bermuda should continue monitoring the remnants of this system in case she regenerates.

...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE INVEST 97-L...
Eastern Atlantic tropical wave is now SW of the Cape Verde Islands. Its cloud pattern continues getting better organized about its surface low pressure spin...but still suggestive of easterly shear on the south side of the paragraph P5 upper ridge. However we are seeing that its t-storm latent heat release is locally inflating this upper ridge into an upper anticyclone overhead that makes the upper outflow more symmetric...which in turn is reducing the easterly vertical shear. Given the rate of organization taking place...I currently expect a 100% chance this will be a tropical cyclone (depression or storm) by sometime on Monday August 27.

...MID-LATITUDES DISCUSSION...
P1...The next upper trough in the mid-latitude westerlies has entered the picture from the upper-left of the above charts. Upper divergence east of this upper trough continues supports a diffuse surface frontal system across central Canada and central US.

P2...Upper trough over the eastern US/Canada persists. Surface frontal cyclone supported by divergence from this upper trough...now located offshore of east Canada and waters SE of Greenland...remains a diffuse 1007 to 1011 mb area. A new 1017 mb frontal low is spinning up offshore of the Carolinas. Warm air advection ahead of this surface frontal system still supports north Atlantic upper ridge. Meanwhile...upper convergence on the back side of this upper trough has been supporting surface ridging...with multiple 1024 to 1020 mb centers across the eastern US and SE Canada.

P3...Upper trough over Atlantic high seas persists. The frontal cyclone moving into the British Isles in the previous discussion has exited the picture...but a cold front extending from the cyclone extends thru the Azores.

P4...Atlantic surface ridge with 1022 mb center is supported by a few upper convergent sources while stretching from the W Atlantic to the waters offshore of SW Europe....including convergence SE of the of the north Atlantic upper ridge (paragraph P2)...and convergence behind Atlantic high seas upper trough (paragraph P3). Albeit the north Atlantic upper ridge has just stacked itself directly above the 1022 mb center such that its SE upper convergence no longer supports this surface ridge. In conjunction with south sides of north Atlantic upper ridge and E Atlantic upper ridge (paragraph P5)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridge across the tropical Atlantic persists. T-storm latent heat release from Isaac has caused this upper ridge to concentrate into an anticyclonic center over the storm. The remainder of the upper ridge is toward west Africa. An sprawling upper vortex persists in relatively lower pressures east of Isaac's upper anticyclone. Tropical wave Invest 97-L continues to locally inflate the upper ridge with its t-storm latent heat release...resulting in yet another embedded upper vortex forming just west of the Cape Verde Islands.

P6...Upper vortex in the tropical Atlantic has entered the western Caribbean Sea...and is merging with fragment of paragraph P2 upper trough that extends across the eastern Gulf of Mexico and SE US.

Updated: 10:25 AM GMT on August 25, 2012

Permalink

2012 Atlantic Hurricane Season Birdseye Discussion #86

By: NCHurricane2009, 10:06 AM GMT on August 24, 2012

...AUGUST 24 2012...6:10 AM EDT...
Tropical Storm Isaac makes the long-awaited west-northwest turn...taking it into Hispaniola this morning...where the risk of flooding and mudslides will take place throughout the day. It will later cross Cuba...brush the Bahamas...affect south Florida and the Florida keys...eventually making its way into the eastern Gulf of Mexico where residents should continue monitoring Isaac's progress. See special Isaac special feature section for details.

Tropical Depression Ten became Tropical Storm Joyce yesterday afternoon...before suddenly sputtering and weakening to a feeble tropical depression this early morning. Although Joyce is weaker...interests in Bermuda should monitor her progress in case she somehow recovers. See Joyce special feature section for details.

The next tropical wave to roll off of Africa continues to be in the NHC tropical weather outlook. Because it is gradually organizing...I am now considering this a special feature on this blog. See third special feature section below for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1916Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM ISAAC...
As of this early morning...Isaac has made the long-awaited WNW turn...which will take him across Hispaniola (Haiti and the Dominican Republic) this morning. Due to the intensity of t-storm clouds seen on infrared satellite...this looks to be a heavy rain maker...and therefore the flash flood and mudslide threat across the mountain slopes of the area is going to be a concern thru the next 24 hours. My latest forecast graphic (versus the NHC's) is shown in Figure 1 below.

Track-wise for Isaac...it is now apparent that in the short-term...I had too much of a south and west bias...and the NHC did better in forecasting the WNW turn right into Hispaniola that we are seeing this morning. In the longer term though...I did better in having a westward bias...showing a more westerly track across the eastern Gulf of Mexico that the NHC has adjusted toward in the last 24 hrs. Now as seen in Figure 1...there is no disagreement between my track and NHC track.

With the paragraph P4 low-level ridge still firmly in place to the north...why has Isaac turned more poleward in track? As mentioned in my previous Isaac section (discussion #85)...the next shortwave upper ridge (in between paragraphs P1 and P2 upper troughs)...coupled with the Gulf portion of upper ridge mentioned in paragraph P2...has stopped-up the westward progress of the paragraph P6 upper vortex. As a result...he has caught up to the upper vortex...and thus it appears this is why he has turned WNW toward Hispaniola (similar to how Joyce continues being pulled poleward by the upper vortex just west of her this morning). Even though he really has not strengthened...I suppose he is near enough to the upper vortex (and that the upper vortex is low enough in the atmosphere) to impart a steering influence on Isaac. Joyce and Isaac this morning are teaching me that if tropical cyclones are not that strong/that tall...as long as they get close to an upper vortex...they get some steering influence from that upper vortex.

Paragraph P6 upper vortex is in the process of merging with paragraph P2 upper trough...creating a large-scale upper ridge weakness still forecasted to have an extension across the Gulf of Mexico and west of Isaac thru most of the forecast period. So expect the WNW track to continue as he stays close to paragraph P6/P2 upper vorticity that is currently keeping the northward component in his steering. Remainder of paragraph P2 upper trough is expected to pass north of Isaac while having a low amplitude thanks to Isaac's impressive upper anticyclone....so I don't see Isaac bending more north toward it (As I stated in my previous two Isaac sections...discussions #84A and #85...the low-amplitude presentation of this upper trough is what led me to a steady WNW to NW track across the east Gulf early on).

Longer-term...by day 5 and beyond...Isaac's low-level circulation teams up with surface frontal system in paragraph P1 in narrowing (but not eliminating) what's left of low-level ridging to the north. Because the low-level ridge does not get a wide weakness...and because it now appears the paragraph P1 upper trough will zoom on by with a low amplitude to the north of Isaac (low amplitude will not help in pulling the storm northward)...he may stall during landfall on the east US Gulf coast...especially as the western upper convergent side of paragraph P1 upper trough builds a blocking low-level ridge due north of Isaac. This solution is what I show at the end of my forecast track in Figure 1 below.

Warnings are dropped across the Lesser Antilles...Puerto Rico...and the Virgin Islands...while warnings/watches remains spread across across Hispaniola (Haiti and DR). Advisories in the Bahamas continue to be spread in...and because of Isaac's WNW turn...the Bahamas are more likely to receive impacts. As anticipated...Cuba has also received advisories in last 24 hrs. Outside of some sea swells...Jamaica and parts of the Caymans should not expect much impact from Isaac thanks to the WNW turn. Advisories in the SW half of Florida and the Florida keys should begin anytime now...and will be spread across the eastern US Gulf shores for the next days...so interests in the east half of the Gulf of Mexico should keep abreast of Isaac.


Figure 1: My forecast for Tropical Storm Isaac this morning

Intensity-wise...Isaac refuses to strengthen...despite warm waters and a grandure upper outflow environment (characterized by enhanced upper outflow into the paragraph P6 upper vortex due west and paragraph P5 upper vortex due east)! This appears to be due to its broadly organized core that simply will not tighten up. This underscores how little fidelity there is in understanding and predicting the changes in a tropical cyclone's core. I weaken him only to 40 mph max winds despite all the land interation...as I think Isaac's broad core/large circulation will allow him to stay in touch with the waters around the islands and keep on going. But the landfalls are not going to help Isaac's broad core consolidate either...so that is why I show a lax strengthening rate when he enters the eastern Gulf of Mexico. Based on latest GFS model runs...I also think he will experience some upper outflow blockage from the paragraph P2/P6 upper vorticity to his west as he gets a bit too close to it...another reason I only show more gradual development in the eastern Gulf of Mexico. I do ramp up the strengthening rate a bit toward the end...as GFS weakens/shrinks (but doesn't eliminate) the lingering upper vorticity west of Isaac around days 4 and 5. At this time...my forecast US Gulf coast landfall intensity is barely a minimal hurricane (75 mph max winds).

Impact swath in Figure 1 begins with the shape/size of the tropical storm wind radius shown at 5 AM NHC advisory. Notice how I keep the impact swath biased toward the right of the storm track...as I think this will be the shape of his cloud bias due to the aforementioned paragraph P6/P2 upper vorticity lingering to his west. I do increase the size a bit to reflect the storm intensifying in the east Gulf....and I also add a bit more symmerty as the lingering upper voriticity could weaken/shrink by days 4 and 5 as said above.

...SPECIAL FEATURE...TROPICAL DEPRESSION JOYCE...
Tropical Depression Ten became Tropical Storm Joyce yesterday afternoon...becoming the 2nd earliest 10th Atlantic tropical storm on record. First place still belong to Jose (Aug 22, 2005)...and Joyce ties the 2nd place record with Jerry (Aug 23, 1995). She then has suddenly sputtered into a poorly-organized tropical depression.

As mentioned previously...Joyce continues interacting with an upper vortex in relatively lower pressures east of Isaac's upper outflow (see paragraph P5). East side of this upper vortex continues to be too close to Joyce such that Joyce is under southerly vertical shear that is currently causing her to collapse. One would expect that a sheared/weak/shallower tropical cyclone south of the paragraph P4 low-level ridge would continue straight west under low-level steering winds...but apparently this upper vortex is low enough in the atmosphere to leverage a more WNW track out of this shallow tropical cyclone. With the GFS timing of the upper vortex's position...I expect the current WNW track to continue through 5 AM Monday (which causes me to have a slight west bias). It should be noted that the upper vortex is south of Joyce by 5 AM Sunday...but what keeps the track going WNW is how the large horizontal circulation of Isaac deforms the low-level winds as he does his WNW track. The reason I have a bit of a west bias is how the GFS shapes the forecasted paragraph P4 low-level ridge weakness...which shows enough low-level ridging remaining to the north in my opinion to keep the track WNW thru 5 AM Mon. The low-level ridge weakness is a superposition of Isaac's large horizontal circulation and an impressive surface frontal cyclone entering the NW Atlantic (and supported by paragraph P1 upper trough) later in the forecast period.

Compared to what I showed in previous discussion #85...this is an overall rightward shift into Bermuda. I suppose a rightward shift makes sense as Isaac's forecast track shifted more northward in the last 24 hrs...and as said above Isaac's circulation plays a role in steering Joyce.


Figure 2: My Tropical Depression Joyce forecast this morning

Intensity-wise...I dissipate Joyce later today (by 5 PM EDT) due to her currently poor satellite appearance. I keep her dissipated thru 5 AM Monday...although after 5 AM Sunday she is already north of the shearing upper vortex and in more favorable upper winds beneath the north Atlantic upper ridge (paragraph P2). However I expect her to be so beaten by the shear that it takes her a while before she can regenerate beneath the more favorable north Atlantic upper ridge. Because she will be sharing this upper ridge with Isaac's tremendous outflow...some northerly shear is possible from Isaac...but I still forecast regeneration into a tropical depression by 5 AM Tue (about the time she passes over or near Bermuda). I do not want to re-strenghten her beyond tropical depression status by day 5 because it appears she will be under immense westerly shear under an upper westerly jet delivered by paragraph P1 upper trough by that time.

...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE...
Another tropical wave has entered the Atlantic tropics from west Africa. Its cloud pattern has gotten better organized about a low pressure spin...but still suggestive of easterly shear on the south side of the paragraph P5 upper ridge. However as we saw with the recent developments of Isaac and Joyce to the west...latent heat release from the t-storms of the wave can locally inflate this upper ridge into an upper anticyclone overhead that makes the upper outflow more symmetric...and reduces the easterly shear. Given the gradual organization taking place...I expect this will become our next Atlantic tropical cyclone...and therefore have granted it a special feature section on this blog.

...MID-LATITUDES DISCUSSION...
P1...The next upper trough in the mid-latitude westerlies is about to enter the picture from the upper-left of the above charts. Upper divergence east of this upper trough supports a difuse surface frontal system across central Canada and central US.

P2...Upper trough over the eastern US/Canada persists. Surface frontal cyclone supported by divergence from this upper trough...now located over east coast of Canada and waters S of Greenland...has weakened to a very diffuse 1007 to 1011 mb area. Warm air advection ahead of this surface cyclone still supports north Atlantic upper ridge. This upper ridge still has a SW-NE tilt...still extending into the Gulf of Mexico (albeit this upper ridge is now split in half by paragraph P6 upper vortex). Meanwhile...upper convergence on the back side of this upper trough has been supporting surface ridging...with multiple 1020 to 1022 mb centers across the eastern US and now SE Canada.

P3...Upper trough regime over Atlantic high seas persists. It supports a relatively new frontal cyclone (upper-right corner of above charts) moving into the British Isles. Weakening surface trough chasing ex-Gordon is now east the Azores as a surface trough. Meanwhile...ex-Gordon has dissipated north of the Canary Islands in last 24 hrs.

P4...Atlantic surface ridge with 1020 mb to 1024 mb centers is supported by a few upper convergent sources while stretching from the Gulf of Mexico to the waters offshore of SW Europe....including convergence SE of the of the north Atlantic upper ridge (paragraph P2)...and convergence behind Atlantic high seas upper trough (paragraph P3). In conjunction with south sides of north Atlantic upper ridge (paragraph P2) and E Atlantic upper ridge (paragraph P5)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridge across the eastern tropical Atlantic persists. T-storm latent heat release from Isaac has caused this upper ridge to concentrate into an anticyclonic center over the storm. The remainder of the upper ridge is toward west Africa. An embedded upper vortex persists in relatively lower pressures east of Isaac's upper anticyclone. The tropical wave in the above 3rd special feature section appears to be locally inflating the upper ridge with its t-storm latent heat release...resulting in yet another embedded upper vortex forming just SW of the Cape Verde Islands.

P6...Upper vortex in the tropical Atlantic is still entering the Caribbean Sea from eastern Cuba.

P7...Former western Gulf of Mexico disturbance Invest 95-L has dissipated in the S Texas/NE Mexico area in last 24 hrs.

P8...Tropical wave entering the central Caribbean in the previous discussion...located west of Isaac...has become indistinct in the west side of Isaac's outer low pressure field. Therefore it will no longer be mentioned on this blog.

P9...Tropical wave that was rolling off of Africa in previous discussion has been moved to its own special feature section. See above 3rd special feature section for details on this system.

Updated: 10:10 AM GMT on August 24, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #85

By: NCHurricane2009, 11:03 AM GMT on August 23, 2012

...AUGUST 23 2012...7:20 AM EDT...
Tropical Storm Isaac churning through eastern Caribbean Sea at this hour....after having delivered impacts to the northern half of the Lesser Antilles. The continued delay in its intensification shifts the major hurricane threat to the northwestern Caribbean....mainly Cuba. I think it is possible for re-intensification into a major hurricane when Isaac reaches the eastern Gulf of Mexico after Cuba. See Isaac special feature section for details.

Area of Interest Invest 96-L...Tropical wave east of Isaac (96-L) has strengthened into Tropical Depression Ten in the last 24 hours...and is expected to track generally northwest into the open Atlantic waters well to the east of Isaac. Interests in Bermuda should monitor this system. See TD 10 special feature section below for additonal details.

Although another tropical wave about to emerge from Africa has already been introduced into the NHC tropical weather outlook...I am not yet considering it a special feature on this blog because its cloud signature shows signs of unfavorable easterly vertical shear. See paragraph P10 for details on this wave.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1928Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM ISAAC...
As of 5 AM EDT (a couple of hours before the release time of this discussion)...Isaac was centered at 15.3N-64.0W...or 255 miles SSE of San Juan Puerto Rico...with maximum sustained winds at the center declining from 45 to 40 mph. My latest forecast graphic (versus the NHC's) is shown in Figure 1 below.

Track-wise for Isaac...I continue to be in a conundrum. Basically...the NHC and computer model consensus show a more north track that I do not quiet agree with. As in special update #84A...I continue to opt for a more southward solution.

One preference to a more southward solution is that since its formation...Isaac's deepest t-storm clouds still have a slight southward bias in satellite pictures...and we have seen that storms in their formative stages (like Isaac) like to regenerate their centers a bit closer to the deepest t-storm clouds...and indeed we have observed this! Because of the slight reformation...I actually have taken my forecast track from #84A and made it a bit more south...which makes me even further from the NHC and model consensus.

A second preference to a more southward track solution is that Isaac remains weaker than expected...which means he is more shallow than expected and thus is less likely to have poleward steering influence from the paragraph P6 upper vortex in the shorter-term. Isaac's large upper anticyclone also is keeping this upper vortex pushed out at a distance. However...the next shortwave upper ridge (in between paragraphs P1 and P2 upper troughs)...coupled with the Gulf portion of upper ridge mentioned in paragraph P1...should stop-up the westward progress of the paragraph P6 upper vortex. So if Isaac strengthens and gets taller as we still forecast...he would catch up to the upper vortex a bit..."feel" the upper vortex as he gets taller...and hence begin the long-awaited poleward bend in his track.

Medium-term...remnants of paragraph P6 upper vortex merge with paragraph P2 upper trough...creating a large-scale upper ridge weakness in days 2 to 4 in the W Atlantic...still forecasted to have a narrow extension across the Carolinas to Gulf of Mexico. With the paragraph P4 low-level ridge firmly to the north...the only thing causing Isaac to continue more poleward during this time is that it feels this upper ridge weakness. Having my track further south/further west by those days is also consistent with my thinking of an aggressively-strengthening Isaac whose upper anticyclonic outflow pushes out/de-amplifies the paragraph P2 upper trough (a less amplified upper trough means that strong/vertically tall Isaac won't feel as much poleward tug from the upper trough). This hunch told me to point Isaac toward the eastern Gulf of Mexico in discussion #84A...and I still believe an east Gulf solution is more likely.

Longer-term...by day 5 and beyond...Isaac's low-level circulation teams up with surface frontal system in paragraph P1 in eroding what's left of surface ridging to the north. There is also a cut-off fragment of the paragraph P2 upper trough in the west Gulf of Mexico...so I expect a track that begins bending more northward as the low-level ridge erodes and as a tall Isaac "feels" the west Gulf upper trough and paragraph P1 upper trough to the NW.

Warnings will be dropped across the Lesser Antilles with improving weather...while warnings/watches remains spread across Puerto Rico and the Virgin Islands...and have been introduced across Hispaniola (Haiti and DR) in last 24 hrs. Advisories in the Bahamas have begun...in accordance with the more north NHC track...but my southerly forecast suggests the Bahamas should go unscathed (nonetheless advisories in the Bahamas should be acted on in case the current NHC solution is the better). Regardless of the solution difference....Cuba looks to be in the crosshairs...so one expects advisories to begin there soon. Jamaica and parts of the Caymans would be more in the crosshairs with a more southerly solution...and may also soon have advisories. For Florida...the NHC solution suggests more significant impacts for the Florida Keys and west coast...while my solution is less intense for the west coast but still significant for the keys. Including Florida...interests in the eastern half of the Gulf of Mexico should monitor this system carefully.


Figure 1: My forecast for Tropical Storm Isaac this morning

Intensity-wise...Isaac has taken longer to strengthen than prevoiusly thought...now due to its broadly organized core. As my previous Isaac discussion said...I believe outside of this...Isaac has almost unlimited strengthening potential with warm waters and a good upper atmosphere. Significant upper outflow enhacement is likely/imminent from:

(1) Paragraph P6 upper vortex to its west.

(2) A second upper vortex that to the east (paragraph P5)

(3) Outflow enhacement to the north from the large-scale paragraph P2 upper trough as it moves into the Atlantic.

My intensity forecast philosophy is to first gradually strengthen Isaac...modeling him as a broadly organized core taking some time to consolidate (which appears to be the case right now). I then ramp up the strengthening rate a bit after 5 PM Saturday (when I believe Isaac's broad core would have finally consolidated). I then have a category 3 landfall for Cuba (at 115 mph max sustained winds)...weakening due to landfall...and then some resurgence as the upper winds remain favorable over Isaac as he marches into the eastern Gulf of Mexico.

Impact swath in Figure 1 begins with the shape/size of the tropical storm wind radius shown at 5 AM NHC advisory...then me imagining the tropical storm wind field growing from there and into a generically-sized strong to major hurricane. With today's skill...it is almost impossible to forecast the true size fluctuations that occur in a tropical cyclone...so the swath size could easily change in the next 5 days. The swath location could also undergo shifts in the next days if there is a good shift in forecast track.

...SPECIAL FEATURE...TROPICAL DEPRESSION TEN...
Tropical wave located well east of Isaac has strneghtened to tropical depression ten in the open tropical Atlantic waters.

As mentioned previously...an upper vortex has been forming in relatively lower pressures east of Isaac's upper outflow (see paragraph P5). Initially I thought this upper vortex would be in the right place to enhance the NW outflow of this tropical cyclone...but it is too close and now providing southerly shear. One would expect that a sheared/weak/shallower tropical cyclone south of the paragraph P4 low-level ridge would continue straight west under low-level steering winds...but apparently this upper vortex is low enough in the atmosphere to leverage a more WNW track out of this shallow tropical cyclone. As Isaac moves west...so will this upper vortex. My Isaac forecast has a west bias...so one would think that my upper vortex forecast would be faster to the west. However I still follow how the GFS moves this upper vortex...as I believe Isaac's outflow will be quiet strong to compensate for my westward bias in his track...such that the upper vortex still tracks on schedule. With the GFS timing...I expect the current WNW track to continue through 5 AM Sunday (which causes me to have a slight west bias)...because TD 10 will continue to be under the east side of this upper vortex and south of the paragraph P4 surface ridge as it is now. Later in the track forecast...the large low-level circulation of Isaac joins up with the low-level ridge weakeness of the paragraph P1 upper trough...and the way GFS shapes the low-level winds east of the weakness tells me a turn to the north is due by day 5. I still maintain a slight west bias by days 4 and 5...accounting for my westward bias in my Isaac forecast track. Because the forecast points to the waters just southwest of Bermuda...and because of the usual errors by day 5...Bermuda should watch this system.


Figure 2: My Tropical Depression Ten forecast this morning

Intensity-wise...despite southerly shear from the upper vortex...there are some impressive t-storms firing on the north half of the circulation...so I believe this is close to tropical storm status. I keep this as a marginal tropical storm thru 5 AM Sunday...accounting for the southerly shear under the east side of the upper vortex. After that time...the north Atlantic upper ridge (paragraph P2) sags southward...and this tropical cyclone will be under that upper ridge and north of the upper vortex such that shear reduces and its upper outflow gets enhanced. Because it will be sharing this upper ridge axis with Isaac's tremendous outflow...some northwesterly shear is possible from Isaac...but I still forecast strengthening by that time into a strong tropical storm. My impact swath in Figure 2 reflects the current southerly shear that TD 10 is under...then is more symmetric about the storm track when the shear reduces...then has a bit of a southeastward bias about the storm track by day 5 to reflect the possible northwesterly shear.

...MID-LATITUDES DISCUSSION...
P1...The next upper trough in the mid-latitude westerlies is about to enter the picture from the upper-left of the above charts. Upper divergence east of this upper trough supports a 1005 mb frontal cyclone over the north-central US.

P2...Upper trough over the eastern US/Canada persists. Surface frontal cyclone supported by divergence from this upper trough...now located over east coast of Canada...has weakened to a very diffuse 1008 to 1010 mb area under the less divergent upper vortex that is a product of its local cool air advection. Warm air advection ahead of this surface cyclone still supports W Atlantic upper ridge which has now spread across the north Atlantic. This upper ridge still has a SW-NE tilt...still extending into the Gulf of Mexico. Meanwhile...upper convergence on the back side of this upper trough has been supporting surface ridging...with multiple 1020 to 1022 mb centers across the eastern US.

P3...Upper trough regime over Atlantic high seas persists. Frontal cyclone just offshore of Europe has exited the picture. Western upper convergence of Atlantic high seas upper trough supports part of Atlantic surface ridge (paragraph P4). Weakening surface cyclone chasing ex-Gordon is now over the Azores as a surface trough. Meanwhile...remnant low of Gordon himself has quickly decayed into a 1022 non-tropical remnant low with zero t-storm activity due to cooler waters north of the Canary Islands and east of the Azores. So far...it shows no signs of surviving under any divergence produced by the Atlantic high seas upper trough. Therefore...Gordon continues to be a gradually dissipating surface feature.

P4...Atlantic surface ridge with 1022 mb to 1023 mb centers is supported by a few upper convergent sources while stretching from the eastern Gulf of Mexico to the waters offshore of W Europe....including convergence SE of the of the north Atlantic upper ridge (paragraph P2)...and convergence behind Atlantic high seas upper trough (paragraph P3). In conjunction with south sides of north Atlantic upper ridge (paragraph P2) and E Atlantic upper ridge (paragraph P5)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridge across the eastern tropical Atlantic persists. T-storm latent heat release from Isaac has caused this upper ridge to concentrate into an anticyclonic center over the storm. The remainder of the upper ridge is toward west Africa...a portion of which is inflated by Tropical Depression Ten's latent heat release. An upper vortex has formed between Isaac's outflow and Ten's outflow...splitting this upper ridge in half.

P6...Upper vortex in central tropical Atlantic is still retrograding westward...thanks to steering about the growing north Atlantic upper ridge (paragraph P2). The upper vortex is currently entering the Caribbean Sea from eastern Cuba.

P7...Western Gulf of Mexico disturbance Invest 95-L is no longer a special feature on this blog. As of 1800Z earlier this evening...it was a surface low of 1011 mb. After having been jammed stationary between the surface ridges in paragraphs P4 and P2...those two surface ridges are becoming more aligned to the northeast such that a low-level southeasterly steering flow is developing...carrying this system slowly NW into NE Mexico/S Texas.

P8...Tropical wave heading for the Yucatan/Bay of Campeche/SE Mexico area in previous discussion is about to exit the picture into the eastern Pacific. Therefore...this is the last mention of this tropical wave on this blog.

P9...Tropical wave entering the eastern Caribbean in the previous discussion is now headed into the central Caribbean. Tropical Storm Isaac continues to gradually strengthen to its east...this wave is losing the battle of low-level inflow against Isaac. Due to strengthening Isaac's proximity to the east...this tropical wave may become indistinct in the west side of Isaac's outer low pressure field.

P10...Yet another tropical wave is about to enter the Atlantic tropics from west Africa. Its cloud pattern suggests it is getting blasted by easterly shear on the south side of the paragraph P5 upper ridge. However as we saw with the recent developments of Isaac and TD 10 to its west...latent heat release from the t-storms of the wave can locally inflate this upper ridge into an upper anticyclone overhead that makes the upper outflow more symmetric...and reduces the easterly shear. Therefore...will watch this wave carefully...but I do not see it well-organized enough yet to make it a special feature on this blog.

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2012 Atlantic Hurricane Season Birdseye Discussion #84B (Special Update)

By: NCHurricane2009, 3:04 PM GMT on August 22, 2012

...AUGUST 22 2012...11:05 AM EDT...
This special update serves as an update statement about tropical wave Invest 96-L...located to the east of Tropical Storm Isaac.

As of 11 AM EDT...the National Hurricane Center has upgraded Invest 96-L to tropical depression ten. If this becomes a named tropical storm by 5 PM EDT today...we will tie with Tropical Storm Jose of 2005 as the earliest tenth Atlantic tropical storm on record. The mention of breaking (or tying) this record was brought up during the intro section of discussion #83.

Return to special update #84A for my most recent thoughts on Tropical Storm Isaac. Up-to-the-minute lastest information on Isaac...including watches and warnings...can be found on www.nhc.noaa.gov...or by local statements made in your area's meteorlogical service and or media.

Return to full discussion #84 for an update on the rest of the Atlantic tropics.

Updated: 3:06 PM GMT on August 22, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #84A (Special Update)

By: NCHurricane2009, 7:27 AM GMT on August 22, 2012

...AUGUST 22 2012...3:30 AM EDT...
This special update is written to provide a more updated Isaac forecast graphic and discussion. Discussion #84 was released with a graphic that was 12 hours older than the discussion itself. Paragraph P numbers below refer to paragraph numbers in the mid-latitude and tropical belt sections of discussion #84.

This special update also includes a statement about tropical wave Invest 96-L...located to the east of Tropical Storm Isaac.

Return to full discussion #84 for an update on the rest of the Atlantic tropics.

...UPDATE ON TROPICAL STORM ISAAC...
Tropical Storm Isaac continues churning toward the Lesser Antilles...Puerto Rico...and Virgin Islands. Conditions will detereorate in this areas over the next several hours...where preparations should be underway. Preparations in the Lesser Antilles will need to near completion sooner as Isaac will strike them first later today. As of 2 AM EDT (an hour before the release time of this discussion)...Isaac was centered at 15.6N-56.4W...or 345 miles east of Guadeloupe in the Lesser Antilles...with maximum sustained winds at the center of 40 mph. My updated forecast versus the NHC's is shown in Figure 1 below.

Track-wise for Isaac...I am in a conundrum at this hour. The latest GFS model runs (as well as the NHC) have been gradually shifting a bit northward...following closer to an older forecast I did in discussion #82A (where I gave more credence to paragraph P6 upper vortex to the west steering the system as Isaac became stronger/taller). In discussion #84...I had been going with a further south solution that was earlier shown by NHC (where I gave less credence to the paragraph P6 upper vortex...because Isaac was weaker/shallower than the #82A solution showed...and after some thought I concluded anyway that a more intense Isaac would have its upper outflow push the paragraph P6 upper vortex more bullishly westward and out of the way).

I still prefer the more southward discussion #84 solution...so all of my track forecast in Figure 1 below is a copy-paste of the track forecast in discussion #84. One preference to that solution is that since its formation...Isaac's deepest t-storm clouds have taken a SW bias in satellite pictures...and we have seen that storms in their formative stages (like Isaac) like to regenerate their centers a bit closer to the deepest t-storm clouds. A second preference to that solution is that it still makes sense to me that an aggressively intensfying tropical cyclone's upper anticyclone will push the paragraph P6 upper vortex out of the way. A third preference is that Isaac's is currently weaker/shallower when compared to what discussion #82A showed by now...so I would rather underplay the paragraph P6 upper vortex steering influence (hence underplay the more north #82A solution).

Longer-term (days 3 to 5)...The Atlantic low-level ridge (paragraph P4) still has no weaknesses far south enough to influence Isaac. So logic says the NW turn shown should then be caused by an upper-level weakness that a very strong/very tall Isaac can feel. By days 3 to 5...models show an upper-level ridge weakness caused by the large-scale paragraph P1 upper trough moving into the W Atlantic...featuring a narrow extension across the Carolinas to Gulf of Mexico. Having my track further south/further west by those days is also consistent with my thinking of an aggressively-strengthening Isaac whose upper anticyclonic outflow pushes out/de-amplifies the paragraph P1 upper trough (a less amplified upper trough means that strong/vertically tall Isaac won't feel as much poleward tug from the upper trough).

The track forecast for days 4 and 5 is threatening to Hispaniola (Haiti and the Dominican Republic)...Jamaica...and Cuba. The next watches or warnings are likely to be raised in these areas...so these areas should get ready to prepare when advisories are raised.

Beyond day 5...the track forecast is suggestive of a Florida threat...so interests in Florida should monitor the progress of Isaac carefully. Upstream of the paragraph P1 large-scale upper trough...the models show a shortwave upper ridge followed by the next major mid-latitude upper trough immediately after. If Isaac is as far NW as shown by day 5...the next mid-latitude upper trough's low-level ridge weakness is far south enough to steer even the weakest/shallowest of tropical cyclones poleward...so a poleward turn is imminent at some point regardless of Isaac's strength or vertical depth. If Isaac remains tall/vertically deep...I interpret the brief shortwave upper ridge would nudge him more west beyond day 5. Therefore...solutions after Florida range from a threat in the east Gulf of Mexico (if the shortwave upper ridge nudges the track west)...or alternatively a track that goes up the US east coast. I prefer the east Gulf solution at this time...in part due to the theory a couple of paragraphs ago that the paragraph P1 upper trough is going to be de-amplified by a strong Isaac upper anticyclonic outflow.


Figure 1: My forecast for Tropical Storm Isaac this early morning.

Intensity-wise...Isaac's ramp up keeps taking longer than I think...due to the dry air in paragraph P4. As old discussion #82A said...I believe outside of dry air...Isaac has almost unlimited strengthening potential with warm waters and a good upper atmosphere. Discussion #82A also suggested upper outflow enhacement from....

(1) Paragraph P6 upper vortex to its west.

(2) A second upper vortex that should form to its east. Indeed paragraph P5 in discussion #84 confirms this second upper vortex is forming.

Even better for Isaac...outflow enhacement to the north is going to come from the large-scale paragraph P1 upper trough as it moves into the Atlantic by days 3 to 5. That is why I still show the same bullish strengthening rate in Isaac that I showed in discussion #84...as I believe Isaac's ramp up is due anytime now (despite the persistent dry air intrusions that have been stopping this so far). Later on...I numb the intensity of Isaac...giving consideration to land interaction.

Impact swath in Figure 1 begins with the shape/size of the tropical storm wind field shown in the NHC advisory as of this writing. Then...I imagine the tropical storm wind field growing into a generically-sized major hurricane. With today's skill...it is almost impossible to forecast the true size fluctuations that occur in a tropical cyclone...so the swath size could easily change in the next 5 days. The swath location could also undergo shifts in the next days if there is a good shift in forecast track.

...UPDATE ON EASTERN ATLANTIC TROPICAL WAVE INVEST 96-L...
The National Hurricane Center at 2 AM EDT has increased the 48-hour tropical cyclone formation probability of this system to 90%...which probably means any time now they will start advisories on yet another Atlantic tropical cyclone. If this becomes a named tropical storm before 5 PM EDT today...we will beat Tropical Storm Jose of 2005 as the earliest tenth Atlantic tropical storm on record. The mention of breaking (or tying) this record was brought up during the intro section of discussion #83.

In general...I expect this to gradually recurve NW over the next days if it becomes an intensifying tropical cyclone. My reasoning for this is found in the Invest 96-L special feature section of discussion #84.

Permalink

2012 Atlantic Hurricane Season Birdseye Discussion #84

By: NCHurricane2009, 4:05 AM GMT on August 22, 2012

...AUGUST 21 2012...10:00 PM EDT...
Note this discussion was intended for release at 10 PM EDT...but has been released a full two hours later. This delayed release is due to technical difficulties with my computer.

Note this discussion has been created approximately 12 hours later than the following atmospheric features chart...thermodynamics chart...and TD 9 (Tropical Storm Isaac) forecast graphic. I will release a special update early tomorrow morning with an updated forecast graphic for Isaac. This special update may also include updates on any other system in the Atlantic discussed here...in case they have changed signficantly since the release of these outdated graphics.

Strong tropical wave Invest 94-L has strengthened into tropical depression 9...then into Tropical Storm Isaac. The delay in its development shifts the major hurricane threat from the northeast Caribbean to the north-central Caribbean. See Isaac special feature section for details.

Area of Interest Invest 95-L...This disturbance remains stalled in the western Gulf of Mexico as expected. See 2nd special feature section for further details.

Area of Interest Invest 96-L...Tropical wave behind Invest 94-L (now Isaac) has also become organized...and is likely to become yet another trpoical storm. See 3rd special feature section for further details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0600Z, and the 0600Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM ISAAC...
Strong tropical wave Invest 94-L has finally strengthened into Tropical Depression Nine, then into Tropical Storm Isaac...as it nears the Lesser Antilles...Puerto Rico...and the Virgin Islands. Conditions will detereorate in this areas over the next several hours...where preparations should be underway. Preparations in the Lesser Antilles will need to near completion sooner as Isaac will strike them first. As of 8 PM EDT (a couple of hours before the release time of this discussion)...Isaac was centered at 15.5N-54.9W...or 453 miles...east of Guadeloupe in the Lesser Antilles...with maximum sustained winds at the center of 40 mph. The forecast graphic (labeled as tropical depression nine) in Figure 1...and associated forecast discussion below...was created a little over 12 hours ago as of this writing. Because of this...I will be doing an updated forecast graphic in a special update in the next several hours (early this upcoming morning).

Track-wise for Isaac...in Figure 1...my first and short-term forecast points basically followed the old forecast I created in special update #82A...because of near-perfection in how this system's center has been following that track over the last days. This leads to a hair of a southward bias in the short-term points in Figure 1.

Medium-term...my track forecast re-joins the NHC's...and represents a southward shift from #82A (which had the system pass closer to Puerto Rico and pass right over Hispaniola). I suppose this makes sense...as #82A stated the northward bend toward these areas would be caused by Isaac being strong/vertically taller and being steered more poleward by paragraph P6 upper vortex...but now the delay in Isaac's formation makes him shallower/less strong than I thought he would be by now. But also...GFS (at the time of Figure 1's creation) was showing a weaker paragraph P6 upper vortex pushed bullishly away by Isaac's growing upper outflow...which also makes it less likely that this upper vortex would have a poleward influence on Isaac's track.

Longer-term (days 4 and 5)...there is an extremely tight model consensus on www.wunderground.com/tropical that makes one shy to disagree with...especially as the NHC official track also agrees with that consensus. That is why in Figure 1 I still follow NHC's track by days 4 and 5. Even by days 4 and 5...the Atlantic low-level ridge (paragraph P4) still has no weaknesses far south enough to influence Isaac. So logic says the NW turn shown should then be caused by an upper-level weakness that a very strong/very tall Isaac can feel. By days 4 and 5...models show an upper-level ridge weakness caused by the large-scale paragraph P1 upper trough moving into the W Atlantic...featuring a narrow extension across the Carolinas to E Gulf of Mexico.

The track forecast for days 4 and 5 is threatening to Hispaniola (Haiti and the Dominican Republic)...Jamaica...and Cuba. The next watches or warnings are likely to be raised in these areas...so these areas should get ready to prepare when advisories are raised.

Beyond day 5...the track forecast is suggestive of a Florida threat...so interests in Florida should monitor the progress of Isaac carefully. Upstream of the paragraph P1 large-scale upper trough...the models show a shortwave upper ridge followed by the next major mid-latitude upper trough immediately after. If Isaac is as far NW as shown by day 5...the next mid-latitude upper trough's low-level ridge weakness is far south enough to steer even the weakest/shallowest of tropical cyclones poleward...so a poleward turn is imminent beyond day 5 regardless of Isaac's strength or vertical depth. If Isaac remains tall/vertically deep...I interpret the brief shortwave upper ridge would nudge him more west beyond day 5. Therefore...solutions after Florida range from a threat in the east Gulf of Mexico (if the shortwave upper ridge nudges the track west)...or alternatively a track that goes up the US east coast.


Figure 1: My forecast for Tropical Depression Nine (TS Isaac) released a little over twelve hours ago.

Intensity-wise...Isaac has taken longer to become a tropical cyclone than prevoiusly thought...due to the dry air in paragraph P4. As old discussion #82A said...I believe outside of dry air...Isaac has almost unlimited strengthening potential with warm waters and a good upper atmosphere. Discussion #82A also suggested upper outflow enhacement from....

(1) Paragraph P6 upper vortex to its west.

(2) A second upper vortex that should form to its east. Indeed paragraph P5 below confirms this second upper vortex is forming.

Even better for Isaac...outflow enhacement to the north is going to come from the large-scale paragraph P1 upper trough as it moves into the Atlantic by days 4 and 5. And coupled with an impressive t-storm cluster at the time of Figure 1's forecast graphic...that is why I suggested a more bullish strengthening rate to 120 mph max winds (category 3 major hurricane)...more bullish than the NHC shows. This means I pose a possible major hurricane scenario in the north-central Caribbean area (Jamaica...Cuba...or Haiti). The good news is after Figure 1 was created...another round of dry air ingestion for Isaac weakened its t-storms and has caused him to already fall behind my intensity forecast schedule.

Impact swath in Figure 1 begins with the shape/size of the impressive t-storm cluster that was over/west of the center at the time Figure 1 was created...then me imagining the tropical storm wind field growing from there and into a generically-sized major hurricane. With today's skill...it is almost impossible to forecast the true size fluctuations that occur in a tropical cyclone...so the swath size could easily change in the next 5 days. The swath location could also undergo shifts in the next days if there is a good shift in forecast track.

...SPECIAL FEATURE...WEST GULF OF MEXICO DISTURBANCE INVEST 95-L...
Outflow from W Atlantic upper ridge (paragraph P1) continues to enhance t-storms in this area...which still remains semi-organized about a surface trough just offshore of Mexico and over W edge of Gulf of Mexico waters. Based on above thermo chart...upper convergence on the back side of the paragraph P1 upper trough has caused some sinking dry air to the north that has infiltrated into this system...numbing tropical cyclone potential for this system for now.

Satellite animations suggests Invest 95-L remains stationary...trapped in a low-level ridge weakness associated with this front...with the Atlantic low-level ridge (paragraph P4) to the east and west US/Canada low-level ridge to the north (paragraph P1) providing conflicting steering. Based on previous discussion #83...I expect 95-L to remain stationary thru a little after 24 hrs...then begin drifting northward to south Texas beyond that time as the blocking low-level ridge to the north gets pushed eastward by the next frontal system in the mid-latitudes. With upper winds staying favorable thru this time...a weak tropical cyclone is possible...so interests along the NE Mexico and south Texas coast should continue monitoring this system carefully. Even if no tropical cyclone development occurs...the risk of persistent rains could cause flooding problems.

...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE INVEST 96-L...
Tropical wave located east of Invest 94-L (now TS Isaac) is now southwest of the Cape Verde Islands...continiuing to feature cyclonic turning and organized t-storm activity to the degree it now looks poised to become yet another Atlantic tropical cyclone.

As mentioned in the old forecast discussed in special update #82A...there is an upper vortex that is expected to form east of 94-L (Isaac)...associated with relatively lower pressures east of Isaac's upper outflow. Latest 200 mb wind barbs in above atmo chart reveal this upper vortex is already forming (and this is concurred by the notes in paragraph P5 below). It looks as though this upper vortex will be just in the right spot to enhance the NW outflow of this tropical wave rather than shear it. Although I have not yet had time to make my own detailed assessment on this system...I do support a gradual NW recurvature into the open Atlantic waters if this system becomes a strengthening tropical cyclone. As it becomes stronger/vertically taller...it is more likely to be steered by the east side of aforementioned upper vortex...and the above Isaac special feature section suggests a large-scale upper ridge weakness which will cause Isaac to initially turn northwest (and the same can be said with this system). Even from a low-level steering perspective...as Isaac bends northward...the east side of Isaac's low-level circulation will also be coaxing this system into a NW track to the east of him.

...MID-LATITUDES DISCUSSION...
P1...Upper trough over the eastern US/Canada persists. Strong surface frontal cyclone supported by divergence from this upper trough...now located over eastern Canada...has weakened to a very diffuse 1006 mb under the less divergent upper vortex that is a product of its local cool air advection. Warm air advection ahead of this surface cyclone still supports W Atlantic upper ridge. W Atlantic upper ridge still has a SW-NE tilt...now stretching from E Mexico (where it supports the upper outflow of 95-L) to the central Atlantic waters E of Bermuda. Meanwhile...upper convergence on the back side of this upper trough has been supporting surface ridging...with multiple 1012 to 1019 mb centers across North America.

P2...Upper trough regime over Atlantic high seas persists. Frontal cyclone just offshore of Europe has exited the picture. Western upper convergence of Atlantic high seas upper trough supports surface ridging south of Greenland that has joined Atlantic surface ridge in paragraph P4. Shortwave upper trough from Atlantic Canada is still merging with Atlantic high seas upper trough (as it passes over the Azores). Over the last days...surface cyclone supported by this shortwave has moved from SE of Newfoundland to the waters W of the Azores while chasing Gordon. Meanwhile...Gordon himself has quickly decayed into a non-tropical remnant low with zero t-storm activity due to its passage over cooler waters north of the Canary Islands and east of the Azores. So far...it shows no signs of surviving under any divergence produced by the Atlantic high seas upper trough. Therefore...Gordon continues to be a gradually dissipating surface feature that will be steered increasingly southward by the above-mentioned-merger between the ridge S of Greenland and paragraph P4 ridge.

P3...Elongated upper trough in Central America in last 24 hrs has moved into the Yucatan as an upper vortex...all as it retrogrades about W Atlantic upper ridge mentioned in paragraph P1.

P4...Atlantic surface ridge with 1018 mb to 1022 mb centers is supported by a few upper convergent sources while stretching from the central Gulf of Mexico to the waters offshore of W Europe....including convergence SE of the of the W Atlantic upper ridge (paragraph P1)...and convergence behind Atlantic high seas upper trough (paragraph P2). In conjunction with south sides of W Atlantic upper ridge (paragraph P2) and E Atlantic upper ridge (paragraph P5)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridge across the eastern tropical Atlantic persists. T-storm latent heat release from tropical wave Invest 94-L (now Isaac) is causing this upper ridge to concentrate into an anticyclonic center over the storm. The remainder of the upper ridge is toward west Africa...a portion of which is inflated by Invest 96-L's latent heat release. An upper vortex is forming between Isaac's outflow and Invest 96-L's outflow...splitting this upper ridge in half.

P6...Upper vortex in central tropical Atlantic is retrograding westward...thanks to steering about the growing W Atlantic upper ridge (paragraph P1) to its north. The upper vortex is currently entering the Caribbean Sea.

P7...Tropical wave that was in the central Caribbean during the previous discussion is now in the western Caribbean heading for the Yucatan/Bay of Campeche/SE Mexico area.

P8...Tropical wave midway between the Cape Verdes and Lesser Antilles is now crossing the Lesser Antilles into the eastern Caribbean Sea. As Invest 94-L strengthened into Tropical Storm Isaac to its east...this wave is losing the battle of low-level inflow against Isaac. Due to strengthening Isaac's proximity to the east...this tropical wave may become indistinct in the west side of Isaac's outer low pressure field.

Updated: 4:24 AM GMT on August 22, 2012

Permalink

2012 Atlantic Hurricane Season Birdseye Discussion #83

By: NCHurricane2009, 1:37 AM GMT on August 20, 2012

...AUGUST 19 2012...9:40 PM EDT...
As expected...Gordon weakens to a category 1 hurricane while bearing down on the southeasternmost Azores (Sao Miguel and Santa Maria) at this hour. Weather conditions in those islands are about to become severe...and not clear up until sometime tomorrow after Gordon passes. See first special feature section below.

The tropical Atlantic is extremely active at this hour. If two of any of the following areas of interest become tropical storms in the next three days...we will be tied or ahead of the record 2005 Atlantic hurricane season...with 10 tropical storms before August 22:

Area of Interest Invest 95-L...Even though Helene and its remnant low have become assimilated into the Mexican-SW US summer monsoon low pressure field...upper winds in the western Gulf of Mexico are still favorable for tropical activity...and we already have a new pop-up disturbance in the western Gulf that has been upgraded to Invest 95-L. See 2nd special feature section for further details.

Area of Interest Invest 94-L...see 3rd special feature section for latest info on this strong tropical wave.

Other Areas of Interest...Pop-up weather in the south-central Caribbean Sea has made landfall in Central America and can no longer develop (paragraph P7). Two additional tropical waves...one ahead of and the second behind Invest 94-L...also have organized t-storm activity and cyclonic turning (see paragraphs P9 and P10). Because the tropical wave in paragraph P10 was introduced into the National Hurricane Center (NHC) tropical weather outlook while I was writing this discussion...I have just granted it a special feature section on this blog. See 4th special feature section for details on paragraph P10 tropical wave.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1916Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...HURRICANE GORDON...
For Hurricane Gordon...my latest forecast (versus the NHC's) is shown in Figure 1 below.

Track-wise for Gordon...earlier in discussions #80 and #81...I had a track that passed Gordon's center over 30W-35N...then pass just south of Santa Maria. NHC at that time was proposing the center to pass north of 30W-35N and then pass between Santa Maria and Sao Miguel. My southward bias at the time was due to me giving more credence to low-level ridge intensification W of Gordon...as the low-level ridge S of Greenland (paragraph P2) merges with low-level Atlantic ridge (paragraph P4)...and by accounts in paragraph P2...this is beginning to occurr. Well tonight...it appears that Gordon's center is tracking in the middle between my and NHC's solutions from that time...which means Gordon's center will pass very near or over Santa Maria in the next hours. This is what is shown by the NHC at this hour...and I also agree with this solution. In Figure 1...the aforementioned intensification of the low-level ridging to the west is what causes the rightward bend in Gordon's track as a non-tropical remnant cyclone.

Intensity-wise...weakening continues as Gordon crossed the 26 deg C isotherm early this morning and into cooler waters. Gordon has been more-or-less following the weaknening rate shown in Figure 1 of discussion #82...so I maintain this weakening rate in Figure 1 below (this is a slightly slower weakening rate when compared to today's 5 PM EDT NHC advisory). This means I am forecasting Gordon to hit Santa Maria with category 1 hurricane force winds in the overnight...with the current track and current hurricane wind radius suggesting the east half of Sao Miguel also getting some hurricane force winds. The frontal zone draped near the Azores has been obliterated as Gordon has moved in...but Gordon's t-storms will gradual collapse over the more stable air over cooler waters such that it still becomes non-tropical.


Figure 1: Forecast for Hurricane Gordon this evening.

Impact swath in Figure 1 is drawn based on the tropical storm wind radius shown at the NHC 5 PM EDT advisory...then extrapolating that along the forecast track. I shrink it later on based on the forecast weakening.

...SPECIAL FEATURE...WEST GULF OF MEXICO DISTURBANCE INVEST 95-L...
While over east-central Mexico...Tropical Depression Helene and its remnant low quickly became ill-defined within the low pressure field of the Mexican/SW US summer monsoon. Despite this...outflow from W Atlantic upper ridge (paragraph P1) continues to enhance t-storms in this area...which have re-organized about a 1010 mb surface low that apperas to be centerd just offshore of Mexico and over W edge of Gulf of Mexico waters...judging by latest visible satellite imagery. Because of how well-organized the low is on visible...I am considering this a special feature on this blog. This low has been classified as Invest 95-L rather than ex-Helene...which tells us that officially this is not recognized to be the remnants of Helene.

Perhaps it is no surprise that disturbed weather in this area lingers...based on previous discussions on this blog (paragraph P6 discusion #81...paragraph P6 discussion #80...paragraph P5 discussion #79). All of these discussions suggested disturbed weather that would linger here for days...and perhaps that the frontal zone from paragraph P1 would dive south into this area and add to the activity...which indeed is beginning to occur. Satellite animations suggest surface low Invest 95-L is stationary...trapped in a low-level ridge weakness associated with this front...with the Atlantic low-level ridge (paragraph P4) to the east and west US/Canada low-level ridge to the north (paragraph P1) providing conflicting steering. Based on low-level GFS model animation...I expect 95-L to remain stationary thru 72 hrs...then begin drifting northward to south Texas beyond that time as the blocking low-level ridge to the north gets pushed eastward by the next frontal system in the mid-latitudes. With upper winds staying favorable thru this time...a tropical depression or storm is possible...so interests along the NE Mexico and south Texas coast shoudl monitor this system carefully. Even if no tropical cyclone development occurs...the risk of persistent rains could cause flooding problems.

...SPECIAL FEATURE...TROPICAL WAVE INVEST 94-L...
Strong tropical wave now WSW of the Cape Verde Islands is still showing impressive t-storm activity and cyclonic turning in those clouds. Within the paragraph P5 upper ridge...t-storm latent heat release has generated a warm core upper anticyclone aloft....which has reducing the easterly shear and enhanced the upper outflow. As a result...tropical cyclone formation is still likely...and I am predicting a 100% chance of a tropical depression or tropical storm out of this system eventually. I predicted tropical cyclone formation from this system sometime today...but it appears this has been delayed due to some level of dry air ingestion (source of dry air in paragraph P4).

In special update #82A...I had provided a 5-day (120 hr) outlook for this system...which is now a 4-day (96 hr) outlook. The paragraph P numbers in that outlook still fit with the mid-latitude and tropical belt discussion P numbers in this dicsussion as well. As a current 4-day outlook...I still stand behind that forecast for now...but I would say that the threat of a major hurricane in the NE Caribbean area is reducing due to the delay in this system becoming a tropical cyclone. So far...the track forecast shown in that outlook is doing quiet well. The more WNW track I show (which brings the storm into Hispaniola by day 4) has a northward bias compared to the main GFS model run...but on the other hand is in the midst of the spread of GFS's ensemble members.

...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE...
Another tropical wave has emerged from Africa recently...located well east of Invest 94-L...southeast of the Cape Verde Islands...and featuring cyclonic turning and organized t-storm activity. As I was writing this discussion...it has been introduced into the NHC tropical weather outlook...and likewise I have voted to give this a special feature section due to its organization.

As mentioned in the Invest 94-L forecast (special update #82A)...there is an upper vortex that is expected to form east of 94-L...associated with relatively lower pressures east of 94-L's upper anticyclone. Latest 200 mb wind barbs in above atmo chart reveal this upper vortex is already forming. Will have to carefully monitor if this emerging upper vortex suppresses or enhances the outflow of this tropical wave...depending on its relative position to this wave. If outflow enhancement occurs...tropical cyclone formation risk from this system is going to increase.

...MID-LATITUDES DISCUSSION...
P1...Upper trough over the eastern US/Canada persists. Strong surface frontal cyclone supported by divergence from this upper trough...located over southern Hudson Bay...has weakened to 1004 mb under the less divergent upper vortex that is a product of its local cool air advection. Warm air advection ahead of this surface cyclone now supports W Atlantic upper ridge. W Atlantic upper ridge still has a SW-NE tilt...now stretching from E Mexico (where it supports the upper outflow of 95-L) to the central Atlantic waters E of Bermuda. Meanwhile...upper convergence on the back side of this upper trough has been supporting surface ridging...with 1020 to 1022 mb centers over the western US/Canada as seen in the upper-left of the above atmo chart.

P2...Upper trough regime over Atlantic high seas persists. Frontal cyclone just offshore of Europe in previous discussion is still performing a meandering loop beneath cyclonic flow of amplified Atlantic high seas upper trough. Western upper convergence of Atlantic high seas upper trough supports a 1023 mb ridge just south of Greenland. Shortwave upper trough from Atlantic Canada is merging with Atlantic high seas upper trough...therefore losing its identity. Surface cyclone supported by this shortwave has moved from SE of Newfoundland to the waters W of the Azores while chasing Hurricane Gordon. Upper covnergence on back side of high seas upper trough is causing the 1023 mb ridge south of Greenland to merge with Atlantic surface ridge in paragraph P4...so either this surface cyclone will become squashed out of existence from this surface ridge development...or alternatively will dive southeastward on the east side of this surface ridge development.

P3...Elongated upper trough in the central Caribbean has been pushed westward into central America in last 24 hrs as it retrogrades about W Atlantic upper ridge mentioned in paragraph P1.

P4...Atlantic surface ridge with 1020 mb to 1023 mb centers is supported by a few upper convergent sources while stretching from the central Gulf of Mexico to the waters offshore of W Europe....including convergence SE of the of the W Atlantic upper ridge (paragraph P1)...and convergence behind Atlantic high seas upper trough (paragraph P2). In conjunction with south sides of W Atlantic upper ridge (paragraph P2) and E Atlantic upper ridge (paragraph P5)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridge across the eastern tropical Atlantic persists. Hurricane Gordon continues to be near the north edge of this upper ridge. T-storm latent heat release from Gordon continues to locally inflate this edge of the upper ridge...which has resulted in an embedded shortwave upper trough south of Gordon. T-storm latent heat release from tropical wave Invest 94-L is causing this upper ridge to concentrate into an anticyclonic center over Invest 94-L.

P6...Upper vortex in central tropical Atlantic is retrograding westward...thanks to growing W Atlantic upper ridge (paragraph P1) to its NW. The upper vortex is currently passing north of the Lesser Antilles.

P7...Visible satellite imagery shows that yesterday's disturbed weather in the south-central Caribbean with light cyclonic turning has made landfall in Central America. The landfall and suppression beneath the paragraph P3 upper vorticity means that tropical cyclone formation is no longer possible with this system. Relatively new E Caribbean upper ridge that enhanced this disturbance yesterday still persists.

P8...Tropical wave that was in the eastern Caribbean in the previous discussion is now in the central Caribbean. It is currently beneath split flow upper divergence between the E Caribbean upper ridge in paragraph P7 and upper vorticity in paragraph P3...so an upheaval in t-storms with this tropical wave is possible during the next hours.

P9...Tropical wave midway between the Cape Verdes and Lesser Antilles continues producing t-storms extending hundreds of miles west of Invest 94-L. There remains enough cyclonic turning is in these clouds such that the NHC TAFB analyzed a 1010 mb low on the south end of this tropical wave as of 1800Z TAFB (and as shown in above atmo chart). Its t-storms remain supported by outflow beneath the E Atlantic upper ridge (paragraph P5)...coupled with enhanced poleward outflow streaming into paragraph P6 upper vortex.

P10...Another tropical wave has emerged from Africa recently...located well east of Invest 94-L...southeast of the Cape Verde Islands...and featuring cyclonic turning and organized t-storm activity. As I was writing this discussion...it has been introduced into the NHC tropical weather outlook...and likewise I have voted to give this a special feature section due to its organization. See 4th special feature section above for details on this system.

Updated: 1:42 AM GMT on August 20, 2012

Permalink

2012 Atlantic Hurricane Season Birdseye Discussion #82A (Special Update)

By: NCHurricane2009, 7:04 AM GMT on August 19, 2012

...AUGUST 19 2012...3:05 AM EDT...
Later this week...a major hurricane is possible for the northeastern Caribbean region from what is now tropical wave Invest 94-L.

Short-term outlook for Invest 94-L from full discussion #82 (paragraph P numbers are associated with full discussion #82):

Strong tropical wave SE of the Cape Verde Islands 48 hrs ago is now SW of the Cape Verde Islands...still showing impressive t-storm activity and now showing cyclonic turning in those clouds. Within the paragraph P5 upper ridge...t-storm latent heat release has been attempting to generate a warm core upper anticyclone aloft....which is reducing the easterly shear on the south side of that upper ridge. As a result...tropical cyclone formation is more likely...and I am predicting a 100% chance of a tropical depression or tropical storm out of this system sometime Sunday (i.e. later today).

My longer-term outlook for Invest 94-L based on 00Z GFS model run of surface and 200 mb (upper-level) dynamics (paragraph P numbers are associated with full discussion #82):

My outlook is summarized in Figure 1 below. Intensity-wise...one impediment against 94L would be the central Atlantic upper vortex (paragraph P6) thereatening to shear the system...but the GFS model has this upper vortex retrograde westward and out of the way thanks to sprawling upper anticyclone over Invest 94L (assuming 94L becomes a strong tropical cyclone) and thanks to W Atlc upper ridge forecast to grow strong in warm air advection ahead of vigorous frontal system mentioned in paragraph P1. GFS also spins up another upper vortex to the east of 94L that tries to chase 94L. I think this second upper vortex is due to relatively lower pressures east of 94L's sprawling upper anticyclone (assuming 94L grows into a strong tropical cyclone)...and perhaps due to some of the shortwave upper vorticity associated with dissipating surface cyclone SE of Newfoundland mentioned in paragraph P2 (the GFS at the surface shows some of this surface cyclone's vorticity swing southeastward to the NE of 94L...which is why I state this).

All-in-all...with the GFS showing a sprawling upper anticyclone persisting with 94L through the forecast period...I expect that both of these upper vortex centers will not shear 94L...but if anything enhance the upper outflow east and west of 94L...and therefore I see almost unlimited strengthening potential in the next 5 days. My intensity forecast in Figure 1 reflects this...bringing 94L to a category 3 115 mph max wind major hurricane by day 4 (5 AM Thu) in the NE Caribbean area...then weakening 94L to a regular hurricane as it crosses the unfavorable mountainous terrain of Hispaniola (Haiti and the Dominican Republic) by day 5 (5 AM Fri). My intensification rate in the short-term starts off a bit slower...representing the gradual development we are used to during tropical cyclone formation...and also due to some potential for 94L to ingest some of the dry air to the northwest (paragraph P4) at it initially spins up into a tropical cyclone.

Track-wise...at the surface...low-level ridging (paragraph P4) is firmly established to the north of 94L thru the forecast period...with all low-level ridge weaknesses shown to be too far north to have any effect of 94L's steering. Therefore...the only thing to pull 94L northward in the next 5 days would be some low-latitude upper vorticity...that is if 94L becomes a strong/vertically tall tropical cyclone capable of "feeling" such upper vorticity. Based on the 00Z GFS model run...thru 96 and 120 hrs...there is upper vorticity lingering over the Bahamas that retrogrades into Florida and E Gulf of Mexico associated with what is now the paragraph P6 upper vortex. The base of the paragraph P1 upper trough is shown lingering over the Carolinas and SW Atlantic by that time as well...so that is why my 96 hr and 120 hr forecast positions are bent more northward...to reflect a strong/vertically tall tropical cyclone "feeling" all this upper vorticity.

Other things to note about my track forecast in Figure 1. If for some reason 94L remains a weak/shallow feature by 96 and 120 hrs...my track forecast will have a northward bias...as the low-level Atlantic ridge (paragraph P4) will be firmly in place to instead steer it straight west thru that time. In addition...my longitudinal positions for 94L are a bit ahead of where the 00Z GFS generally places 94L...as the initial 7-hourly motion shown in Figure 1 suggests to me that GFS was carrying this system a bit too slowly to the west.


Figure 1: My best guess track and intensity forecast for tropical wave Invest 94-L this early morning. TS = Tropical Storm, H = Hurricane, MH = Major Hurricane

Return to full discussion #82 for an assessment on the rest of the Atlantic tropics.

Updated: 7:27 AM GMT on August 19, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #82

By: NCHurricane2009, 11:56 PM GMT on August 18, 2012

...AUGUST 18 2012...8:00 PM EDT...
While still moving eastward toward southern Azores region...Tropical Storm Gordon has rapidly intensified into a category 2 hurricane in the last 36 hours as stated in special update #81B. As a result...the southern Azores are much more likely to see a fully-tropical Gordon...and see more intense winds than previously thought. Gordon is still expected to reach the southern Azores by late Sunday (late tomorrow) through part of Monday. See the 1st special feature section below for further details.

Elsewhere...remnant of tropical depression seven strengthened to Tropical Storm Helene yesterday afternoon in the Bay of Campeche...just offshore of Mexico...as stated in special update #81A. It has since made landfall and weakened to Tropical Depression Helene...and additional details on Helene are in the 2nd special feature section below. See 3rd special feature section for info on strong tropical wave Invest 94-L in the eastern tropical Atlantic. I am also monitoring pop-up weather in the south-central Caribbean Sea (see paragraph P8 for details).

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 0130Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...HURRICANE GORDON...
Gordon is confounding at this hour. While my previous forecast did say he would briefly become a hurricane today...he rapidly intensified into a category 2 of 105 mph winds as of 5 PM EDT. As predicted during the Gordon special feature section in discussion #81...the two upper troughs in paragraph P2 are merging...with a more zonal upper westerly jet connecting the south sides of both upper troughs and overspreading Gordon's general region. So why is Gordon doing so well when there is a 200 mb upper westerly jet right above it that should be shearing it? Perhaps its entire vertical structure (surface spin and upper outflow) has become tucked below the 200 mb upper winds. The last time we saw this was during Hurricane Chris in June (see Chris special feature section in discussion #33). But we know at one point Gordon was tall enough to be sensitive to 200 mb shearing...as we cited westerly shear in the Gordon special feature section of discussions #81 and #80.

It is also possible that the cool temps of paragraph P2 upper troughing are de-stabilizing the atmosphere despite surface water temps that are lukewarm...which is also something similar we saw during Chris. Furthermore...it is possible that the eastward acceleration in Gordon's track makes it more resilient to the 200 mb upper westerly jet's shear.

My latest forecast (versus the NHC's) is shown in Figure 1 below.

Track-wise for Gordon...beyond 24 hrs...we are still expecting the two upper troughs in paragraph P2 to be fully merged...with upper convergence behind the merged upper trough causing the 1024 mb ridge S of Greenland (paragraph P2) and Atlantic surface ridge (paragraph P4) to strengthen west of Gordon (this development is also discussed in paragraph P2). In discussions #80 and #81...I had a southward bias relative to NHC as I gave more credence to this low-level ridge development W of Gordon. Today...the NHC track has become aligned with the track forecast I showed in discussions #80 and #81...so I see no reason today to disagree with the NHC track.

Intensity-wise...weakening has got to be imminent as Gordon will still be crossing the 26 deg C isotherm into cooler waters by 5 AM Sun. Since I am confounded by the recent intensification to category 2...I am just going to go along with the NHC's weakning rate shown in their advisory at 5 PM EDT. He is still en route to merge with the frontal zone draped near the Azores (hence eventually becoming a non-tropical frontal low)...courtesy of the Atlantic high seas upper trough/surface cyclone offshore of Europe (paragraph P2). How strong Gordon is during its non-tropical phase depends on how much upper divergence he sees from the east side of the high seas upper trough.


Figure 1: Forecast for Hurricane Gordon this afternoon.

Impact swath in Figure 1 is drawn based on the tropical storm wind radius shown at the NHC 5 PM EDT advisory...then extrapolating that along the forecast track. I shrink it later on based on the forecast weakening.

...SPECIAL FEATURE...TROPICAL DEPRESSION HELENE...
Yesterday afternoon...warm waters in the Bay of Campeche...coupled with supportive upper outflow from W Atlantic upper ridge (paragraphs P1 and P2)...allowed the remnants of depression Seven to finally regenerate into Tropical Storm Helene (special update #81A). She has since made landfall in eastern Mexico today...where she has weakened to a tropcial depression. As of 5 PM EDT today...a small t-storm burst just west of her center caused the NHC to maintain advisories on this system...and scattered t-storms in her circulation continue to bring a flood/mudslide threat along the mountain slopes of east-central Mexico. As a weaker/shallow type of tropical cyclone...she has been tracking WNW to NW in last 36 hrs...steered by low-level flow around the low-level ridge in paragraph P4...and also by the low-level ridge building beneath the convergent back side of paragraph P1 upper trough. As paragraph P1 mentions...this low-level ridge has not been steadily-defined...which is why Helene has been tracking slowly and erratically.

Her continually degrading satellite appearance tells me she will dissipate as a tropical cyclone anytime now...and I think her remnant low will quickly become ill-defined within the low pressure field of the Mexican/SW US summer monsoon. That is why I am not doing a forecast or prolonged outlook for Helene (like I did for Gordon).

...SPECIAL FEATURE...TROPICAL WAVE INVEST 94-L...
Strong tropical wave SE of the Cape Verde Islands 36 hrs ago is now SW of the Cape Verde Islands...still showing impressive t-storm activity and now showing cyclonic turning in those clouds. Within the paragraph P5 upper ridge...t-storm latent heat release has been attempting to generate a warm core upper anticyclone aloft....which is reducing the easterly shear on the south side of that upper ridge. As a result...tropical cyclone formation is more likely...and I am predicting a 100% chance of a tropical depression or tropical storm out of this system sometime Sunday (i.e. in the next 24 hrs). Due to the high levels of tropical activity elsewhere in the Atlantic basin...I have not had time to make my own assessment on a longer-term outlook for this system. However...latest computer model spread on www.wunderground.com/tropical are gung-ho about pushing this system west to WNW into the Lesser Antilles and northern Caribbean region in the next days...so interests in these regions should begin monitoring this system carefully.

...MID-LATITUDES DISCUSSION...
P1...Upper trough over central US/Canada is now over eastern US/Canada. Strong surface frontal cyclone supported by divergence from this upper truogh...located over southern Hudson Bay...has intensified further to 995 mb. Local cool air advection has caused an upper low to form along the upper trough axis...and the surface cyclone is now nearly-stationary beneath this upper low. Warm air advection ahead of this surface cyclone now supports W Atlantic upper ridge that used to be supported by warm air advection ahead of paragraph P2 system (see paragraph P2 for current state of this W Atlantic upper ridge). Meanwhile...upper convergence on the back side of this upper trough has been supporting surface ridging...but so far the surface ridging has not been steady...with one surface ridge tracking from the west US to the Great Lakes (is currently 1018 mb over SE Michigan in the above atmo chart)...and another surface ridge developing over the NW US.

P2...Upper trough regime over eastern Canada remains is now all consolidated in the Atlantic high seas. Frontal cyclone just offshore of Europe in previous discussion is still performing a meandering loop beneath cyclonic flow of amplified Atlantic high seas upper trough. Western upper convergence of Atlantic high seas upper trough supports a 1024 mb ridge just south of Greenland. Shortwave upper trough over Atlantic Canada is merging with Atlantic high seas upper trough...therefore losing its identity. Surface cyclone supported by this shortwave has moved from E Maine to SE of Newfoundland in last 36 hrs...and in next 24 hrs should dissipate as upper covnergence on back side of high seas upper trough causes the 1024 mb ridge south of Greenland to merge with Atlantic surface ridge in paragraph P4 (hence squashing out the existence of this surface cyclone). W Atlantic upper ridge south of this upper trough regime has gained a SW-NE tilt in last 48 hrs...now stretching from E Mexico (where it supports the upper outflow of TD Helene) to the central Atlantic waters E of Bermuda. Because this W Atlantic upper ridge is now supported by warm air advection ahead of 995 mb cyclone in paragraph P1...this W Atlantic upper ridge will be moved to that system's paragraph in next discussion.

P3...Elongated upper trough E of Bermuda persists...which still extends into the central Caribbean. An upper vortex crowns the N end of this elongated upper trough...embedded in the midst of W Atlantic upper ridge mentioned in paragraphs P1 and P2.

P4...Atlantic surface ridge with 1018 mb to 1022 mb centers is supported by a few upper convergent sources while stretching from the northern Gulf of Mexico to Canary Islands....including convergence SE of the of the W Atlantic upper ridge (paragraphs P1 and P2)...convergence behind the upper trough in paragraph P1...and convergence behind Atlantic high seas upper trough (paragraph P2). In conjunction with south sides of W Atlantic upper ridge (paragraph P2) and E Atlantic upper ridge (paragraph P5)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridge across the eastern tropical Atlantic persists. Hurricane Gordon continues to be near the NW edge of this upper ridge. T-storm latent heat release from Gordon continues to locally inflate this corner of the upper ridge...which has resulted in an embedded shortwave upper trough W of Gordon. This shortwave upper trough W of Gordon was also noted in paragraph P5 of previous discussion #81...and I am still not sure if this shortwave upper trough was the result of Gordon's cyclonic circulation advecting in some of the mid-upper level cooler air associated with the upper vorticity mentioned in paragraph P3.

P6...A new upper vortex has spun up in the central tropical Atlantic...thanks to relatively lower pressures in between the E Atlantic upper ridge in paragraph P5 and W Atlantic upper ridge mentioned in paragraph P1 and P2.

P7...Remnant of depression seven in the Bay of Campeche became Tropical Cyclone Helene just offshore of E Mexico last afternoon (special update #81A)...and therefore has been granted its own special feature section above. See above Helene special feature section for further details on this system.

P8...South fragment of pre-Gordon tropical wave in eastern Caribbean area was removed from NHC TAFB analyses yesterday (see paragraph P7 of discussion #81). Since then...there has been a flare up of t-storms with light cyclonic rotation in the south-central Caribbean...which I think maybe associated with this tropical wave fragment. This area of disturbed weather is located east of the upper vorticity in paragraph P3...and west of a new east Caribbean upper ridge (devloping between the paragraph P3 and paragraph P6 upper vorticities). Split flow upper divergence between the paragraph P3 upper vorticity and new E Caribbean upper ridge is also supporting the t-storms. So far...no computer models develop this system...but I will be watching this system carefully due to the favorable upper divergence. If signs of development occur before this moves into Central America (and before my next full discussion)...then I will write a special update.

P9...Tropical wave expected to cross the Lesser Antilles in the previous discussion has done so...and is now in the eastern Caribbean. Although it has escaped the dry air (paragraph P4)...westerly shear on north side of E Caribbean upper ridge (paragraph P8) is preventing development with this system.

P10...Tropical wave headed into the waters midway between the Cape Verdes and Lesser Antilles has arrived there. It has been producing t-storms extending hundreds of miles west of Invest 94-L. There is enough cyclonic turning is in these clouds such that the NHC TAFB analyzed a 1013 mb low on the south end of this tropical wave as of 1200Z TAFB (and as shown in above atmo chart). The development of its t-storms is due to supportive outflow beneath the E Atlantic upper ridge (paragraph P5)...coupled with enhanced poleward outflow streaming into paragraph P6 upper vortex.

Updated: 12:01 AM GMT on August 19, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #81B (Special Update)

By: NCHurricane2009, 9:07 PM GMT on August 18, 2012

...AUGUST 18 2012...5:10 PM EDT...
This special update is written concerning Gordon...which has unexpectedly strengthened into a category 2 hurricane while en route to the southern Azores region. While Gordon was expected to possibly become a hurricane today...this much intensification was not anticipated. As a result...the southern Azores are much more likely to see a fully-tropical Gordon...and see more intense winds than previously thought. Gordon is still expected to reach the southern Azores by late Sunday (late tomorrow) through part of Monday. See www.nhc.noaa.gov for latest warnings and forecasts on Gordon.

In the next couple of hours...I will be releasing my next full discussion of the Atlantic tropics. Some theories on how Gordon was able to unexpectedly strengthen will be discussed. This next full discussion has taken quiet a bit longer to produce due to the fairly busy activity in the Atlantic tropics...with Hurricane Gordon...Helene in eastern Mexico...and strong tropical wave Invest 94-L in the eastern tropical Atlantic.

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2012 Atlantic Hurricane Season Birdseye Discussion #81A (Special Update)

By: NCHurricane2009, 10:36 PM GMT on August 17, 2012

...AUGUST 17 2012...6:35 PM EDT...
Tropical storm warnings are suddenly required for the east coast of Mexico...as a recon aircraft investigation finds the remnants of Tropical Depression Seven in the Bay of Campeche have regenerated into Tropical Storm Helene. This is contrary to my earlier statement on this system in full discussion #81...where I forecasted this to become close to a tropical depression (but not quiet get there) before landfall in Mexico. If this remains a tropical cyclone by the time I write my next full discussion (late this evening or early tomorrow)...then it will be awarded a special feature section and forecast on my blog.

Latest information on Helene...including tropical storm warnings and forecasts...can be found on the National Hurricane Center website (www.nhc.noaa.gov)

Figure 1 below is a live satellite image of Tropical Storm Helene

Return to full discussion #81 for an assessment on the rest of the Atlantic tropics.


Figure 1: Live satellite image of Tropical Storm Helene

Updated: 10:38 PM GMT on August 17, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #81

By: NCHurricane2009, 11:14 AM GMT on August 17, 2012

...AUGUST 17 2012...7:15 AM EDT...
Tropical Storm Gordon trucking eastward toward the southern Azores region...likely to affect the southern islands with some tangible weather (gusty winds and rain) as a non-tropical system late Sunday through part of Monday. See special feature section for details.

Elsewhere...remnant of tropical depression seven has arrived to the Bay of Campeche while becoming a bit better organized...but I am not yet re-upgrading it to a special feature on this blog until I see a more imminent trend of tropical cyclone reformation (see paragraph P6 for details). A strong tropical wave has just emerged from Africa...and is already upgraded to disturbance Invest 94-L. I believe this tropical wave will eventually become a tropical cyclone...and therefore have added it as a special feature on my blog this morning. See second special feature section for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0130Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM GORDON...
Tropical Storm Gordon initially surprised us in the last 24 hrs by strengthening more than expected...but now has weakened a bit from earlier-than-expected westerly shear induced by SW side of Atlantic high seas upper trough (paragraph P2)...and induced by shortwave upper trough just west of Gordon (paragraph P5). My latest forecast (versus the NHC's) is shown in Figure 1 below.

Track-wise for Gordon...beyond 48 hrs...models still show east US shortwave upper trough (now entering Atlantic Canada) joining Atlantic high seas upper trough (both upper troughs mentioned in paragraph P2)...with upper convergence behind the merged upper trough causing the 1026 mb ridge S of Greenland (paragraph P2) and Atlantic surface ridge (paragraph P4) to strengthen west of Gordon. By 96 thru 120 hrs (5 AM Tue thru 5 AM Wed)...GFS still shows an impressive low-level ridge stomping in from the west and into the Azores. Therefore...I still show my southward bias in the later part of the track forecast...as I still am giving more credence to the forecast low-level ridge development just west of Gordon. The NHC has also done some southward adjustment of the short-term track forecast as of 5 AM EDT...as they note Gordon will be exposed to some deep-layered NW flow on the back side of the Atlantic high seas upper trough/surface cyclone offshore of Europe (paragraph P2). This also increases my confidence in showing a southward bias in the later part of the track forecast.

Intensity-wise...warm enough waters and paragraph P5 upper ridge outflow initially strengthened Gordon more than expected such that he got ahead of my previous intensity forecast. This early morning...we are finding that earlier-than-anticipated westerly shear (1st paragraph of this special feature section) is weakening Gordon...so now he has just fallen behind my previous intensity forecast. My intensity forecat in Figure 1 below is similar to my previous...but a little less vigorous (given that he has just fallen behind). My intensity forecast below suggests strengthening to a minimal hurricane (75 mph max winds) by 24 hrs...as I think the shortwave high-latitude uppper ridge between the two upper troughs in paragraph P2 could transiently reduce the westerly shear. After 24 hrs...the GFS model 200 mb (upper) wind forecast shows the two upper troughs merged...with a more zonal upper westerly jet connecting the south sides of both upper troughs and overspreading Gordon's general region. So after 24 hrs...westerly shear will pick up in earnest...so I forecast Gordon to weaken after 24 hrs. By 5 AM Sun (48 hrs)...Gordon will be crossing the 26 deg C isotherm into cooler waters...and with a frontal zone already draped across the Azores this morning...we can expect Gordon to quickly become a non-tropical frontal low around or just after that time. The frontal zone is courtesy of Atlantic high seas upper trough/surface cyclone offshore of Europe (paragraph P2)...and how strong Gordon becomes as a non-tropical low depends on how much upper divergence he sees from the east side of the high seas upper trough. An alternate scenario was shown by 00Z GFS this morning...which shows Gordon as a dissipating surface trough located N of the Canary Islands as the forecast strong low-level ridge stomps in from the west.


Figure 1: Forecast for Tropical Storm Gordon this morning.

Impact swath in Figure 1 is drawn based on my forecast track. Its initial shape is based on the NE-biased storm canopy seen on infrared imagery...then I extrapolate that along my forecast track. Taking my impact swath literally means only the southeasternmost Azores islands get clipped by tangible weather from Gordon...but the central Azores may get some effect if it follows closer to the NHC's forecast track. Mention of strongest winds being biased south of center is due to Gordon's forecast brisk eastward track...and we know that cyclonic circulations tracking eastward in the northern hemisphere have their winds enhanced on the south side and weakened on the north side.

...SPECIAL FEATURE...TROPICAL WAVE INVEST 94-L...
Yet another tropical wave has just come off of Africa into the Atlantic tropical belt...currently SE of the Cape Verde Islands with impressive t-storm activity. Within the paragraph P5 upper ridge...t-storm latent heat release has been attempting to generate a warm core upper anticyclone aloft. For now...most of this latent heat release has been south of this upper ridge axis...resulting in intensified upper easterly winds shearing the t-storms west of the tropical wave axis. Once an upper anticyclone becomes more established overhead of the wave and within the paragraph P5 upper ridge...the shear will reduce and tropical cyclone formation will become likely.

...MID-LATITUDES DISCUSSION...
P1...Upper trough from W Canada has arrived to central US/Canada. Divergence east of this upper trough is supporting a vigorous 998 mb surface frontal cyclone just south of Hudson Bay. Expect the upper trough to support this frontal cyclone over the next days...and in turn cool air advection behind this frontal cyclone will cause this upper trough to amplify southward across the eastern half of North America in the next days.

P2...Upper trough regime over eastern US and eastern Canada remains spread into all of the Atlantic high seas. Frontal cyclone just offshore of Europe in previous discussion is performing a meandering loop beneath cyclonic flow of amplified Atlantic high seas upper trough. Cool air advection behind this cyclone continues enforcing this Atlantic high seas upper trough...whose western upper convergence supports a 1026 mb ridge just south of Greenland. Shortwave upper trough over the eastern US is now pushing into Atlantic Canada. Divergence east of this shortwave upper trough has intensified the NE US surface cyclone in the previous discussion...which is now 1004 mb over E Maine and spreading into the NW Atlantic. Low-level warm air advection ahead of this cyclone supports W Atlantic upper ridge.

P3...Mid-ocean upper vorticity persists. West Gulf upper vortex has been pushed into E Mexico via upper anticyclonic outflow of the remnants of Seven (paragraph P6). Elongated upper trough E of Bermuda persists...which still extends into central Caribbean upper vorticity.

P4...Atlantic surface ridge with 1016 mb to 1023 mb centers is supported by a few upper convergent sources while stretching from the northern Gulf of Mexico to waters offshore of W Europe....including convergence SE of the of west Atlantic upper ridge (paragraphs P2)...convergence behind the shortwave upper trough entering Atlantic Canada (paragraph P2)...and convergence behind Atlantic high seas upper trough (paragraph P2). In conjunction with south sides of W Atlantic upper ridge (paragraph P2) and E Atlantic upper ridge (paragraph P5)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridge across the eastern tropical Atlantic persists. Its northwestern corner continues aiding the upper outflow of Tropical Storm Gordon. T-storm latent heat release from Gordon has locally inflated this corner of the upper ridge...which has resulted in an embedded shortwave upper trough W of Gordon and some upper vorticity SE of Gordon. I am not sure of the shortwave upper trough W of Gordon was the result of Gordon's cyclonic circulation advecting in some of the mid-upper level cooler air associated with the upper vorticity mentioned in paragraph P3.

P6...Remnant tropical wave of depression seven has entered the Bay of Campeche...and gained some hints of organization on satellite imagery. Highly divergent SW quad of W Atlantic upper ridge (paragraph P2) is helping to produce t-storms over the remnants. The NHC has increased its probability of tropical cyclone reformation from this system given the developing organization...and may send out a recon airplane to investigate later today. Currently...I am forecasting the ridge weakness associated with the paragraph P1 system will have no effect...and therefore I expect this to continue WNW into eastern Mexico in the next 36 hrs while steered by the low-level ridge in paragraph P4 (and also by the low-level ridge building beneath the convergent back side of paragraph P1 upper trough). I am also predicting this to become close to tropical depression status (but not quiet get there) before landfall. GFS model is not delivering a clear-cut solution...somewhat showing a surface low tracking WNW into E coast of Mexico...then keeping a semi-permanent surface low along the E coast of Mexico/near S Texas. Also...surface cold front from paragraph P1 system is expected to dive toward the Gulf of Mexico in the next days...which could also add to disturbed weather in this area.

P7...South fragment of pre-Gordon tropical wave in NE Caribbean area has been dropped from NHC TAFB maps in last 24 hrs...as if it dissipated.

P8...Tropical wave midway between the Cape Verdes and Lesser Antilles in the previous discussion continues rapidly westward...and will be crossing the Lesser Antilles later today. Dry air (paragraph P4) is suppressing this tropical wave.

P9...Tropical wave that passed over the Cape Verde Islands in the previous discussion is now west of the islands...already headed into the waters midway between the Cape Verdes and Lesser Antilles. Dry air (paragraph P4) is suppressing this tropical wave.

P10...Yet another tropical wave has just come off of Africa into the Atlantic tropical belt...currently SE of the Cape Verde Islands with impressive t-storm activity. Unlike the previous two tropical waves (paragraphs P8 and P9)...this wave is doing much better against dry air and looks poised to become the next Atlantic tropical cyclone. It has also been upgraded to disturbance Invest 94-L. See 2nd special feature section above for further details on this system.

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2012 Atlantic Hurricane Season Birdseye Discussion #80

By: NCHurricane2009, 10:33 AM GMT on August 16, 2012

...AUGUST 16 2012...6:35 AM EDT...
Tropical disturbance Invest 93-L has strengthened into tropical depression eight...then into Tropical Storm Gordon...in the last 24 hours. Interests in the Azores should monitor this system carefully. See special feature section for further details.

Elsewhere...remnant of tropical depression seven is currently crossing the Yucatan Peninsula....headed for southeast Mexico and Bay of Campeche. It is not clear if the system could contribute to gradual tropical development in the Bay of Campeche/west Gulf of Mexico area over the next days. See paragraph P6 for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0125Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM GORDON...
Tropical depression eight from special update #79B has strengthened to Tropical Storm Gordon. Origin of this system briefly mentioned in paragraph P7 below. Figure 1 below shows my versus the NHC's forecast for Tropical Storm Gordon as of 5 AM EDT this morning.

Track-wise...I agree with the NHC's forecast track thru first 72 hrs. An eastward turn is imminent...as models show abundant westerly flow between 30N and 35N on north side of paragraph P4 surface ridge by 24 hrs. Although the 1022 mb ridge S of Greenland (paragraph P2 below) could retard eastward progress...its south side is not beyond 40N...and its influence will be wedged even further north and away from Gordon by NE US surface cyclone (paragraph P2) pushing in. Beyond 72 hrs...east US shortwave upper trough will be joining Atlantic high seas upper trough (both upper troughs mentioned in paragraph P2)...with upper convergence behind the merged upper trough causing the 1022 mb ridge S of Greenland (paragraph P2) and Atlantic surface ridge (paragraph P4) to strengthen west of Gordon. In fact by 120 hrs (5 AM Tue)...GFS shows NW low-level flow from this ridge at a location west of 20W...and the forecast below shows Gordon still a tad west of 20W by that time. Given the GFS is on the south side of current model consensus...and because the NHC notes the model consensus shifted south as of 5 AM EDT...I am inclined to be biased south of the NHC's track after 72 hrs...giving more credence to the NW low-level flow produced by strengthening surface ridge west of Gordon.

Intensity-wise...dry air (paragraph P4) is not a problem...as upper outflow supplemented by E Atlc upper ridge (paragraph P5) and atmospheric instability over warm enough waters is allowing Gordon to create its own environment. Moreover...the above thermo chart shows that Gordon is generally headed toward moist air in the higher latitudes...with all the dry air staying south of the higher latitudes. One thing that impedes Gordon's intensity in the very-near-term is SW shear evident by storm clouds biased NE of the center as seen on this morning's infrared. SW shear is caused by paragraph P3 upper vorticity just to Gordon's west. It is assumed that the forecast eastward-curving track will quickly allow Gordon to distance himself away from the shearing upper vorticity. However...if the SW shear becomes more of a problem for Gordon than expected...I will be doing a special update later today downgrading my intensity forecast.

With that said...I am assuming continued warm waters and paragraph P5 upper ridge outflow will continue to allow Gordon to strengthen...and my strengthening rate is at or slightly above the NHC's thru 5 AM Sat (48 hrs out). By 48 hrs..the GFS model 200 mb (upper) wind forecast shows the east US shortwave upper trough (paragraph P2) zooming eastward toward the Atlantic high seas upper trough (paragraph P2)...with a more zonal upper westerly jet connecting the south sides of both upper troughs and overspreading Gordon's general region. So after 48 hrs (after 5 AM Sat)...I assume some westerly shear from this jet will begin weakning Gordon. By 5 AM Sun (72 hrs)...Gordon will be crossing the 26 deg C isotherm into cooler waters...and with continued westerly shear...I expect him to have weakened back down to a strong 70 mph tropical storm (this is more aggressive weakening than NHC's...which still has him a 75 mph minimal hurricane by that time). Transition to non-tropical status after 72 hrs is expected by NHC...and this makes sense to me too...with Gordon being over cooler waters. How strong Gordon remains as a non-tropical cyclone will depend on how much upper divergence he sees from the base of (or east side of) the Atlantic high seas upper trough.


Figure 1: Forecast for Tropical Storm Gordon this morning.

Impact swath in Figure 1 is drawn based on my forecast track. Its initial shape is based on the NE-biased storm canopy seen on infrared imagery...then I extrapolate that along my forecast track. Taking my impact swath literally means only the southeasternmost Azores islands barely get clipped by tangible weather from Gordon...but all of the Azores should remain cautious in case there needs to be a northward adjustment in forecast later on. Mention of strongest winds being biased south of center is due to Gordon's forecast brisk eastward track...and we know that cyclonic circulations tracking eastward in the northern hemisphere have their winds enhanced on the south side and weakened on the north side.

...MID-LATITUDES DISCUSSION...
P1...Next upper trough in the mid-latitudes has entered the upper-left corner of atmo chart from W Canada. Divergence east of this upper trough is supporting a vigorous 999 mb surface frontal cyclone in south-central Canada. Expect the upper trough to support this frontal cyclone over the next days...and in turn cool air advection behind this frontal cyclone will cause this upper trough to amplify southward across the eastern half of North America in the next days.

P2...Upper trough regime over eastern US and eastern Canada remains spread into all of the Atlantic high seas. Frontal cyclone that spun up along front offshore of Europe (in previous discussion #79) has intensified....and remains just offshore of Europe. Cool air advection behind this cyclone continues enforcing a longwave upper trough covering the entire Atlantic high seas...whose western upper convergence supports a 1022 mb ridge south of Greenland. Shortwave upper trough over the eastern US remains supported by cool air advection from Hudson Bay surface cyclone which has just recently exited the picture. Divergence east of this shortwave upper trough now supports a developing 1008 to 1010 mb surface cyclone over the NE US. Low-level warm air advection ahead of this NE US cyclone supports W Atlantic upper ridge.

P3...Mid-ocean upper vorticity persists. One upper vortex persists in the western Gulf of Mexico. Elongated upper trough E of Bermuda persists...a portion of which has broken off into a new Central Caribbean inverted upper trough.

P4...Atlantic surface ridge with 1018 mb to 1020 mb centers is supported by a few upper convergent sources while stretching from Gulf of Mexico to waters offshore of W Europe....including convergence SE of the of west Atlantic upper ridge (paragraphs P2)...convergence behind east US shortwave upper trough (paragraph P2)...and convergence behind Atlantic high seas upper trough (paragraph P2). In conjunction with south sides of W Atlantic upper ridge (paragraph P2) and E Atlantic upper ridge (paragraph P5)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridge across the eastern tropical Atlantic persists.

P6...Remnant tropical wave of depression seven is crossing the Yucatan Peninsula...heading toward Bay of Campeche and SE Mexico. Highly divergent SW quad of W Atlantic upper ridge (paragraph P2) is helping to produce plenty of t-storms over and east of the remnants. Unlike 24 hrs ago...GFS computer model is no longer explicit in suggesting tropical cyclone formation in the Bay of Campeche/W Gulf of Mexico. However...watching to see if remnant tropical wave will contribute to anything before moving into the eastern Pacific...or alternatively if the t-storms east of the remnant wave (supported by divergent SW quad of W Atlc upper ridge) develop something. Also...surface cold front from paragraph P1 system is expected to dive toward the Gulf of Mexico in the next days...which could also add to disturbed weather in this area.

P7...North half of tropical wave Invest 93-L contributed to formation of Tropical Storm Gordon (NE of the Lesser Antilles and east of Bermuda) in the last 24 hrs...see above special feature section for details on Gordon. Meanwhile...south fragment of tropical wave has crossed the Lesser Antilles and is headed toward the northern Caribbean sea Islands. At this time...dry air (paragraph P4) and SW shear from paragraph P3 upper vorticity is suppressing this tropical wave fragment.

P8...Tropical west of the Cape Verde Islands in previous discussion is now midway between the Cape Verdes and Lesser Antilles moving rapidly westward. Dry air (paragraph P4) is suppressing this tropical wave.

P9...Suspect tropical wave moving offshore of Africa in previous discussion has been added to NHC TAFB surface maps during the last 24 hrs. As of 0000Z...the tropical wave had passed over the Cape Verde Islands and is just west of the islands. Despite the favorable environment provided by E tropical Atlantic upper ridge (paragraph P5)...dry air (paragraph P4) is likely to suppress this tropical wave...similar to what is now happening with the tropical wave in paragraph P8.

Updated: 10:07 AM GMT on August 17, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #79B (Special Update)

By: NCHurricane2009, 9:01 PM GMT on August 15, 2012

...AUGUST 15 2012...5:03 PM EDT...
This special update is written concerning tropical low Invest 93-L mentioned in full discussion #79...which has just been upgraded to tropical depression eight...centered at 29.9N-55.1W per first public advisory from the National Hurricane Center (NHC). This development is very close to what I forecasted in special update #79A earlier today.

My next full update on the Atlantic tropics will be sometime very late this evening or early tomorrow morning. Return to full discussion #79 for details on the rest of the Atlantic tropics.


Figure 1: Live satellite image of tropical depression eight

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2012 Atlantic Hurricane Season Birdseye Discussion #79A (Special Update)

By: NCHurricane2009, 3:49 PM GMT on August 15, 2012

...AUGUST 15 2012...11:55 AM EDT...
This special update is written concerning tropical low Invest 93-L mentioned in full discussion #79...which I estimate is centered at 54.5W-28.5N as of 11:55 AM EDT using visible satellite imagery. This center fix is further north than what I saw using this morning's infrared imagery during discussion #79. This means the center will likely pass over or very near 55W-30N later this afternoon...contrary to my earlier statement which said the center would pass west of that location.

Based on the continually improving satellite appearance (live satellite stream is shown below)...I expect a 100% chance that this tropical low will be upgraded to a tropical depression or tropical storm at 5:00 PM EDT this afternoon.

Return to full discussion #79 for details on the rest of the Atlantic tropics.


Figure 1: Live satellite image of tropical low Invest 93-L

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2012 Atlantic Hurricane Season Birdseye Discussion #79

By: NCHurricane2009, 10:30 AM GMT on August 15, 2012

...AUGUST 15 2012...6:31 AM EDT...
Remnant of tropical depression seven is currently crossing Central America and the western Caribbean. Land interaction with Central America is preventing this system from regenerating into a tropical cyclone...but it could reemerge into the Bay of Campeche and western Gulf of Mexico in the next days. See paragraph P5 for details.

Elsewhere...tropical wave Invest 93-L has rapidly organized in the last 24 hours...and is at high risk of tropical cyclone formation in the next 24 hours. Interests in the Azores should monitor the progress of 93-L as computer models suggest it near or just south of the area in the next days. See special feature section below for further details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1929Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL LOW INVEST 93-L...
Low pressure spin of tropical wave Invest 93-L has arrived to the waters NE of the Lesser Antilles. Finally...the favorable upper outflow beneath paragraph P4 E Atlc upper ridge...coupled with increased instability over increasingly warm waters...has allowed this system to overcome dry air(source of dry air mentioned in paragraph P3). The low pressure spin has rapidly organized in the last 24 hrs with increasing spiral bands...and is likely to become a tropical cyclone anytime in the next 24 hrs.

Short-term motion of this low pressure center is straight NW according to Figure 1...as it is currently recurving into a large-scale deep-layered ridge weakness caused by upper vorticity in paragraph P2...and caused by surface frontal and upper troughal activity in paragraph P1. This NW track should allow this system to safely pass east of Bermuda. Computer model consensus shown on www.wunderground.com/tropical has a good grip on the short-term NW motion...but I specifically prefer the models that show a track passing a little west of 55W-30N rather than over 55W-30N (due to the angle of current short-term motion). In the longer-range...the computer model consensus leans toward a recurving track that would take it south of the Azores rather than over the Azores...but interests in the Azores should watch this system.


Figure 1: Short term motion of tropical surface low Invest 93-L this morning based on 0045Z and 0745Z center fixes using satellite imagery.

...MID-LATITUDES DISCUSSION...
P1...Upper trough regime over central US and central Canada has spread into eastern areas and all of the Atlantic high seas. Frontal cyclone heading into Europe in the previous discussion has made landfall...and yet another frontal cyclone has spun up along the cold front of this frontal cyclone. This new and rapidly developing frontal cyclone is also heading into Europe. Cool air advection behind these cyclones continues enforcing a longwave upper trough covering the entire Atlantic high seas...whose western upper convergence supports a 1024 mb ridge SW of Greenland. E Canada surface cyclone has decayed into surface troughs and east-west front across the northwest Atlantic....with its shortwave upper trough passing south of Greenland. Shortwave upper trough over the central US is pushing into the eastern US...supported by cool air advection from frontal system diving in from W Canada...currently centered with a 1007 mb low in southern Hudson Bay. Low-level warm air advection ahead of this 1007 mb cyclone supports W Atlantic upper ridge...while SW US upper ridge persists behind this regime.

P2...Mid-ocean upper vorticity persists. One upper vortex persists in the Gulf of Mexico. Elongated upper vortex E of Bermuda has weakened into an upper trough reaching into the north-central Caribbean. Central Caribbean inverted upper trough has dissipated due to tremendous latent heat release from the remnants of tropical depression seven (paragraph P5).

P3...Atlantic surface ridge with 1019 mb to 1024 mb centers is supported by a few upper convergent sources while stretching from Gulf of Mexico to waters offshore of W Europe....including convergence SE of the of west Atlantic and SW US upper ridges (paragraphs P1)...and convergence behind Atlantic high seas upper trough (paragraph P1). In conjunction with south sides of W Atlantic upper ridge (paragraph P1) and E Atlantic upper ridge (paragraph P4)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P4...Upper ridge across the eastern tropical Atlantic has grown into the east half of the Caribbean.

P5...Remnant tropical wave of depression seven has entered the western Caribbean and Central America. Due to high split flow upper divergence between the E Atlc tropical upper ridge (paragraph P4) and W Atlantic upper ridge (paragraph P1)...this system has developed a significant increase in size and intensity of t-storm activity. Latent heat release from the t-storm activity has punched out the unfavorable central Caribbean inverted upper trough (paragraph P2)...and moisture from the t-storm activity has eradicated the dry air in the area (source of dry air in paragraph P3). Despite all of this...tropical cyclone reformation is prevented currently by land interaction with Central America. Beginning 90 hrs from now...GFS computer model has been insisting that the remnants could eventually contribute to tropical cyclone formation in the Bay of Campeche/western Gulf of Mexico. GFS shows the tropical cyclone curving slowly NW toward S Texas/NE Mexico thru 183 hrs from now...steered about the SW quad of a low-level ridge that builds behind a frontal system (this frontal system is not yet in the scope of the above charts...but will soon enter from the upper-left of the charts in next 24 hrs).

P6...Tropical wave Invest 93-L is currently NE of the Lesser Antilles. Due to its high potential for tropical cyclone formation in the next 24 hrs...it has been moved to its own special feature section above. See special feature section above for further details on this system.

P7...Tropical wave E of the Cape Verde Islands in the previous discussion is now west of the islands...and continues to show disorganized t-storm activity despite the favorable environment provided by E tropical Atlc upper ridge mentioned in paragraph P4. On the other hand...dry air is just to the northwest of this tropical wave (source of dry air mentioned in paragraph P3). For now...this dry air is likely to stall any development potential with this tropical wave.

P8...Satellite imagery suggests the next tropical wave is emerging from west Africa into the tropical belt of the Atlantic (see lower-right corner of above atmo chart).

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2012 Atlantic Hurricane Season Birdseye Discussion #78

By: NCHurricane2009, 3:48 AM GMT on August 14, 2012

...AUGUST 13 2012...11:49 PM EDT...
Remnants of tropical depression seven...currently in the central Caribbean Sea...still shows no signs of regeneration. See paragraph P6 for details.

Elsewhere...in the past 24 hrs...there has been an increase in concentrated t-storms southwest of Bermuda (end of paragraph P1) and the southwestern Gulf of Mexico (paragraph P5). However...conditions for tropical development in these locations are not showing.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1928Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...MID-LATITUDES DISCUSSION...
P1...Upper trough regime over central US and central Canada has spread into eastern areas and all of the Atlantic high seas. Frontal cyclone from Greenland in the previous discussion is pushing into Europe...with cool air advection behind this cyclone enforcing a longwave upper trough covering the entire Atlantic high seas whose western upper convergence supports a 1021 mb ridge S of Greenland. E Canada deep-layered cyclone has decomposed into an upper trough and 1008 mb surface cyclone. Shortwave upper trough over the central US remains supported by cool air advection from frontal system diving in from W Canada...which is currently centered with a 1004 mb low in southern Hudson Bay. Low-level warm air advection ahead of eastern Canada deep-layered cyclone supports W Atlantic upper ridge...while SW US upper ridge persists behind this regime. T-storms have fired up SW of Bermuda in last 24 hrs...with support of divergent W Atlantic upper ridge...and from a satellite perspective appears to be associated with cold front of E Canada cyclone. The nearest surface trough to these t-storms has been analyzed west rather than under these t-storms...suggesting a system under westerly vertical shear of E Canada cyclone's upper trough. Therefore...no signs of tropical development are occuring here at this time.

P2...Mid-ocean upper vorticity persists....still featuring a few upper vortex centers. One upper vortex is moving from Gulf of Mexico into NE Mexico while weakening to an east-west upper trough. A second and quiet elongated upper vortex is E of Bermuda. Central Caribbean inverted upper trough in previous discussion continues retrograding westward about W Atalntic upper ridge (paragraph P1). Third upper vortex SSW of the Azores has weakened into an upper trough south of the Azores in last 24 hrs.

P3...Atlantic surface ridge with 1020 mb to 1025 mb centers is supported by a few upper convergent sources while stretching from Gulf of Mexico to waters offshore of W Europe....including convergence SE of the of west Atlantic and SW US upper ridges (paragraphs P1)...and convergence behind Atlantic high seas upper trough (paragraph P1). In conjunction with south side of W Atlantic upper ridge (paragraph P1)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P4...Upper ridging across Atlantic tropics remains concentrated into two areas. Upper ridge is in the western Caribbean has moved to SE Mexico...remaining supported by relatively higher pressures between the Gulf of Mexico upper vortex and central Caribbean inverted upper trough mentioned in paragraph P2. Tropical upper ridging remains firmly established in eastern tropical Atlantic.

P5...Tropical wave moved into eastern Pacific from SE Mexico in paragraph P6 of previous discussion....but t-storms have fired up right behind...starting over the Yucatan peninsula last evening and working there way into the SW Gulf of Mexico in last 24 hrs. I am not sure if these t-storms have association with this tropical wave...but a study of upper winds shows these t-storms started thanks to split flow upper divergence between SE Mexico upper ridge (paragraph P4) and Gulf upper vortex (paragraph P2). The SE Mexico upper ridge has inflated thanks to latent heat release of these t-storms...reducing the westerly shear from the Gulf upper vortex. However...surface pressures in the Gulf are high thanks to surface ridge discussed in paragraph P3...so there are no signs of tropical development in this area at this time.

P6...Remnant tropical wave of depression seven has entered the central Caribbean. Despite semi-favorable conditions described in special feature section of previous discussion #77...there are no signs of regeneration into a tropical cyclone...so I have dropped this as a special feature on this blog. As described previously...semi-favorable conditions continue to be from divergent west end of E Atlc tropical upper ridge (paragraph P4)... as well as reduced westerly shear thanks to vast upstream upper easterlies in the central Caribbean inverted upper trough (paragraph P2). Dry air to the west (origin of which mentioned in paragraph P3) is supported by upper convergence on the east side of SE Mexico upper ridge (paragraph P4). This system could outrun the favorable E Atlc tropical upper ridge...and into the much less favorable dry air and inverted upper trough. Atlernatively...this system could ingest the neighboring dry air if it spins up at the surface.

P7...Tropical wave Invest 93-L continues pushing west...and its large low pressure spin is centered far WNW of the Cape Verde Islands and far ENE of the Lesser Antilles. Even though it is still under favorable upper outflow beneath paragraph P4 E Atlc upper ridge...and its westward track is still taking it into increasingly warmer waters...I still am not re-considering this a special feature on this blog due to weak t-storm activity caused by ingestion of dry air (source of dry air mentioned in paragraph P3).

P8...Tropical wave that emerged from Africa in previous discussion is now just E of the Cape Verde Islands...and continues to show disorganized t-storm activity despite the favorable environment provided by E tropical Atlc upper ridge mentioned in paragraph P4.

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2012 Atlantic Hurricane Season Birdseye Discussion #77

By: NCHurricane2009, 10:47 PM GMT on August 12, 2012

...AUGUST 12 2012...6:48 PM EDT...
Conditions for tropical depression seven to regenerate are increasing. Interests in the western Caribbean Sea should monitor the progress of the remnants. See special feature section below for further details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1930Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...REMNANTS OF SEVEN...
Remnant tropical wave of depression seven crossed the Lesser Antilles last night...and is currently moving westward across the eastern Caribbean Sea. The remnants have overcome the first unfavorable factor for tropical cyclone regeneration...which is unfavorable southwesterly shear from paragraph P3 upper vorticity retrograding into the Caribbean Sea. This is either due to faster-than-expected breakup of the upper vorticity on its own accord...or latent heat release from the t-storms expanding the paragraph P5 E Atlc upper ridge into the east Caribbean to facilitate the breakup...or a combination of both theories. There are no more unfavorable southwesterly shearing upper winds in the upstream environment...instead less hostile easterly upper winds across the central Caribbean inverted upper trough (paragraph P3) and west Caribbean upper ridge (paragraph P5). Coupled with the westward expansion of E Atlc upper ridge (paragraph P5) which is ventilating the system quiet well at this hour...the remnants have a better opportunity for tropical cyclone regeneration than I thought in the previous discussion.

However...there is no imminent threat of tropical cyclone regeneration at this hour...with some lingering unfavorable factors that still lie upstream of the remnants of Seven. The east side of the paragraph P5 W Caribbean upper ridge promotes upper convergence (like a typical mid-latitude upper ridge wave)...which is strengthening the dry air to Seven's west (source of dry air mentioned in paragraph P4). It is still possible that the remnants either ingest this dry air as it tries to spin up...or that the brisk west motion of the remnant tropical wave causes it to outrun the favorable E Atlc upper ridge and move into the dry air and less favorable central Caribbean inverted upper trough (paragraph P3).

...MID-LATITUDES DISCUSSION...
P1...Upper trough regime over central US and central Canada has spread into eastern areas and all of the Atlantic high seas. Frontal cyclone SE of Greenland in previous discussion is heading into western Europe (upper-right corner of above atmo chart)...while another frontal cyclone (1002 mb as of 1930Z HPC) dives into the top-center of above atmo-chart from Greenland. Both frontal cyclones' cool air advection support a longwave upper trough covering the entire Atlantic high seas...whose western upper convergence supports a 1023 mb ridge on the SE Canada coast. Deep-layered cyclone over E Great Lakes in previous discussion is moving NE into eastern Canada...and upper convergence behind this system supports a 1018 mb ridge over the SE US and 1017 mb ridge over Minnesota. Yet another shortwave upper trough has developed over the central US in this regime...supported by cool air advection from frontal system diving in from W Canada. Low-level warm air advection ahead of eastern Canada deep-layered cyclone supports W Atlantic upper ridge...while SW US upper ridge persists behind this regime.

P2...NE Atlantic upper trough in previous discussion has exited the picture while moving into Europe.

P3...Mid-ocean upper vorticity persists....still featuring a few upper vortex centers. One upper vortex is moving from Gulf of Mexico into NE Mexico while retrograding around paragraph P1 SW US upper ridge. A second and quiet elongated upper vortex is digging southwestward around the W Atlantic upper ridge (paragraph P1)...a fragment of which has split off and retrograded into the central Caribbean as an inverted upper trough. A third upper vortex still shows faintly in the above atmo chart 200 mb wind barbs...located SSW of the Azores.

P4...Atlantic surface ridge with 1018 mb to 10XX mb centers is supported by a few upper convergent sources while stretching from Gulf of Mexico to waters offshore of W Europe....including convergence SE of the of west Atlantic and SW US upper ridges (paragraphs P1)...and convergence behind Atlantic high seas upper trough (paragraph P1). In conjunction with south side of W Atlantic upper ridge (paragraph P1)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridging across Atlantic tropics remains concentrated into two areas. An upper ridge is in the western Caribbean as a southward extension of W Atlantic upper ridge in paragraph P1...and is supported by relatively higher pressures between the Gulf of Mexico upper vortex and central Caribbean inverted upper trough mentioned in paragraph P3. Tropical upper ridging remains firmly established in eastern tropical Atlantic.

P6...Tropical wave over SE Mexico/Bay of Campeche in previous discussion has moved into the eastern Pacific in last 24 hrs...and therefore will no longer be mentioned on this blog.

P7...Remnants of Florence have dissipated north of the Bahamas in last 24 hrs.

P8...Remnants of tropical depression seven crossed the Lesser Antilles last evening...and are currently in the eastern Caribbean. Due to increased risk of regenerating into a tropical cyclone that was not previously seen on this blog...this system has been moved into its own special feature section. See above special feature section for further details on this system.

P9...Tropical wave Invest 93-L continues pushing west...and its large low pressure spin is now far WNW of the Cape Verde Islands and far ENE of the Lesser Antilles. Even though it is still under favorable upper outflow beneath paragraph P5 E Atlc upper ridge...and its westward track is still taking it into increasingly warmer waters...I still am not re-considering this a special feature on this blog due to weak t-storm activity caused by ingestion of dry air (source of dry air mentioned in paragraph P4).

P10...A tropical wave with currently disorganized t-storm activity is emerging from the west coast of Africa into the Atlantic tropical belt (lower-right corner of above atmo chart).

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2012 Atlantic Hurricane Season Birdseye Discussion #76

By: NCHurricane2009, 11:57 PM GMT on August 11, 2012

...AUGUST 11 2012...8:05 PM EDT...
Atlantic tropics have rapidly quieted in last 36 hours. For info on the remnants of Florence...remnants of tropical depression seven...and strong tropical wave Invest 93-L...see paragraphs P7 thru P9 in tropical belt discussion.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1330Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...MID-LATITUDES DISCUSSION...
P1...Upper trough regime over central US and central Canada has spread into eastern areas and Atlantic high seas. Frontal cyclone that crossed Greenland in previous discussion leaves behind front over north Atlantic with a 1008 mb cyclone SE of Greeland. Local cool air advection behind this 1008 mb cyclone has generated a shortwave upper trough whose western convergence supports 1022 mb ridge centers E of Newfoundland. Intensifying frontal cyclone headed toward eastern Great Lakes has become stationary while its cool air advection has caused an upper vortex to develop overhead of itself. With the upper vortex overhead...this Great Lakes cyclone is now deep-layered and at 1005 mb as of 1200Z earlier today. Central North America surface ridge is re-building with support of upper convergence on back side of this upper trough regime...as seen with 1020 to 1023 mb centers in upper-left corner of above atmo birdseye chart stretching from the SW US to eastern Canada. Low-level warm air advection ahead of Great Lakes cyclone supports W Atlantic upper ridge...while SW US upper ridge persists behind this upper trough regime.

P2...Deep-layered vortex north of the Azores is shifting east into western Europe...and its upper vortex has de-amplified into a NE Atlantic upper trough.

P3...Mid-ocean upper vorticity persists....now featuring a few upper vortex centers. One upper vortex is moving NW from W Caribbean and into E Gulf of Mexico. A second and quiet large upper vortex is digging southwestward around the W Atlantic upper ridge (paragraph P1)...now reaching the eastern Caribbean Sea. A third upper vortex shows faintly in the above atmo chart 200 mb wind barbs...located SSW of the Azores.

P4...Atlantic surface ridge with 1022 mb to 1026 mb centers is supported by a few upper convergent sources while stretching from W Atlantic to waters offshore of W Europe....including convergence SE of the of west Atlantic upper ridge (paragraphs P1)...and convergence behind NE Atlantic upper trough (paragraph P2). In conjunction with south side of W Atlantic upper ridge (paragraph P1)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridging across Atlantic tropics has re-structured in last 36 hrs. An upper ridge is developing in the central Caribbean as a southward extension of W Atlantic upper ridge in paragraph P1...and is supported by relatively higher pressures between the W Caribbean upper vortex and E Caribbean upper vorticity mentioned in paragraph P3. Tropical upper ridging remains firmly established in eastern tropical Atlantic.

P6...Tropical wave in central Caribbean during the previous discussion has rapidly moved westward into Central America and now into SE Mexico/Bay of Campeche. Westerly shear from the W Caribbean/E Gulf of Mexico upper vortex (paragraph P3) has separated all t-storm activity from this tropical wave. This t-storm activity...currently in the western Caribbean...remains supported by split flow divergence between this upper vortex and new upper ridging developing in the central Caribbean Sea (paragraph P5).

P7...Remnants of Florence is currently located just north of the Bahamas. Remnants are shown in NHC TAFB maps as an increasingly weaker surface trough tracking WNW toward deep-layered ridge weakness associated with east Great Lakes deep-layered cyclone in paragraph P1. Despite being under favorable W Atlantic upper ridge (also in paragraph P1)...remnants of Florence not showing signs of regeneration...so I no longer have this as a special feature on this blog. I expect the remnants to either dissipate...or to continue curving more northward in track such that it passes east of the US shoreline and west of Bermuda...perhaps merging with cold front of Great Lakes cyclone expected to push into the Atlantic.

P8...Tropical depression seven has collapsed into a remnant low/tropical wave in last 36 hrs due to excessive rapid westward motion. See recent discussion #72 Ernesto section on how rapid westward motion can dissipate a weak tropical cyclone. Low-level cloud motions in visible satellite...and rapid motion of tropical wave in paragraph P6...suggest the remnants of seven will continue rapidly westward as it crosses the Lesser Antilles tonight and across the Caribbean Sea in the next days. In addition (as previous TD 7 discussion highlighted)...remnant of seven is beginning to encounter unfavorable southwesterly shear from paragraph P3 upper vorticity retrograding into the Caribbean Sea. The only thing going for TD 7 to regenerate is that its rapid westward motion could allow it to outrun the paragraph P3 upper vorticity and into more favorable paragraph P5 central Caribbean upper ridge. Then again...east side of this Caribbean upper ridge could promote upper convergence (like a typical mid-latitude upper ridge wave)...which would strengthen the dry air to Seven's northwest and go against its regeneration (source of dry air mentioned in paragraph P4).

P9...Strong tropical wave Invest 93-L has crossed the Cape Verde Islands...and its large low pressure spin is now centered just NW of those islands. Its t-storm activity has reduced due to unfavorable thermodynamics...namely ingestion of dry air to its northwest and having its north side cross over waters below 26 deg C. While it is currently under favorable upper outflow beneath paragraph P5 upper ridge...and its westward track will take it to increasingly warm waters...I have stopped considering this a special feature on this blog until I see how well it fares against its recent dry air ingestion (source of this dry air mentioned in paragraph P4).

Updated: 11:58 PM GMT on August 11, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #75

By: NCHurricane2009, 10:41 AM GMT on August 10, 2012

...AUGUST 10 2012...6:45 AM EDT...
Ernesto turned southwestward more than expected...hence making landfall into southern Mexico yesterday afternoon a little earlier than expected. This prevented the tropical storm from becoming a hurricane...but a significant flood threat looms for southern Mexico. See Ernesto special feature section for further details.

Elsewhere...Florence's remnant still being watched for re-genesis into a tropical cyclone...and tropical wave Invest 92-L has strengthened into tropical depression seven during the last 24 hours. Cape Verde Islands need to monitor strong tropical wave that has emerged from Africa...which has been upgraded to Invest 93-L.

See all four special feature sections for further details on Ernesto...Florence...Tropical Depression Seven...and strong tropical wave Invest 93-L. To have four special feature sections simultaneously by far makes this the most active day in the 2012 Atlantic Hurricane season thus far.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1929Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL DEPRESSION ERNESTO...
Atlantic surface ridge (paragraph P4) has linked with central US surface ridge (paragraph P1) thru bridging together in the Gulf of Mexico. Coupled with the fact that the central US surface ridge is strengthening to the north of Ernesto...this pushed Ernesto southwestward from the Bay of Campeche and into southern Mexico last afternoon. The more southwestward track made the landfall a bit earlier than expected...so Ernesto never re-gained hurricane strength. At 1 AM CDT...Ernesto has been downgraded to a tropical depression. Even though Ernesto is now a dissipating tropical cyclone pushing SW into southern Mexico...its remnant low looks to be a significant inland flood threat as evidenced by tremendously intense t-storm clouds in its large circulation shown by infrared satellite this morning.

As Ernesto is exiting the scope of the above charts and Atlantic region...this will likely be the last discussion of Ernesto on this blog. There is a distinct possibility of Ernesto continuing into the eastern Pacific from southern Mexico...perhaps regenerating into an eastern Pacific tropical cyclone (NHC E-pacific outlook cites 40% chance of this occurring in next 48 hours as of this writing). If this were to occur...I will not be covering its regeneration there.

...SPECIAL FEATURE...REMNANTS OF FLORENCE...
Florence's remnant tropical wave is tracking WNW...currently positioned just north of Puerto Rico and east of the Bahamas. It is still directly under the paragraph P5 tropical upper ridge axis for better upper outflow...and prolonged favorable upper outflow for the next several hours is also possible due to persistent W Atlantic upper ridge mentioned in paragraph P1. In fact...the favorable upper outflow allowed the remnant tropical wave to produce an impressive t-storm cluster last afternoon.

The next obstacle noted for Florence's remnant continues to be southwesterly vertical shear due to mid-ocean upper vorticity (paragraph P3). As of this writing...we are roughly at the 72-hour point in the 200-mb outlook released in discussion #73...and this outlook is important as it showed an aggressive break-up of this upper vorticity that favors Florence's regeneration. I must say...the current 200-mb wind barbs in above atmo chart match fairly well to the cited 200-mb outlook...so I still think Florence has a chance to regenerate.

Computer models show a deep-layered ridge weakness moving into NW Atlantic by 72 and 96 hrs...associated with upper trough and surface frontal activity in paragraph P1. Florence appears north enough to be influenced by the weakness...and the deep-layered nature of the weakness suggests Florence will recurve northward whether she stays shallow/weak or becomes stronger/taller. The timing of the weakness suggests Florence will turn northward such that it passes west of Bermuda and east of the US coast.

...SPECIAL FEATURE...TROPICAL DEPRESSION SEVEN...
Tropical wave Invest 92-L...located east of Florence's remnant and well WSW of the Cape Verde Islands...has strengthened to tropical depression seven during the last 24 hours. Its t-storm driven latent heat release continues to locally inflate the paragraph P5 upper ridge such that the depression exhibits upper anticyclonic outflow. Its thunderstorms continue to be sufficient in keeping out paragraph P4 dry air mass. While all of this suggests potential for strengthening...I am hesitant in my forecast in Figure 1 to suggest much strengthening due to the rapid westward motion this tropical depression is beginning to take. Recently for example...the early phases of Ernesto struggled while it was moving quickly westward...see Ernesto section in recent discussion #72. A quick westward motion of this depression in particular makes it more susceptible to unexpectedly outrunning its t-storm canopy and moving into the paragraph P4 dry air mass that lies just to its northwest.

I actually suggest weakening beginning in 48-hours...due to models suggesting the paragraph P3 upper vorticity will dump in yet another upper vortex that retrogrades southwest into the Caribbean Sea. This upper vortex is expected to be in the east Caribbean Sea by 48 hrs (Sun morning)...central Caribbean by 72 and 96 hrs (Mon and Tue morning)...then western Caribbean by 120 hrs (Wed morning). Looking at Figure 1...the forecast locations of this upper vortex is coincident with the depression's forecast positions...suggesting unfavorable southerly or westerly shear from the upper vortex.


Figure 1: Forecast for tropical depression seven this morning. Interests in the central Lesser Antilles may experience tropical storm conditions as the system passes through late Saturday and on Sunday.

With a hesitance to suggest much development...my track forecasting assumes that this system will be weak/shallow and hence steered by lower-level flow. For the next 120 hours (5-days)...upper trough over the eastern US (paragraph P1) is forecast by models to break up into two upper troughs not strong enough to deflect the W Atlantic upper ridge. Therefore...this upper trough activity will be deflected northward around the upper ridge...and hence any low-level ridge weakness generated by upper trough divergence will be too far north to affect the steering of the tropical cyclone as it stays weak/shallow. Therefore...I predict a straight west track in Figure 1...which is similar to the NHC track shown in Figure 1 but with a southward bias.

...SPECIAL FEATURE...STRONG TROPICAL WAVE INVEST 93-L...
As paragraph P7 in the tropical belt discussion states...a tropical wave that has recently emerged from Africa is doing so with impressive t-storm activity and a low pressure spin. Its t-storm latent heat release has allowed it to establish a favorable upper anticyclonic cell of outflow within the tropical Atlantic upper ridge (paragraph P5 feature)...with an unfavorable Cape Verde upper vortex being pushed westward and away by the upper anticyclone (as stated in paragraphs P2 and P5). With the Cape Verde upper vortex warded off...and with the center of this tropical wave over waters at least 26 deg C...all of this suggests to me that this tropical wave has a high chance of becoming a tropical depression or even a tropical storm later today...passing over the Cape Verde Islands tonight.

If this tropical system gets strong/vertically tall enough...it could bend on a more WNW track in the short-term while feeling the influence of upper-layers of vorticity presently north of the Azores (paragraph P2). This more north track would take it toward increasingly cooler waters and the dry air mass described in paragraph P4...ultimately re-weakening/re-shallowing the system such that it returns on a westward track guided by the low-level Atlantic ridge (paragraph P4 feature). Regardless of strength/vertical depth of this system...a westward track is favored in the long term anyway...all as the upper vorticity north of the Azores de-amplifies...and this system becomes embedded in deep-layered easterlies produced by W Atlantic upper ridge (paragraph P1) and Atlantic surface ridge (paragraph P4).

...MID-LATITUDES DISCUSSION...
P1...Upper trough over central US and central Canada is spreading into eastern areas. Frontal cyclone in waters just W of Greenland has crossed Greenland in last 24 hrs...and left the scope of the above charts. With supportive divergence E of this upper trough...another frontal cyclone is spinning up over the central US and heading toward the eastern Great Lakes. Central US surface ridge is re-building with support of upper convergence on back side of this upper trough regime...as seen with 1027 to 1020 mb centers in upper-left corner of above atmo birdseye chart. Low-level warm air advection ahead of developing cyclone headed toward Great Lakes supports W Atlantic upper ridge...while SW US upper ridge persists behind this upper trough regime.

P2...Surface frontal depression (cyclone) persists north of the Azores at 1005 mb. Large upper vortex remains stacked over the surface cyclone. Whatever vorticity that is not above the 1005 mb cyclone is in the form of a cut-off upper vortex just north of the Cape Verde Islands. Latent heat release from strong tropical wave Invest 93-L is pushing this Cape Verde upper vortex westward as an inverted upper trough embedded in tropical Atlantic upper ridge. See paragraph P5 for further details.

P3...Mid-ocean upper vorticity persists....now featuring a couple of upper vortex centers. One upper vortex is entering the west Caribbean Sea while retrograding SW about W Atlantic upper ridge (paragraph P1). The second upper vortex is located well SE of Newfoundland and well east of Bermuda.

P4...Atlantic surface ridge with a 1023 mb center is supported by a few upper convergent sources while stretching from Gulf of Mexico/SE US to waters offshore of W Europe....including convergence SE of the of west Atlantic upper ridge (paragraphs P1)...and convergence behind NE Atlantic upper deep-layered vortex (paragraph P2). In conjunction with south side of W Atlantic upper ridge (paragraph P1)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics. However...this dry air has been eradicated across the Caribbean Sea and Gulf of Mexico due to moisture from Ernesto...the remnants of Florence...and moisture produced by interaction of Caribbean upper vortex (paragraph P3) with a tropical wave (paragraph P6). Newly-formed tropical depression seven and strong tropical wave Invest 93-L are helping to reduce this dry air in the eastern tropical Atlantic.

...TROPICAL BELT DISCUSSION...
P5...Upper ridge across Atlantic tropics has been split in half by paragraph P3 upper vorticity retrograding SW into the Caribbean Sea. West half is concentrated over Ernesto...providing great anticyclonic outflow over the storm. East half of this upper ridge is being locally inflamed into cells of upper anticyclonic outflow via t-storm latent heat release from the remnants of Florence...tropical depression seven...and strong tropical wave Invest 93-L...with upper troughs forming between these cells. E Caribbean upper trough has formed to the SW of Ex-Florence upper anticyclonic cell. Inverted upper trough E of the Lesser Antilles is between the anticyclonic cells of Ex-Florence and tropical depression seven. Cape Verde upper vortex in paragraph P3 has transitioned into an inverted upper trough between Invest 93-L and tropical depression seven upper anticyclonic cells.

P6...Eastern Caribbean tropical wave in previous discussion has moved into the central Caribbean. The tropical wave continues interacting with divergent southwesterly flow on SE flank of W Caribbean upper vortex (paragraph P3 feature). This supports t-storm activity in this region. Despite the t-storm activity...no tropical cyclone development is expected due to westerly vertical shear across the south side of this upper vortex.

P7...Satellite imagery shows that the strong tropical wave from Africa in the previous discussion has emerged off the coast with a well-organized surface low pressure spin headed toward the Cape Verde Islands. It has been upgraded to Invest 93-L due to potential for tropical cyclone formation...and therefore is given a special feature section on this blog. See above Invest 93-L special feature section for further details on this situation.

Updated: 11:01 AM GMT on August 10, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #74

By: NCHurricane2009, 9:55 AM GMT on August 09, 2012

...AUGUST 9 2012...6:10 AM EDT...
Ernesto crossing the Bay of Campeche waters...expected to re-strengthen into a hurricane shortly...and make its final landfall into mainland Mexico later this afternoon.

Elsewhere...Florence's remnant still being watched for re-genesis into a tropical cyclone...and tropical wave Invest 92-L is likely to become the next Atlantic tropical cyclone.

See all three special feature sections for further details on Ernesto...Florence...and Invest 92-L.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1933Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM ERNESTO...
Since previous discussion #73 (8:01 PM EDT Aug 7)...Ernesto strengthened to 85 mph winds for its Yucatan landfall...weakened only marginally to 45 mph max winds during the afternoon of August 8...and has quickly regained strength while crossing the Bay of Campeche waters this early morning. Favorable conditions of warm waters...low shear...and grand anticyclonic upper outflow continue to persist with this sytem...and therefore the NHC expects Ernesto to regain hurricane strength before its final landfall into mainland Mexico expected this afternoon. As of 1 AM CDT this morning...I am predicting this afternoon's landfall intensity to be 80 mph max winds (as shown in Figure 1 below). This is slightly below the 85 mph peak Ernesto had for its first Yucatan landfall...because I see a ragged core of t-storms on infrared satellite rather than a tightly-wound-up organzied core.


Figure 1: Tropical Storm Ernesto forecast graphic created early this morning.

Track forecasting has been more challenging with Ernesto. Just after I released my previous forecast in discussion #73 (which had a north bias compared to NHC's)...Ernesto suddenly turned straight west and made landfall in the Yucatan further south than expected. Low-level steering solution in discussion #73 cited that Atlantic surface ridge (paragraph P5 feature) was going to re-build...but I had failed to interpret this would translate to a more west track as opposed to a continued WNW track.

I also cited (in discussion #73 low-level steering solution) that a low-level ridge weakness would redevelop from paragraph P1 upper trough/surface frontal system...growing over the eastern US by 11 PM Thursday (tonight)...then growing into the Gulf of Mexico by 11 PM Fri thru 11 PM Sat. At the time I used that forecasted ridge weakness to suggest a more northward and slower track compared to NHC's. The more westerly-than-expected track of Ernesto has made him out-of-phase to feel any influence from this ridge weakness...I may have also considered this ridge weakness too much...and the models also show a less imperssive low-level ridge weakness...now keeping the Atlantic surface ridge (paragraph P5) linked with central US surface ridge (paragraph P1). Evidence that the central US surface ridge is re-building is also described in paragraph P1. In this forecast (shown in Figure 1 above)...I simply agree with the NHC forecast this time around...and also agree with the WSW bend shown after landfall...which would occur if the central US low-level ridge continues re-building to Erensto's north.

...SPECIAL FEATURE...REMNANTS OF FLORENCE...
Florence's remnant low has been downgraded to a remnant tropical wave tracking WNW...passing just NE of the Lesser Antilles. It is still directly under the paragraph P6 tropical upper ridge axis for better upper outflow...allowing the remnants to continue firing t-storms.

Judging by above thermo birdseye chart...Florence has exited the dry air region that caused her to dissipate in the first place (source of dry air in paragraph P5). The next obstacle noted for Florence's remnant continues to be southwesterly vertical shear due to mid-ocean upper vorticity (paragraph P3). As of this writing...we are roughly at the 48-hour point in the 200-mb outlook released in discussion #73...and this outlook is important as it showed an aggressive break-up of this upper vorticity that favors Florence's regeneration. I must say...the current 200-mb wind barbs in above atmo chart match fairly well to this 200-mb outlook...so I still think Florence has a chance to regenerate. If the forecasted break-up of mid-ocean upper vorticity begins falling behind schedule...then southwesterly shear would indeed stop Florence from regenerating. Due to the southwesterly shear potential...the NHC cites a 0% chance of regeneration in next 48 hours.

Track-wise...discussion #72 forecast for Florence's remnant suggested a more WNW track longer-term due to erosion of low-level Atlantic ridge from paragraph P1 upper trough/surface frontal system...and this gradual WNW turn is occuring with Florence's remnant at this time. The stronger/vertically deeper Florence becomes (assuming we have regeneration)...she could track even more north while feeling influences from mid-ocean upper vorticity in paragraph P3 and paragraph P1 upper trough emerging from the eastern US.

...SPECIAL FEATURE...TROPICAL WAVE INVEST 92-L...
This tropical wave...located east of Florence's remnant and well WSW of the Cape Verde Islands...has been continuously organizing since it left Africa. Its t-storm driven latent heat release has successfully locally inflated the south side of paragraph P6 upper ridge such that upper easterly shear is reducing...and more of an upper anticyclonic outflow continues to be established. Its thunderstorms continue to be sufficient in keeping out paragraph P5 dry air mass. With this reasoning...I continue to expect this tropical wave to become our next Atlantic tropical cyclone. W Atlantic upper ridge (paragraph P1) is nearly vertically stacked with Atlantic surface ridge (paragraph P5) for deep-layered easterly flow very supportive of a continued west track. In the longer-term...we shall see how this system interacts with any ridge weaknesses of paragraph P1 upper trough/surface frontal system...which would help in a northward bend or turn at some point. Interests in the Lesser Antilles and northern Caribbean Islands should monitor this system carefully as it is unclear if it will track north enough to hit or miss these islands.

...MID-LATITUDES DISCUSSION...
P1...Upper trough over central US and central Canada persists...and is spreading into eastern areas. First frontal cyclone just SW of Greenland has crossed Greenland and left the scope of above birdseye charts. Second frontal cyclone has swung from from Hudson Bay into waters just W of Greenland in the last 48 hrs...strengthening from 1002 to 998 mb in that timeframe. Central US surface ridge continued to weaken while being pushed into eastern US ahead of second frontal cyclone...joining the Atlantic ridge (paragraph P5) with a 1017 mb center over the SE US. But this central US surface ridge is re-building behind the second surface cyclone with support of upper convergence on back side of this upper trough regime...as seen with 1020 to 1021 mb centers in upper-left corner of above atmo birdseye chart. Low-level warm air advection ahead of second cyclone supports W Atlantic upper ridge...while SW US upper ridge persists behind this upper trough regime.

P2...Longwave upper trough regime across Atlantic high seas has concentrated into the NE Atlantic. Surface frontal depression (cyclone) diving SE toward Azores has become stationary just north of those islands....its cool air adveciton wrapping around a good bulk of the longwave upper trough into a large upper vortex stacked over the surface cyclone. Whatever did not get wrapped above the surface cyclone is a cut-off upper low just north of the Cape Verde Islands.

P3...Mid-ocean upper vorticity persists....now featuring a couple of upper vortex centers...one entering the Caribbean Sea while retrograding SW about W Atlantic upper ridge (paragraph P1)...the second located well SE of Newfoundland.

P4...All cut-off upper vorticity in Gulf of Mexico has retrograded westward into Mexico....pushed vigorously by the strong upper anticyclonic outflow of Tropical Storm Ernesto.

P5...Atlantic surface ridge with a 1025 mb center is supported by a few upper convergent sources while stretching from Gulf of Mexico/SE US to waters offshore of W Europe....including convergence SE of the of west Atlantic upper ridge (paragraphs P1)...and convergence behind NE Atlantic upper vortex (paragraph P2). In conjunction with south side of W Atlantic upper ridge (paragraph P1)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics. However...this dry air has been eradicated across the Caribbean Sea and Gulf of Mexico due to moisture from Ernesto...the remnants of Florence...and moisture produced by interaction of Caribbean upper vortex (paragraph P3) with a tropical wave (paragraph P7). Tropical wave Invest 92-L is helping to reduce this dry air in the eastern tropical Atlantic. Within this surface ridge...a surface trough E of Bahamas had been supported by by upper divergence between mid-ocean upper vorticity in paragraph P3 and W Atlantic upper ridge (paragraph P1). This surface trough is dissipating...what is left of it is SE of Bermuda at this hour.

...TROPICAL BELT DISCUSSION...
P6...Upper ridge across Atlantic tropics has been split in half by paragraph P3 upper vorticity retrograded SW into the Caribbean Sea. West half is concentrated over Ernesto...providing great anticycloinc outflow over the storm as noted in above Ernesto special feature section. East half of this upper ridge is being locally inflamed into cells of upper anticyclonic outflow via t-storm latent heat release from the remnants of Florence and tropical wave Invest 92-L...with upper troughs forming between these cells. For instance...there are inverted upper trough signatures between Invest 92-L and Florence...located east of the Lesser Antilles.

P7...Tropical wave in eastern Caribbean Sea is interacting with divergent southwesterly flow on SE flank of Caribbean upper vortex (paragraph P3 feature). This has greatly increased t-storm activity in this region. There is also increased t-storm activity ahead of this tropical wave in the west Caribbean/Central America...supported by split flow upper divergence between this upper vortex and Ernesto's upper anticyclone. Despite all this t-storm activity...no tropical cyclone development is expected due to westerly vertical shear across the south side of this upper vortex.

P8...Satellite imagery over Africa shows a very impressive tropical wave about to enter the eastern tropical Atlantic waters...as marked in the lower-right corner of above atmo birdseye chart.

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2012 Atlantic Hurricane Season Birdseye Discussion #73

By: NCHurricane2009, 12:01 AM GMT on August 08, 2012

...AUGUST 7 2012...8:01 PM EDT...
This discussion has been released 17 hours after the following graphics were created. This is due to another attempt at experimental 200-mb and surface outlook graphics that are time-consumptive for me to generate. Therefore...I have decided this is the last time I will attempt these outlooks.

Ernesto becomes a hurricane this afternoon...set to make landfall across Mexico's Yucatan peninsula beginning tonight. Later this week...Ernesto is expected to cross the Bay of Campeche waters and make a second and final landfall into mainland Mexico. Meanwhile...Florence's remnant low is at a higher risk of regenerating than previously thought..and yet another tropical wave (recently classified as Invest 92-L) is at risk of tropical cyclone formation. See all three special feature sections for further details on Ernesto...Florence...and Invest 92-L.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0121Z-released HPC analysis.
----->In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.
----->In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.


5-day surface outlook created at 1800Z yesterday. The streamlines represent lower-level atmospheric flow...ONLY drawn at the well-defined perimeter of surface ridge flows. Cyclonic flow is assumed to be occurring between the surface ridge flows. 0-hour flow is based on initialization of GFS computer model surface to 10-meter depth (shown on NOAA's MAG page). Flow beyond 0-hour is based on GFS model wind barb display for its surface to 10-meter high forecast (also shown on NOAA'S MAG page).


5-day 200-mb (upper-level) outlook created at 1200Z. The streamlines represent upper-level atmospheric flow...ONLY drawn at the well-defined perimeter of upper-level trough/vortex flows. Anticyclonic flow is assumed to be occuring between the upper-level trough/vortex flows. 0-hour flow is based on 200-mb wind barbs in above atmospheric birdseye chart. Flow beyond that is based on GFS model wind barb display for its 200 mb forecast...shown on NOAA's MAG page.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.
----->Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...HURRICANE ERNESTO...
Since previous discussion #72 (4:30 AM EDT Aug 6)...Ernesto was battling inverted upper trough aloft and dry air to his west. Yesterday (Aug 6) afternoon...Ernesto's thunderstorm latent heat release defeated the inverted upper trough earlier than I expected...and he quickly strengthened to 65 mph winds. Last evening (Aug 6)...Ernesto consumed the remainder of the dry air to his west...which halted intensification beyond 65 mph winds. Due to the dry air ingestion...my forecast graphic in Figure 1 below (generated early this morning) barely had Ernesto become a 75 mph max wind hurricane before making landfall in the Yucatan. Otherwise the rest of the intensity forecast in Figure 1 below is a copy-paste of my previous intensity forecast from discussion #72.

Predicting exactly when the dry air would mix out is difficult...but he has done so ahead of schedule and quickly become an 80 mph max wind hurricane this afternoon. So the only intensity forecast correction I am going to make now is to say that the 11 PM Tue point in Figure 1 (representing tonight's landfall in Yucatan) should be upped from 75 to 80 mph max winds. Ernesto is running out of time for additonal strengthening as his center is near the Yucatan shore as of this writing.


Figure 1: Tropical Storm (now Hurricane) Ernesto forecast graphic created early this morning.

Low-level steering solution (using above 5-day surface outlook).......Current ex-91L/paragraph P1 cold front surface ridge weakness shows as an inverted E Gulf of Mexico trough wedged between central US ridge (paragraph P1) and Atlantic ridge (paragraph P5). This inverted trough is shown collapsing thru 48 hr (11 PM Wed) as Atlantic ridge rebuilds in. Due to second impulse in paragraph P1...ridge weakness re-establishes itself across eastern US starting at 72 hr (11 PM Thu)...the weakness growing into the N Gulf of Mexico thru 96 and 120 hrs (11 PM Fri and 11 PM Sat). In conclusion...from surface perpsective...dissipation of E Gulf inverted trough suggests steady WNW steering thru 48 hrs (11 PM Wed)...weakening in WNW steering by 72 hrs (11 PM Thu)...then collapse in steering in 96 and 120 hrs (11 PM Fri and 11 PM Sat).

Upper-level steering soliuton (using above 5-day 200 mb outlook).......Paragraph P4 upper vortex is being pushed away from Ernesto by its vigorous upper anticyclonic outflow. Therefore...I keep this upper vortex a non-factor in Ernesto's upper-level steering solution. This leaves SW US upper ridge (paragraph P1) and upper trough (also in paragraph P1). The paragraph P1 upper trough amplifies southward and erodes SW US upper ridge thru 48 hrs (almost to suggest gradual collapse in upper steering solution thru 48 hrs...which is 11 PM Wed based on outlook's released time). The 200 mb 5-day outlook hints the SW US upper ridge recovers and noses in from the west after 48 hrs (after 11 PM Wed)...which would help in a westward upper-level steering solution by that time and onwards.

In Figure 1...my track forecast still has a northward bias compared to NHC's...because of Ernesto's tendency to be tracking more north than expected over last several hours...but also due to how I blend the above upper-level and surface steering solutions. Since the above upper-level steering solution is collapsed thru 11 PM Wed...I will be using the first 48 hrs worth of above lower-level steering solution regardless of how strong Ernesto gets...which is a steady WNW push. Between 11 PM Wed and 11 PM Thursday...I slow down Ernesto (behind NHC's suggested speed) as the above low-level steering solution starts weakening. There is also a westward bend in track beginning 11 PM Thu and onwards...as I believe Ernesto will be re-strengthening over Bay of Campeche waters...allowing him to feel more of the above upper-level steering solution. With 96 and 120 hr (11 PM Fri and 11 PM Sat) low-level steering solution collapsed...by that time I am depending fully on Ernesto's ability to continue strengthening and feeling the upper-level steering solution...which would take him westward into mainland east-central Mexico by Friday/Saturday. Not sure about NHC's forecasted WSW bend by Friday/Saturday...unless the SW US upper ridge will be strong enough to give a southward component to his track by that time.

...SPECIAL FEATURE...REMNANT LOW OF FLORENCE...
Florence's remnant low has moved directly under paragraph P6 upper ridge axis for better upper outflow...and combined with warmer water temps...she has begun re-flaring t-storms over her center. Despite the presence of dry air...these current dynamics are why I initially had kept Florence strong in my first forecasts during discussions #71 and #71A.

The dry air that caused Florence to weaken into a remnant low in the first place (source of dry air in paragraph P5) is still plentiful...keeping the new t-storm flares only intermittent. The next obstacle noted for Florence's remnant in previous discussion #72 is southwesterly vertical shear due to mid-ocean upper vorticity (paragraph P3). Above 200-mb outlook suggests the SW shear is parked stationary thru 24 hrs (which is 11 PM tonight due to the release time of above graphic). This stationary shear is located at 50W longitude and westward...and Florence is currently nearing that longitude. However...prospects for Florence to regenerate into a tropiacal cyclone are increasing...as above 200-mb outlook is aggressive in weakening the mid-oceanic upper vorticity. When comparing the above 96 hr 200-mb outlook to the discussion #72 120 hr 200-mb outlook...the weakening forecast for this upper vorticity is more aggressive than ever...which would reduce the SW shear and increase chances for Florence to regenerate. On the other hand...if the above 200-mb outlook does not pan out (i.e. the upper vorticity does not weaken as fast)...then Florence will fail to regenerate due to southwesterly shear.

Track-wise...so far Florence has been closely following my forecasted straight west track shown in Figure 2 of the previous discussion...as her shallow structure is steered westward by paragraph P5 surface ridge. Albeit...she looks like she has re-gained some vertical depth and appears to be bending more north in track...gravitated toward paragraph P3 upper vorticity. As a rule of thumb...the stronger Florence becomes...the more north she will track due to the paragraph P3 upper vorticity. The longer she stays weaker...the more west she will track due to paragraph P5 surface ridge.

...SPECIAL FEATURE...TROPICAL WAVE INVEST 92-L...
A tropical wave in the eastern tropical Atlantic has been continuously organizing since it left Africa...and is currently located WSW of the Cape Verde Islands and east of Florence's remnant low. Its t-storm driven latent heat release has been trying to locally flare up the paragraph P6 upper ridge and establish an upper outflow structure. Because most of this latent heat release has occurred on the south side of the upper ridge...this has increased the speed of upper eastelry winds...hence increasing the easterly vertical shear.

Its thunderstorms...supported by whatever upper outflow structure it has now...have been sufficient in keeping out paragraph P5 dry air. Due to SW-NE orientation of paragraph P6 upper ridge in this part of the Atlantic...with time the continued west track of the tropical wave will take it more directly beneath the upper ridge axis for less shear/more symmetrical upper outflow...similar to what is now occurring with Florence's remnant low. With this reasoning...I expect this tropical wave to become a tropical cyclone in the next days. It will track more WNW toward paragraph P3 upper vorticity the sooner it develops into a stronger/vertically deeper tropical cyclone. Atlernatively...if no development occurs...it will continue straight west under the guidance of the Atlantic (paragraph P5) surface ridge.

...MID-LATITUDES DISCUSSION...
P1...Upper trough over central US and central Canada persists....with the frontal cyclone it supports swinging east across E Canada and into the Atlantic high seas SW of Greenland while intensifying to 989 mb. A second impulse...another frontal cyclone from W Canada...has swung into Hudson Bay with a 1002 mb center...its cool air advection re-enforcing this upper trough. Central US surface ridge has weakened from 1025 to 1018 mb due to incoming 1002 mb cyclone...but this surface ridge should re-position/re-build behind this surface cyclone with support of upper convergence on back side of the upper trough. Low-level warm air advection ahead of 989 mb cyclone supports W Atlantic upper ridge...while SW US upper ridge persists behind this upper trough.

P2...Longwave upper trough regime continues across Atlantic high seas. Deep-layered low has pushed into west Europe...but its upper trough lingers behind in NE Atlantic. Frontal depression S of Greenland is diving SE toward the Azores....as its supporting shortwave upper trough swings SE and merges with NE Atlantic upper trough.

P3...Mid-ocean upper vorticity persists....now featuring a few upper vortex centers. In past 36 hrs...shortwave upper trough S of Greenland (paragraph P2 above) contributed to and re-enforced this upper vorticity.

P4...Cut-off upper vorticity in Gulf of Mexico is retrograding westward toward S Texas and Mexico....pushed vigorously by the strong upper anticyclonic outflow of Hurricane Ernesto.

P5...Atlantic surface ridge of 1023 to 1025 mb centers is supported by a few upper convergent sources while stretching from Gulf of Mexico to waters offshore of W Europe....including convergence SE of the of west Atlantic upper ridge (paragraphs P1)...and convergence behind NE Atlantic upper trough (paragraph P2). South side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics. However...this dry air is reduced due to moisture from Hurricane Ernesto and tropical wave Invest 92-L. Within this surface ridge...surface trough was recently supported by upper divergence between mid-ocean upper vortex in paragraph P3 and W Atlantic upper ridge (paragraphs P1). Now...this surface trough is supported by split flow divergence between two of the upper vortices in paragraph P3. This surface trough has recently been steered SW by this surface ridge and toward the waters E of the Bahamas. Decaying front extending from W Europe deep-layered cyclone (paragraph P2) also cuts into this surface ridge...with a 1020 mb low near the Azores...and 1016 mb low in south Spain.

...TROPICAL BELT DISCUSSION...
P6...Upper ridge continues to cover all of the Atlantic tropics. T-storm latent heat release from Hurricane Ernesto...occasionally from the remnants of Florence...and tropical wave Invest 92-L...continue to locally inflated this upper ridge into cells of upper anticyclonic outflow...with upper troughs forming between these cells. The upper trough E of the Lesser Antilles has finally merged with the mid-ocean upper vorticity mentioned in paragraph P3. Inverted upper trough between Florence's anticyclone and Invest 92-L's developing upper anticyclone persists...now located midway between the Lesser Antilles and Cape Verde Islands. Based on how low-amplitude this inverted upper trough has been...now I think it has too much representation in the above 200-mb outlook...which shows this inverted upper trough gradually going into the SE Caribbean over the next 5-days.

P7...Tropical wave fragment (surface trough) over north Florida...formerly tropical disturbance Invest 91-L...has dissipated several hours ago.

P8...Tropical wave approaching Lesser Antilles in previous discussion has crossed those islands into the eastern Caribbean Sea. Westerly vertical shear from paragraph P3 upper vorticity is preventing any tropical development here.

P9...Based on satellite imagery...next tropical wave from Africa has been continuously organzing into a low pressure spin while passing south of the Cape Verde Islands. It has been upgraded to tropical disturbance Invest 92-L. Hence this tropical wave has received its own special feature section above. See that special feature section for additional details on this situation.

Updated: 9:54 AM GMT on August 08, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #72

By: NCHurricane2009, 8:37 AM GMT on August 06, 2012

...AUGUST 6 2012...4:30 AM EDT...
Ernesto and Florence each are much lesser threats than they appeared to be previously. Ernesto has passed south of Jamaica and Cayman Islands without tropical storm impacts reaching the islands...but threat of tropical storm conditions shifts to southern Yucatan/Belize/NE Guatemala region...as well as to northern Honduras. See special feature sections for more details on Ernesto and Florence.

This discussion has taken a long time to get out due to experimental surface and 200 mb (upper-level) outlooks shown in atmospheric birdseye chart section. I may not continue such a product because of how much time it takes me to create them. My updated forecasts for Ernesto and Florence...as well as some other features...are heavily dependent on these outlooks. If these forecasts do not work...another reason for me to stop or adjust the methods used in these outlooks.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1330Z-released HPC analysis.
----->In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.
----->In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.


5-day surface outlook created at 1200Z. The streamlines represent lower-level atmospheric flow...ONLY drawn at the well-defined perimeter of surface ridge flows. Cyclonic flow is assumed to be occurring between the surface ridge flows. 0-hour flow is based on HPC's North America map wind barbs. Flow beyond that is based on GFS model wind barb display for its surface to 10-meter high forecast...shown on NOAA's MAG page.


5-day 200-mb (upper-level) outlook created at 1200Z. The streamlines represent upper-level atmospheric flow...ONLY drawn at the well-defined perimeter of upper-level trough/vortex flows. Anticyclonic flow is assumed to be occuring between the upper-level trough/vortex flows. 0-hour flow is based on 200-mb wind barbs in above atmospheric birdseye chart. Flow beyond that is based on GFS model wind barb display for its 200 mb forecast...shown on NOAA's MAG page.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.
----->Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM ERNESTO...
Ernesto has been accelerating rapidly west across the Caribbean Sea...in strong low-level trade wind flow driven by wind going from the Atlantic surface ridge (paragraph P5) and into the ITCZ to his south. For a tropical cyclone that is not well-established like Ernesto...rapid westward motion is not good for a few reasons:
(1) A cyclonic circulation that moves quickly westward has winds accelerate on its north side...but winds that are slowed on its south side. If the cyclonic circulation is not yet well-established...the south side of circulation may open and the system no longer qualify as a tropical cyclone.
(2) Rapid westward motion in deep tropics is sympomatic of strong low-level inflow into ITCZ...so the tropical cyclone is competing for low-level inflow against the ITCZ and hence may not build as much t-storm activity necessary for development.
(3) The force of Ernesto's upper anticyclone had been pushing the E Caribbean inverted upper trough into central Caribbean (paragraph P6)...and the ventilation provided by the upper anticyclone had been producing t-storms protecting Ernesto from dry air (see paragraph P5 for source of dry air). With the rapid acceleration to the west...Ernesto's center left behind the favorable upper anticyclone...sliding right into the dry air and beneath the unfavorable inverted upper trough.

The Gulf portion of Atlantic surface ridge (paragraph P5) has weakened due to low pressure field of ex-Invest 91L (paragrpah P7) and cold front of mid-latitude system in paragraph P1. This surface ridge weakness has caused Ernesto to finally slow down...allowing its trailing t-storm complex to catch up. A very recent check of upper winds reveals Ernesto is still battling inverted upper trough directly overhead...and above 200-mb outlook suggests this inverted upper trough won't be punched out by Ernesto's t-storm latent heat release till 24 hrs or a little later...so I forecast no strengthening for next 24 hrs in Figure 1. After 24 hrs...the above 200 mb outlook shows Ernesto's growing upper anticyclone pushing Gulf upper vortex (paragraph P4) westward...with possible grand anticyclonic outflow in between Gulf upper vortex to the west and paragraph P3 mid-oceanic upper vorticity encroaching from the east. With great outflow and warm waters...the only inhibiting factor for strengthening after 24 hrs would be land interaction...so my intensity forecast in Figure 1 shows a decent strengthening rate while Ernesto is over water...and a weakening rate when Ernesto is over land.

Low-level steering solution (using above 5-day surface outlook).......Current ex-91L/paragraph P1 cold front surface ridge weakness shows as an inverted E Gulf of Mexico trough wedged between central US ridge (paragraph P1) and Atlantic ridge (paragraph P5). This inverted trough grows into SE US from 0 hr to 72 hr...its growth spurt in 72 hr due to W Canada impulse (paragraph P1) swinging in by this time. Inverted trough grows into a complete SW-NE ridge weakness thru 96 and 120 hr...but south of that weakness the Atlantic ridge is shown bulging west from 96 to 120 hr. From surface perpsective...inverted trough suggests steady WNW steering thru 72 hrs...then collapse in steering in SW-NE ridge weakness by 96 hrs (as Atlantic ridge tries to push Ernesto north while central US ridge blocks this)...then perhaps a WNW nudge by 120 hrs as Atlantic ridge bulges west as stated above.

Upper-level steering soliuton (using above 5-day 200 mb outlook).......Paragraph P4 states that the only reason upper vortex in Gulf will move out of the way is due to a strengthening Ernesto's upper anticyclone. If the upper vortex did not move out of the way...it would shear apart Ernesto and keep him shallow...so I can't imagine a scneario at this point where a vertically-coupled/strong Ernesto would be steered by this upper vortex. Therefore...I keep this upper vortex a non-factor in Ernesto's upper-level steering solution. This leaves SW US upper ridge (paragraph P1 & P2) and upper trough (paragraph P1)...but the paragraph P1 upper trough amplfies southward and erodes SW US upper ridge thru 72 hrs (almost to suggest gradual collapse in upper steering solution thru 72 hrs). The 200 mb 5-day outlook hints the SW US upper ridge recovers and noses in from the west by 96 and 120 hrs...which would help in a westward upper-level steering solution.

In Figure 1...since the above upper-level steering solution is collapsed thru first 72 hrs...I will be using the first 72 hrs worth of above lower-level steering solution regardless of how strong Ernesto gets. Between 72 hrs and 96 hrs (11 PM Wed to 11 PM Thu)...am forced to still use low-level steering soltuion as Ernesto should be weaker/shallow from Yucatan landfall. Again...the low-level solution at 96 hrs is a weak one...while Atlantic ridge tries to push Ernesto north while central US ridge blocks this. Ernesto should be re-strengthening between 96 hrs and 120 hrs...allowing me to blend in more of the 120 hr upper-level steering solution (low-level solution at that time is westward bulging Alantic ridge giving WNW nudge...upper-level solution at that time is SW US upper ridge nosing in from the west and providing a more westward push).


Figure 1: Forecast for Tropical Storm Ernesto this morning.

...SPECIAL FEATURE...TROPICAL STORM FLORENCE...
Alas...after reaching a peak strength of 60 mph winds...Florence has weakened quiet a bit by couhing on dry air (mentioned in paragraph P5). As noted in discussion #71 and #71A...this dry air threat to Florence always loomed...but it has been finally realized.

The weakening has caused Florence to become more shallow...hence causing her to turn westward while losing touch with upper-level ridge weakness (paragraph P3) and becoming more steered by low-level ridge (paragraph P5). Previous forecast adjustment in discussion #71A was suggestive of WNW heading all the way after Florence unexpectedly had strengthened...the theory being that she would always feel the upper-level ridge weakness because of her refusal at the time to weaken from dry air. In hindsight...even if she had maintained her strength/vertical depth...she still would have turned more westward anyway as...
(1) Convergence behind British Isles/W Europe upper trough strengthened the low-level ridge to her north (paragraph P5).
(2) Comparison of yesterday's and today's atmo birdseye chart shows a westward shift in upper-level ridge weakness (paragraph P3 feature).

My forecast in Figure 2 is now aggressive in dissipating Florence into a tropical depression then remnant low in next 24 hrs. Above 5-day 200 mb outlook suggests semi-permanence of mid-ocean upper vorticity (paragraph P3)...so by 48 hrs Florence's current westward pace would begin to bring her to southwesterly vertical shear from this upper vorticity...which is why I do not forecast her remnant low to regenerate into a tropical cyclone. By 96 to 120 hrs (days 4 and 5)...above 200 mb outlook begins breaking up the upper vorticity into one vortex over Bahamas/W Caribbean...and another vortex to the NE...with a col of more favorable upper winds in between (E of Bahamas). This col would roughly be where I expect Florence's remnant low by 5-days...but I think it too early to speculate on re-generation of the remnant low this far out.

My remnant low track forecast in Figure 2 is rather far south of the NHC's...as I take a strict approach using the above 5-day surface outlook to predict her steering. This strict approach assumes the remnant will be too shallow to experience any gravitation toward the mid-oceanic upper vorticity shown in the 200 mb 5-day outlook. From 48 to 120 hrs...the surface outlook shows a surface ridge weakness propagating from Great Lakes to the eastern US and west edge of Atlantic...associated with what is now a second impulse of paragraph P1 frontal system currently over W Canada. I finally bend the track of remnant low more northward by 120 hours in response to this weakness...placing it E of the Bahamas by that time.


Figure 2: Tropical Storm Florence forecast

...MID-LATITUDES DISCUSSION...
P1...Frontal system and upper trough from W Canada has crossed the central US and central Canada in last 24 hours. Vigorous frontal surface cyclone supported by E divergence of upper trough has moved from Manitoba to S Hudson Bay while deepening from 999 to 996 mb in last 24 hrs. Upper convergence on back side of upper trough supports central US 1025 mb surface ridge. Low-level warm air advection ahead supports W Atlantic upper ridge...while SW US upper ridge persists behind this upper trough. Another shortwave upper trough and frontal cyclone over W Canada will soon dive SE and re-enforce this upper trough.

P2...Longwave upper trough regime continues across Atlantic high seas. W Atlantic and SW US upper ridges used to flank this system...but now have been moved to paragraph P1. Deep-layered low and upper trough continue to gradually push into the British Isles and western Europe (upper-right corner of above atmo birdseye chart). Frontal depression (now 1000 mb) and upper trough S of Greenland persists. Upper convergence behind this upper trough supports a surface ridge over E Newfoundland at 1024 mb. According to above 5-day 200 mb (upper-level) outlook...upper trough S of Greenland should merge with mid-ocean upper vorticity (paragraph P3) and British Isles/west Europe upper trough thru next 96 hours...the merger's upper divergence supporting SE-diving surface cyclone over and north of Azores as suggested by flow in upper-right corner of above surface 5-day outlook. I expect this SE-diving surface cyclone to be associated with aformentioned 1000 mb cyclone currently south of Greenland.

P3...Mid-ocean upper vortex persists. It continues to have a westward upper trough extension thru waters south of Bermuda....the extension locked between upper anticyclonic outflow of Ernesto to the south and W Atlantic upper ridge (paragraphs P1 and P2) to the north.

P4...Cut-off upper vorticity over west tip of Cuba has retrograded west into Gulf of Mexico. According to above 200-mb (upper-level) outlook...this upper vorticity will continue to retrograde west into Texas/Mexico by 72 hours. Considering this same outlook thru 72 hrs shows upper trough in paragraph P1 diving south and eroding SW US upper ridge (paragraphs P1 and P2)...it is not the SW US upper ridge that would drive this upper vortex into Texas/Mexico...but the upper anticyclone of Ernesto. With that said...if Ernesto turns out to be weaker than expected...then upper vortex may not retrograde as fast as shown in outlook. By 96 and 120 hours out...the outlook hints at SW US upper ridge re-flaring behind paragraph P1 upper trough...so the upper vortex retrogression by that time would be assisted if Ernesto's upper anticyclone fails to do the job.

P5...Atlantic surface ridge of 1023 to 1027 mb centers is supported by a few upper convergent sources while stretching from SE US to NW Africa....including convergence SE of the of the SW US and west Atlantic upper ridges (paragraphs P1 and P2)...and convergence behind British Isles/W Europe upper trough (paragraph P2) that has strengthened this ridge. South side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics. However...this dry air is reduced due to moisture from Tropical Storm Ernesto...Tropical Storm Florence...and tropical wave emerging from Africa (paragraph P9). Within this surface ridge...mid-ocean surface trough is still supported by upper divergence between mid-ocean upper vortex in paragraph P3 and W Atlantic upper ridge (paragraphs P1 and P2). This mid-ocean surface trough is further SW and located now just SE of Bermuda while moving with this upper divergence maximum. Decaying front extending from British Isles/W Europe cyclone (paragraph P2) also cuts into this surface ridge.

...TROPICAL BELT DISCUSSION...
P6...Upper ridge continues to cover all of the Atlantic tropics. T-storm latent heat release from Tropical Storm Ernesto...Tropical Storm Florence...and tropical wave emerging from Africa (paragraph P9)...continue to locally inflated this upper ridge into cells of upper anticyclonic outflow...with upper troughs forming between these cells. Inverted upper trough over E Caribbean has been pushed into central Caribbean by Ernesto's upper anticyclone...but this inverted upper trough did disrupt Ernesto (see Ernesto special feature section above for details). The upper trough WSW of the Cape Verde Islands is now E of the Lesser Antilles...and should merge with the mid-ocean upper vorticity mentioned in paragraph P3 when observing the above 200 mb 5-day outlook. Another inverted upper trough has formed SW of the Cape Verde Islands in between Florence's anticyclone and paragraph P9 tropical wave anticyclone...but after 24 hrs I dissipate this inverted upper trough in the above 5-day 200 mb outlook with Florence expected to diminish.

P7...Tropical wave fragment was over north Florida earlier at 1200Z...formerly tropical disturbance Invest 91-L. It was producing intermittent t-storms supported by split flow upper divergence between upper vorticity in paragraph P3 and upper vortex in paragraph P4.

P8...Tropical WSW of Cape Verde Islands in previous discussion is approaching Lesser Antilles. It continues struggling while ingesting dry air described in paragraph P5. Upper trough E of the Lesser Antilles (paragraph P6) is also suppressing this tropical wave.

P9...Based on satellite imagery...next tropical wave (with currently disorganized t-storms) is emerging from Africa into the eastern tropical Atlantic.

Updated: 10:12 PM GMT on August 07, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #71A (Special Update)

By: NCHurricane2009, 11:00 PM GMT on August 04, 2012

...AUGUST 4 2012...7:00 PM EDT...
This special update serves to address recent developments in Tropical Storm Ernesto...newly-developed Tropical Storm Florence...and tropical disturbance Invest 91-L moving from the Bahamas into Florida. Ernesto and Florence each have special feature sections in full discussion #71...while 91-L was discussed in paragraph P7 of discussion #71.

Concerning Tropical Storm Ernesto...he weakened from 60 to 50 mph shortly after discussion #71 was released...which baffled the National Hurricane Center (and me as well) considering his impressive upper outflow structure and favorable conditions. He has re-strengthened to 60 mph winds this afternoon...but the short-lived weakening has made Ernesto behind my intensity forecast in discussion #71. Given the favorable outlook on Ernesto in discussion #71...I expect him to catch up...so I will not change my intensity forecast...which currently passes his center south of Jamaica as a high-end category 1 hurricane and has him make landfall as a category 3 across the Yucatan. Trackwise...Ernesto is a bit behind schedule...but more importantly for Jamaica a bit more north. This means my discusison #71 impact swath (based on current tropical storm wind radius from NHC which I dilate in time to represent a strengthening storm) is more likely to cover all of Jamaica instead of the island's south half. Ernesto being slightly behind schedule means Jamaica impacts arriving tomorow evening instead of tomorrow late afternoon. All of Jamaica is now under a tropical storm warning...so all interests in Jamaica should be preparing.

Concerning Tropical Storm Florence...she has strengthened more than forecast in discussion #71. The small storm canopy once biased to the SW side of center has become very symmetric over the center. Like any tropical cyclone south of a tropical upper ridge axis...Florence has been trying to reduce easterly vertical shear and increase symmetry in its upper anticyclonic outflow by trying to locally pump up the upper ridge with t-storm latent heat release...and she appears to be successful as evidenced by the new symmetry in the storm canopy. It looks as though her low-level circulation has shrunken while tightening with this small storm canopy...which maybe why her north side has avoided adverse effects of dry air ingestion and water temps at/below 26 deg C. My updated thinking on track is now straight WNW as she has been...because she will now be strong/deep-layered enough to always feel the upper-level ridge weakness (caused by mid-ocean upper vortex) given that she does not weaken from dry air ingestion and/or cooler waters to the north (any significant weakening means that she will turn west while losing "feel" of upper-level ridge weakness). My updated intensity forecast is listed below...showing strengthening...then flattening due to possible dry air effects...then some more strengthening as I am still gambling on warmer waters/symmetric upper outflow allowing Florence to win against dry air.

5 PM Sat Aug 04 2012...50 mph...initial
5 AM Sun Aug 05 2012...60 mph...12 hr
5 PM Sun Aug 05 2012...65 mph...24 hr
5 AM Mon Aug 05 2012...65 mph...36 hr
5 AM Tue Aug 06 2012...65 mph...60 hr
5 AM Wed Aug 07 2012...75 mph...84 hr
5 AM Thu Aug 08 2012...75 mph...108 hr

Some final notes about Florence. Because I now think she will continue straight WNW (instead of stair-stepping W due to weakening shown in previous forecast)...this may place her closer to mid-ocean upper vortex...in which case she could get sheared by it later on. Will re-assess computer model upper wind forecasts by my next full discussion. Also...impact swath shown in discussion #71 Figure 2 needs to be more symmetric about storm track in short-term to account for increased symmetry in storm canopy...but can be left at same initial size based on 5 PM EDT NHC tropical storm wind radius and radius of storm canopy on satellite.

Concerning tropical wave fragment Invest 91-L...it has developed a surface 1016 mb low pressure spin and organizing t-storms in its east half...but is about to make landfall in Florida. Given high surface pressures from Atlantic surface ridge (paragraph P5...discussion #71) and land interaction...I still am not expecting tropical cyclone formation. Westward retrograding of upper vortex (paragraph P4...discusison #71) into Gulf of Mexico is still on pace with GFS model's forecast from this morning...which should mean this disturbance gets suppressed by the upper vortex as it continues WNW into the NE Gulf of Mexico...leaving behind the favorable outflow of upper anticyclonic cell to its east that had allowed its east half to organize. Because of surface ridge expected to build behind next upper trough (paragraph P1...discussion #71)...this disturbance should then continue WNW into Louisiana...and from there it would act to enhance shower and t-storm activity along tail end of upper trough's associated surface cold front.

Some final notes on Invest 91-L. If the upper vortex retrogrades west faster than expected for any reason...then 91-L would stay in touch with favorable upper anticyclonic cell to its east and tropical development would need to be re-assessed.

Updated: 4:56 AM GMT on August 05, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #71

By: NCHurricane2009, 2:35 PM GMT on August 04, 2012

...AUGUST 4 2012...10:40 AM EDT...
Tropical Storm Ernesto has begun strengthening earlier than expected...and poses a major threat in the Caribbean Sea for the next days. See first special feature section below for further details.

Tropical wave in eastern Atlantic strengthens to tropical depression six and then Tropical Storm Florence. See second special feature section below for further details on this system.

Tropical wave fragment over Cuba and Bahamas strengthened to tropical disturbance Invest 91-L in last 24 hours...but no tropical cyclone formation is expected at this time. See paragraph P7 in tropical belt discussion.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0600Z, and the 0726Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM ERNESTO...
Since previous discussion...and as this discussion was being written...Tropical Storm Ernesto has actually strengthened instead of weakened. I was expecting Ernesto to weaken slightly thru this morning...then start strengthening again later today...but it is obvious this strengthening has occurred ahead of schedule. As mentioned before...Ernesto's upper anticyclone has been trying to merge with upper anticyclone of upstream tropical wave (Invest 91-L...paragraph P7). This merger has been delayed as the tropical wave had unexpectedly intensified the identity of its upper anticyclone...and so the unfavorable east Caribbean inverted upper trough (paragraph P6) remains defined between these two upper anticyclones. 24 hrs ago...NW shear from the tropical wave's upper anticyclone was allowing dry air (paragraph P5) to enter from the NW. So one would think that the delayed merger between the upper anticyclones would cause persistent NW shear/dry air...as well as E Caribbean inverted upper trough...that would all go against Ernesto's favor. But clearly...Ernesto's t-storm canopy has gotten very impressive and symmetrical...latent heat release from which has allowed for its own upper anticyclone to push out all these unfavorable effects/features.

My intensity forecast is shown in Figure 1...and is definitely upped from the previous due to the earlier than expected strengthening. With warm waters...low shear...and good upper anticyclonic outflow in all directions...there is a possibility of rapid intensification in this forecast period. I show a steady strengthening rate...but then slow it after 5 AM Tuesday simply out of conservativeness (in case I am wrong to show aggressive strengthening). Weakening is shown by 5 AM Thu due to landfall across Yucatan peninsula. For now...my intensity forecast shows a category 3 hurricane moving into the Yucatan by day 4...and then a category 1 hurricane entering the SW Gulf of Mexico by day 5 due to weakening from landfall.

Trackwise...I see no reason to disagree with the steady west-to-WNW track shown by NHC for the next five days...given there will be deep-layered easterly flow on the south sides of the W Atlantic upper ridge (paragraph P2) and Atlantic surface ridge (paragraph P5). Upper trough in paragraph P1 (and another upper trough that will dig in right behind it from Hudson Bay) will erode the Gulf portion of the Atlantic surface ridge beginning day 2 and later. However...there will be another surface ridge supported by convergence behind the upper trough...so this surface ridge in conjunction with the Atlantic surface ridge would create more of a NW track rather than a total turn to the north. Previously I wanted to show a more northward track than NHC by day 5...out of respect for upper vortex in paragraph P4 that I thought this strong/deep-layered tropical cyclone would "feel." Because of the surprise flare up in upstream tropical wave's upper anticyclone (paragraph P7...Invest 91-L)...GFS now suggests this upper vortex will get a head start in moving west away from Ernesto to suggest that Ernesto will no longer interact with it.

Blue-dashed impact swath in Figure 1 is initially based on tropical storm wind radius shown at 5 AM EDT NHC advisory. It is generally symmetric about the forecast track to represent a well-organized and symmetric t-storm canopy...gradually growing size to represent the strengthening storm I forecast. Notice the swath becomes slightly smaller by the end....to account for landfall and weakening as Ernesto crosses the Yucatan. All of Jamaica is under a tropical storm watch...but taking my forecast impact swath literally means only south half of Jamaica would get notable impacts. However...if Ernesto tracks more north than expected or grows bigger than expected...all of Jamaica would be covered...so all of Jamaica should be preparing for tropical storm conditions as the watch suggests.


Figure 1: Forecast for Tropical Storm Ernesto this morning.

...SPECIAL FEATURE...TROPICAL DEPRESSION SIX (TROPICAL STORM FLORENCE)...
This feature strengthened from tropical depression six to Tropical Storm Florence as I was writing this discussion. However...I still refer to it as tropical depression six. I will be fully referring to it as Florence in my next discussion.

Tropical wave south of the Cape Verde Islands has moved WNW in past 24 hours while strengthening into tropical depression six. Its t-storm driven latent heat release continues to be south of the paragraph P6 upper ridge axis...which has locally increased the speed of upper easterly winds as it tries to locally inflate this upper ridge. This has maintained the easterly vertical shear over the tropical cyclone...as evidenced by the organized t-storm activity taking a SW bias in satellite pictures.

Over the next 5-days...mid-ocean upper vortex mentioned in paragraph P3 will be key in how this tropical cyclone behaves. GFS model 200 mb (upper) wind forecast suggests upper vortex will gradually merge with longwave upper trough regime in paragraph P2. As it does so...upper vortex is forecast to be stationary for next 48 hours...begin lifting NW by 72 hours...and then be merged in mid-latitude westerlies by 120 hours (while leaving behind another upper vortex retrograding SW into Hispaniola and well ahead of this tropical cyclone). All of this suggests the upper vortex will be far enough away to not shear this tropical cyclone...and instead be an upper-level ridge weakness such that the tropical cyclone tracks more WNW when stronger and deeper-layered...and tracks more W when weaker and shallower.

On these notes...my and NHC's forecast is shown in Figure 2 below. Based on its impressive satellite appearance...I agree with the intensity forecast shown by NHC for first 36 hrs...predicting a 45 mph max wind tropical storm. Because of the SW bias in storm clouds mentioned above...this makes the depression susceptible to swallowing dry air mentioned in paragraph P5 and shown "knocking on the door" of this tropical cyclone's north half in above thermo chart. Having the cyclone's north half passing over waters 26 deg C or below for the next couple of days is no help in allowing for t-storms in the north half. So I also agree with the NHC's forecast weakening between 36 hrs to 72 hrs. I do not dissipate the system like NHC does after 72 hrs...as I am gambling that warmer waters and very symmetrical upper outflow will help by then. Philosophy for symmetric upper outflow is because of SW-NE orientation of paragraph P6 upper ridge presently in this part of the Atlantic...so the cyclone's W-to-WNW heading will take it more directly under the upper ridge axis with time.

Track forecast in Figure 2 between me and NHC is same thru first 72 hrs...agreeing that a stronger and more deep-layered tropical cyclone will track WNW while "feeling" upper-level ridge weakness mentioned in 3rd paragraph of this special feature section...then agreeing that it will no longer "feel" this weakness while the tropical cyclone itself becomes shallower and weaker (hence turning more west). After 72 hrs...my intensity forecast is more aggressive...so my track forecast is more north than NHC after 72 hrs as I think it will "re-feel" the upper ridge weakness as it regains strength and hence vertical structure.

Impact swath in Figure 2 is on the basis of current small cloud canopy biased SW of the center...followed by gradual shrinking of impact swath diameter due to forecast weakening...and then a resurgence of the diameter as I forecast re-strengthening. The impact swath is also more symmetric about my forecast track in the end as I expect fully-symmetric upper outflow structure by then...which will lead to more symmetric cloud canopy.


Figure 2: Tropical depression six forecast issued earlier this morning.

...MID-LATITUDES DISCUSSION...
P1...Next frontal system and its upper trough is entering the picture from W Canada. It features a vigorous 999 mb surface cyclone over Manitoba whose cold front is generating strong t-storms over south-central Canada and north-central US this morning. This frontal system will be interacting with western Atlantic tropical activity over the next days...see Tropical Storm Ernesto section above and paragraph P7 below for further details on this.

P2...Longwave upper trough regime continues across NE North America and Atlantic high seas...with low-level warm air advection ahead supporting a west Atlantic upper ridge...while another upper ridge exists behind over the SW US. Deep-layered low and its upper trough is beginning to push into the British Isles (upper-right corner of above atmo birdseye chart). Surface ridge E of Atlantic Canada is now SE of Greenland...supported by upper convergence on the back side of deep-layered low and currently greater-than-1024 mb. Accelerationally divergent westerly jet on north side of SW US upper ridge appears to support a disorganized 1005 mb frontal depression over the central US soon to merge with cold front of system in paragraph P1. Frontal depression from E Canada has moved offshore to waters S of Greenland while strengthening to 1001 mb with support of divergence on the east side of longwave upper trough regime. Upper convergence behind the longwave supports a surface ridge just SE of Hudson Bay that has strengthened to 1026 mb in last 24 hrs. Frontal depression moving into Azores and its upper trough is merging with British Isles system mentioned at the beginning of this paragraph.

P3...Mid-ocean upper vortex persists. Western fragment has broken off and stationary south of Bermuda while quickly orbiting strong W Atlantic upper ridge mentioned in paragraph P2.

P4...Cut-off upper vorticity over west tip of Cuba persists.

P5...Atlantic surface ridge of 1025 to 1026 mb centers is supported by a few upper convergent sources while stretching from the Gulf of Mexico to the waters offshore of west Europe....including convergence SE of the of the SW US and west Atlantic upper ridges in paragraph P2. South side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics. However...this dry air is reduced due to moisture from Tropical Storm Ernesto...tropical disturbance Invest 91-L (paragraph P7)...and newly-formed tropical depression six. Within this surface ridge...mid-ocean surface trough is still supported by upper divergence between mid-ocean upper vortex in paragraph P3 and W Atlantic upper ridge in paragraph P2. This mid-ocean surface trough is further SW and located now just SE of Bermuda while moving with this upper divergence maximum.

...TROPICAL BELT DISCUSSION...
P6...Upper ridge continues to cover most of the Atlantic tropics. T-storm latent heat release from Tropical Storm Ernesto...tropical disturbance Invest 91-L (paragraph P7)...and from newly-formed tropical depression six...continue to locally inflated this upper ridge into cells of upper anticyclonic outflow...with inverted upper troughs forming between these cells. Inverted upper troughs are over E Caribbean Sea...south-central Caribbean Sea...and WSW of Cape Verde Islands. The inverted upper trough WSW of the Cape Verde Islands may merge with the mid-ocean upper vorticity mentioned in paragraph P3.

P7...Tropical wave over Central America in previous discussion has moved across Guatemala into the eastern Pacific. It leaves behind a surface trough and t-storm activity over Cuba and the Bahamas...continuing to be supported by divergent southwesterly jet between paragraph P4 upper vortex and anticyclonic cell of paragraph P6 upper ridge. The t-storm activity had inflated the upper anticyclonic cell with latent heat release in last 24 hours...which has reduced the southwesterly shear...in effect forcing this to become an area of interest in the NHC outlook and be upgraded to Invest 91-L. Based on latest GFS model output...I forecast this surface trough to make landfall over Florida in next 24 hours and move directly under the paragraph P4 upper vorticity...leaving behind the favorable upper outflow beneath the anticyclonic cell. I expect this cell to then merge with strengthening Ernesto's anticyclonic outflow by 48 hours...and Ernesto's outflow would then push the paragraph P4 upper vortex across the Gulf of Mexico right as this surface trough continues WNW into the Gulf. This would keep the surface trough beneath the unfavorable upper vortex...so I do not expect tropical cyclone formation from this surface trough. Surface trough likely to continue WNW into Louisiana via steering from surface ridge that builds on back side of paragraph P1 upper trough...perhaps enhancing SE US rainfall on the tail end of the paragraph P1 cold front.

P8...Tropical wave SW of the Cape Verde Islands is now WSW of the islands. It continues struggling while ingesting dry air described in paragraph P5 while simultaneously competing for low-level inflow with strong tropical wave to its east that has strengthened into tropical depression six in last 24 hrs. Inverted upper trough WSW of Cape Verde Islands (paragraph P6) is also suppressing this tropical wave.

P9...Tropical wave south of the Cape Verde Islands in previous discussion has strengthened to tropical depression six in last 24 hrs. See second special feature section above for further details on this system.

Updated: 4:19 AM GMT on August 06, 2012

Permalink

2012 Atlantic Hurricane Season Birdseye Discussion #70

By: NCHurricane2009, 11:10 AM GMT on August 03, 2012

...AUGUST 3 2012...7:15 AM EDT...
Tropical depression five has strengthened into Tropical Storm Ernesto during the previous 24 hours...and is now crossing the Lesser Antilles into the Caribbean Sea. Ernesto is expected to strengthen further...and interests in the Caribbean Sea should continue monitoring this system very carefully. See first special feature section below for further details.

Tropical wave rolling off of Africa has quickly organized over the last several hours. See second special feature section below for further details on this system.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0130Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM ERNESTO...
Tropical depression five became Tropical Storm Ernesto of 50 mph max sustained winds just before crossing the Lesser Antilles...and is now weakening to 45 mph as it crosses the islands into the Caribbean Sea. As advertised in the previous discussion...Ernesto is beginning the delicate process of having its upper anticyclonic outflow merge with the anticyclonic cell of the paragraph P6 tropical wave (located to the NW). The tropical wave's upper anticyclonic cell is blocking outflow and hence limiting t-storm activity in the NW half of Ernesto. This has allowed dry air mentioned in paragraph P4 to work its way into the NW half of the storm. The strength of Ernesto's upper anticyclonic outflow is keeping the E Caribbean inverted upper trough (paragraph P5) at bay to the west...but still watching carefully to see if Ernesto catches up to the inverted upper trough in which case vertical shear out of the south would also become a problem for the storm.

My intensity forecast is shown in Figure 1. Thru today and into tomorrow morning...I expect Ernesto to slightly weaken further from the unfavorable effects described above. Beyond that...I expect Ernesto's upper anticyclone to have finished merging with the anticyclonic cell to the NW...which would create a more symmterical and robust upper outflow environment over Ernesto that would squash out the E Caribbean inverted upper trough and dry air. My intensity forecast toward the latter part of the forecast is dampened as I expect possible southerly vertical shear from the paragraph P3 upper vorticity...which the GFS model sluggishly retrogrades out of the way. However...I think the center of Ernesto would be in split flow upper divergence between its upper anticyclonic cell and this upper vortex...and also have plenty of eastern upper outflow dump into the anticyclonic cell...so I still show a slow strengthening rate by then. My intensity forecast by the end of 5-days in Figure 1 is a category 2 hurricane.

Trackwise...I see no reason to disagree with the steady west-to-WNW track shown by NHC for the next three days...given there will be deep-layered easterly flow on the south sides of the W Atlantic upper ridge (paragraph P1) and Atlantic surface ridge (paragraph P4). Upper trough over W Canada previous discussion is about to enter the upper-left corner of above atmo birdseye chart. Based on model presentations of this upper trough (and another upper trough that digs right behind it from Hudson Bay)...it now appears it will erode the Gulf portion of the Atlantic surface ridge beginning day 3 and later. However...there will be another surface ridge supported by convergence behind the upper trough...so this surface ridge in conjunction with the Atlantic surface ridge would create more of a NW track rather than a total turn to the north. I am a little north of the NHC forecat track in Figure 1 by day 5 as I am expecting a vigorous hurricane that is deep-layered enough to feel some steering influence from the paragraph P3 upper vortex.

Blue-dashed impact swath in Figure 1 is initially based on tropical storm wind radius shown at 5 AM EDT NHC advisory...coupled with the small t-storm canopy presently over and just south of the center. The swath is then drawn generally symmetric about the forecast track in a gradually growing size to represent the strengthening storm I forecast. Notice the swath becomes increasingly biased to the north of my forecast track by the end....to represent possible southerly vertical shear from the paragraph P3 upper vortex later on. Interests in the Caribbean Sea should monitor this system very carefully.


Figure 1: Forecast for Tropical Storm Ernesto this morning.

...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE...
Tropical wave currently south of the Cape Verde Islands has been well-organized since leaving west coast of Africa yesterday. Its t-storm driven latent heat release has been mainly south of the paragraph P5 upper ridge axis...which has locally increased the speed of upper easterly winds as it tries to locally inflate this upper ridge. This has slightly increased the easterly vertical shear over the tropical wave...as evidenced by the organized t-storm bands taking a westward bias in satellite pictures. If the strong t-storms continue to locally inflate the upper ridge...then the upper flow over the tropical wave will become less easterly and more anticyclonic...making tropical cyclone formation more likely.

Over the next 5-days...mid-ocean upper vortex mentioned in paragraph P2 will be the inhibiting factor for development. GFS model 200 mb (upper) wind forecast suggests the upper vortex may stay parked long enough to begin imparting unfavorable southwesterly shear on this tropical wave. After 5-days...the upper vortex is forecast by GFS to lift out while it re-links with the mid-latitude westerlies...returning upper winds to a favorable state for tropical development. At this time...no computer models develop this tropical wave into a tropical cyclone.

...MID-LATITUDES DISCUSSION...
P1...Longwave upper trough regime continues across eastern North America and Atlantic high seas...with low-level warm air advection ahead supporting a west Atlantic upper ridge...while another upper ridge builds behind over the SW US. Deep-layered low remains parked just offshore of the British Isles (upper-left corner of above atmo birdseye chart). Surface ridge E of Atlantic Canada has strengthened from 1020 to 1026 mb while supported by upper convergence on the back side of deep-layered low. Accelerationally divergent westerly jet on north side of SW US upper ridge appears to support a disorganized 1002 mb frontal depression over the central US...while 1004 mb depression has accelerated from south-central Canada to E Canada while becoming supported by divergence on the east side of longwave upper trough regime. Upper convergene behind the longwave supports a 1015 mb surface ridge just SW of Hudson Bay. Frontal depression approaching Azores has amplified its supporting shortwave upper trough into a small upper vortex thanks to local cool air advection on its back side. Nova Scotia/Newfoundland frontal depression has weakened into surface troughs stretching into NW Atlantic.

P2...Mid-ocean upper vortex persists. Western fragement has broken off and is passing south of Bermuda while quickly orbiting strong W Atlantic upper ridge mentioned in paragraph P1. It will soon merge with longwave upper trough regime discussed in paragraph P1.

P3...Cut-off upper vorticity over west tip of Cuba persists.

P4...Atlantic surface ridge of 1014 to 1025 mb centers is supported by a few upper convergent sources while stretching from the Gulf of Mexico to the waters offshore of west Europe....including convergence SE of the of the SW US and west Atlantic upper ridges in paragraph P1. South side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics. However...this dry air is reduced due to moisture from Tropical Storm Ernesto...and the tropical waves in paragraphs P6 and P8. Within this surface ridge...mid-ocean surface trough is still supported by upper divergence between mid-ocean upper vortex in paragraph P2 and W Atlantic upper ridge in paragraph P1.

...TROPICAL BELT DISCUSSION...
P5...Upper ridge continues to cover most of the Atlantic tropics. T-storm latent heat release from paragraph P6 and P8 tropical waves...as well as from Tropical Storm Ernesto....continue to locally inflated this upper ridge into cells of upper anticyclonic outflow...with inverted upper troughs forming between these cells. Inverted upper troughs are over E Caribbean Sea and WSW of Cape Verde Islands.

P6...Tropical wave in the central Caribbean continues has moved west into Central America. Its t-storm activity...and associated upper anticyclonic cell of paragraph P5 upper ridge (driven by t-storm latent heat release)...remains displaced to the east thanks to upper vorticity in paragraph P3. Divergent southwesterly upper jet between paragraph P3 upper vorticity and upper anticyclonic cell is also helping to support these t-storms...as well as a surface trough over Cuba and the Bahamas that appears to be a north fracture of this tropical wave.

P7...Tropical wave SW of the Cape Verde Islands is struggling while ingesting dry air described in paragraph P4 while simultaneously competing for low-level inflow with strong tropical wave to its east (strong tropical wave discussed in 2nd special feature section above and paragraph P8 below).

P8...Tropical wave rolling off of Africa in the previous discussion (now south of the Cape Verde Islands) has been introduced into the National Hurricane Center Tropical Weather Outlook in last 24 hours. See second special feature section above for further details on this system.

Permalink

2012 Atlantic Hurricane Season Birdseye Discussion #69

By: NCHurricane2009, 10:30 AM GMT on August 02, 2012

...AUGUST 2 2012...6:30 AM EDT...
Tropical wave Invest 99-L has become tropical depression five...but the newly-formed depression is currently in a fragile state. Nonetheless...interests in the central Lesser Antilles should prepare for tropical storm conditions...and interests in the Caribbean Sea should monitor this system carefully. See special feature section for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0131Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL DEPRESSION FIVE...
Tropical wave WSW of the Cape Verde Islands...designated as Invest 99-L...has strengthened to tropical depression five in last 24 hrs...and is accelerating west toward the Lesser Antilles this morning. During the previous discussion...the outlier CMC model suggested the upper vorticity in paragraph P2 (mid-lat discussion) would split over the next 5-days while a fragment retrograded around the W Atlantic upper ridge (paragraph P1). The CMC then calculated a respectable deeper-layered tropical cyclone that would "feel" the western split of the upper vorticity and hence track more northward. Current strength of W Atlantic upper ridge suggests it will act as a fortress against any splitting of paragraph P2 upper vorticity...so I don't think this CMC scenario will happen. In fact...the CMC has increasingly stopped shwoing this outlier solution and come into agreement with the remainder of the computer models.

Even though the paragraph P2 upper vorticity will not split...it has surprised us by becoming more amplfied east of the W Atlantic upper ridge...hence splitting the favorable upper anticyclone over tropical depression five into once cell to the WNW and another to the ENE (see paragraph P5). This had increased the westerly shear over the depression...and made the depression quickly appear "sickly." Very recently though...a t-storm complex has fired over/south of the center...as it enters the favorable upper anticyclonic cell to the WNW. Moreover...GFS 200 mb (upper wind) forecast suggests this cell will merge with the anticyclonic cell of the paragraph P6 tropical wave...and then the merged cell continuing west with the tropical cyclone. My intensity forecast in Figure 1 is more aggressive than the NHC forecast...on the basis that if the GFS 200 mb forecast verfies...I see a prolonged exposure to favorable upper anticyclonic outflow that will strengthen this system thru the next 5 days. I initially dampen the intensity forecast today and Friday...given the poor initial structure of the depression and that it may encounter light southerly shear from E Caribbean inverted upper trough mentioned in paragraph P5. I then accelerate the intensity Saturday and Sunday...but then I go a bit more conservative by Monday and Tuesday. My intensity forecast by the end of 5-days in Figure 1 is a high-end category 1 hurricane.

Trackwise...I see no reason to disagree with the steady west-to-WNW track shown by NHC...given there will be deep-layered easterly flow on the south sides of the W Atlantic upper ridge (paragraph P1) and Atlantic surface ridge (paragraph P4). Upper trough over Alaska in previous discussion is now in W Canada (but still not yet in the scope of mid-latitude discussion below). Based on model presentations of this upper trough (and another upper trough that digs right behind it from Hudson Bay)...it does not look as though it will sufficiently erode the Atlantic surface ridge (paragraph P4) nor the SW US upper ridge (paragraph P1)...so a steady west-to-WNW track is possible beyond 5 days.

Blue-dashed impact swath in Figure 1 is initially based on very recent t-storm complex that has fired over and just south of center. The swath is then drawn generally symmetric about the forecast track in a generic size...but it is hard to predict what the actual size fluctuations of this storm are going to be over the next 5 days. Tropical storm advisories are in effect for central Lesser Antilles islands...where tropical storm preparations should be taking place. Beyond that...interests in the Caribbean Sea should monitor this system very carefully.


Figure 1: Forecast for tropical depression five this morning.

...MID-LATITUDES DISCUSSION...
P1...Longwave upper trough regime continues across eastern North America and Atlantic high seas...with low-level warm air advection ahead supporting a west Atlantic upper ridge...while another upper ridge builds behind over the SW US. Upper trough from Hudson Bay has shifted SE into eastern US while what's left of its frontal depression is an east-west front in eastern US. Yet another Hudson Bay upper trough and surface frontal depression has sweeped in. Convergence between this longwave upper trough regime and SW US upper ridge supports a 1020 mb west US surface ridge...while 1013 mb and 1020 mb surface ridges over the Great Lakes and E of Atlantic Canada is supported by upper convergence on the back sides of embedded shortwave upper troughs. Accelerationally divergent westerly jet on north side of SW US upper ridge appears to support a disorganized 1004 mb frontal depression over the central US...and another 1004 mb depression moving quickly E across south-central Canada. Surface frontal cyclone and its shortwave upper trough heading toward Europe is about to make landfall in the British Isles...and its satellite appearance suggests its shortwave upper trough has become vertically stacked with the surface cyclone to create a deep-layered low. Frontal depression offshore of Newfoundland (and its supporting shortwave upper trough) has moved quickly east toward Azores in last 24 hrs. Finally...frontal depression has moved NE from NC/VA coast and into Nova Scotia/Newfoundland in last 24 hrs.

P2...Mid-ocean upper vortex persists.

P3...Cut-off upper vorticity over west Cuba is retrograding slowly west toward the Yucatan while steeered about SW US upper ridge mentioned in paragraph P1.

P4...Atlantic surface ridge of 1016 to 1023 mb centers is supported by a few upper convergent sources while stretching from the Gulf of Mexico to the waters offshore of west Europe....including convergence SE of the of the SW US and west Atlantic upper ridges in paragraph P1. South side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics. However...this dry air is reduced due to moisture from tropical depression five...and the tropical waves in paragraphs P6...P8....and P9. Within this surface ridge...mid-ocean surface trough is still supported by upper divergence between mid-ocean upper vortex in paragraph P2 and W Atlantic upper ridge in paragraph P1.

...TROPICAL BELT DISCUSSION...
P5...Upper ridge continues to cover most of the Atlantic tropics. T-storm latent heat release from paragraph P6....P8....and P9 tropical waves...as well as from tropical depression five....continue to locally inflated this upper ridge into cells of upper anticyclonic outflow...with inverted upper troughs forming between these cells. Inverted upper troughs are over Panama...E Caribbean Sea...and SW of Cape Verde Islands. Upper anticyclonic cell over tropical depression five is split into one cell to its WNW and the other to its ENE...thanks to amplified upper vorticity in paragraph P2.

P6...Tropical wave in the central Caribbean continues west. Its t-storm activity...and associated upper anticyclonic cell of paragraph P5 upper ridge (driven by t-storm latent heat release)...remains displaced to the east thanks to upper vorticity in paragraph P3. Therefore...no tropical cyclone development is possible with this tropical wave.

P7...Tropical wave WSW of the Cape Verde Islands...designated as Invest 99-L...has been upgraded to tropical depression five in last 24 hrs. See special feature section for further details.

P8...Tropical wave south of the Cape Verde Islands in the previous discussion is now SW of the islands. The low pressure swirl of this wave is not as well-defined as 24 hrs ago...but there is still some cyclonic turning of the clouds in satellite animation. T-storm activity in NW half of this tropical wave is limited by dry air mentioned in paragraph P4.

P9...Satellite imagery suggests the next tropical wave is rolling off of Africa...as remarked in the lower-right corner of above atmo birdseye chart.

Updated: 7:10 AM GMT on August 03, 2012

Permalink

2012 Atlantic Hurricane Season Birdseye Discussion #68

By: NCHurricane2009, 8:57 AM GMT on August 01, 2012

...AUGUST 1 2012...5:00 AM EDT...
Tropical disturbance Invest 99-L has become slightly better organized in the last 24 hours. Interests in the Caribbean Sea and Bahamas should watch this system. See special feature section for further details.

Elsewhere...t-storm activity that has entered the NE Gulf of Mexico not expected to develop...especially as it diminshes quickly this early morning (see end of paragraph P1). One of the Caribbean tropical waves has become an area of interest in the National Hurricane Center outlook...but is not expected to develop (paragraph P7). Impressive tropical wave with low pressure swirl is south of the Cape Verde Islands...but dry air appears to be inhibiting development (paragraph P9).

Of side note...July has ended with no Atlantic tropical cyclones for the month. This is the first July since 2009 that had zero Atlantic tropical cyclones.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1329Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL WAVE INVEST 99-L...
Tropical wave southwest of the Cape Verde Islands...designated as Invest 99-L...is continuing west and is now WSW of the islands. Its surface low pressure is still marked in NHC TAFB maps. This surface low and t-storms has favorable conditions thanks to established upper anticyclonic outflow as mentioned in paragraph P5 of tropical belt discussion. National Hurricane Center Outlook now suggests a 50% chance of tropical cyclone formation in next 48 hours. Given the increase of organization seen in last 24 hours...I am more confident that a tropical cyclone will develop and am now putting this as a special feature on this blog.

The CMC and GFS model are still the only two global models suggesting any development out of this system. The CMC continues to present a stronger tropical cyclone and more WNW track...while the GFS continues to present a weaker tropical cyclone and more W track. Surface and upper-level outputs of both models for the next 120 hours (5 days) are summarized in Figures 1 and 2. Features in those figures are marked with paragraph (P) numbers corresponding to paragraph numbers in the tropical belt and mid-latitude discussions.

As shown in Figure 1a...the CMC model is initialized with a position north of 10N latitude when in reality Invest 99-L is along 10N latitude judging by latest satellite image. This might explain some of its northward bias. CMC also suggests the upper vorticity in paragraph P2 (mid-lat discussion) splits over the next 5-days while a fragment retrogrades around the W Atlantic upper ridge (Figure 1b). Coupled with the fact that CMC calculates a more rapid strengthening rate (and hence a deeper-layered tropical cyclone capable of "feeling" upper features)...it suggests it will "feel" the western split of the upper vorticity and hence track more WNW. CMC model WNW track would take it over the northern Lesser Antilles and eventually the Bahamas.

The GFS model (Figure 2) is better initialized than CMC...showing Invest 99-L along 10N latitude rather than just north of 10N. It shows a weaker tropical cyclone and straight west track across the Lesser Antilles and into the Caribbean. Its more southward solution is due to calculating a weaker (and hence more shallow) tropical cyclone not as capable of feeling upper features. Even if 99-L did become stronger in the GFS solution...the GFS does not split the paragraph P2 upper vorticity like the CMC does such that it cannot track more northward toward a split.


Figure 1: 00Z CMC model output this morning. (a) is 0-hour...(b) is 120-hour. The left panes are surface pressure fields...the right panes are 500 mb (upper-level) pressure fields.


Figure 2: 00Z GFS model output this morning. (a) is 0-hour...(b) is 120-hour. The left panes are surface pressure fields...the right panes are 500 mb (upper-level) pressure fields.

Due to a more accurate initial position in GFS...coupled with slow rate of organization 99-L has had...I am leaning toward the weaker tropical cyclone and further south track shown by GFS. I think it is possible the current GFS output thru 5-days maybe too weak given the favorable widespread upper anticyclonic outflow currently over 99-L. So if 99-L becomes stronger and more deep-layered...and if the upper vorticity splits as shown in CMC...then it could "feel" a western split in the upper vorticity and track more WNW like CMC. I think such a WNW track would be further south than the current CMC since the CMC was initialized a tad too far north.

In the very long range...both models show the next upper trough in the mid-latitudes along eastern North America by day 5. While currently over Alaska...this upper trough is not yet in the scope of the mid-latitudes and tropical belt discussion. From current GFS and CMC outputs...it looks as though this upper trough is not going to be far south enough to erode the surface ridge (paragraph P4 of mid-lat discussion). If the surface ridge stays enforced...99-L would continue its west or west-northwest track beyond 5 days.

...MID-LATITUDES DISCUSSION...
P1...Longwave upper trough regime continues across eastern North America and Atlantic high seas...with low-level warm air advection ahead supporting a west Atlantic upper ridge...while another upper ridge builds behind over the SW US. Upper trough from Hudson Bay continues supporting a surface frontal depression...1008 to 1007 mb and now just SE of the bay. Convergence between this longwave upper trough regime and SW US upper ridge supports a 1020 to 1017 mb west US surface ridge extending to Minnesota...while a 1017 to 1022 mb surface ridge from West Virginia to Atlantic Canada is supported by upper convergence on the back sides of embedded shortwave upper troughs. Accelerationally divergent westerly jet on north side of SW US upper ridge appears to support a disorganized 1011 mb frontal depression over the central US. Surface frontal depression (extratropical low) heading toward Europe is still doing so as seen in upper-right of above atmo chart...and frontal depression offshore of Newfoundland is moving slowly NE. Finally...shortwave upper trough over SE US continues supporting a frontal depression along the coast of the Carolinas and Virginia...which is now heading slowly NE. Split flow upper divergence between this SE US shortwave upper trough and SW US upper ridge earlier had triggered strong t-storms over the SE US that have migrated into the NE Gulf of Mexico. There is a surface trough associated with this activity...but land interaction...high surface pressures from the surface ridge in paragraph P4...and recent decrease in this activity suggests no tropical development expected.

P2...Upper vortex just SE of the Azores has merged with shortwave upper trough supporting the surface frontal depression (extratropical low) heading toward Europe mentioned in paragraph P1. Mid-ocean upper vortex persists.

P3...Cut-off upper vorticity in the Caribbean Sea consists of an upper vortex over west Cuba.

P4...Atlantic surface ridge of 1018 to 1025 mb centers is supported by a few upper convergent sources while stretching from the Gulf of Mexico to the waters offshore of west Europe....including convergence SE of the of the SW US and west Atlantic upper ridges in paragraph P1. South side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics. However...this dry air appears to be reducing due to enhancement of tropical waves as described in paragraphs P5....P7....and P8. Within this surface ridge...mid-ocean surface low has weakened into a surface trough...but still supported by upper divergence between mid-ocean upper vortex in paragraph P2 and W Atlantic upper ridge in paragraph P1.

...TROPICAL BELT DISCUSSION...
P5...East tropical Atlantic upper ridge finally has linked with south-central Caribbean upper ridge....and now it covers most of the Atlantic tropics. T-storm latent heat release from tropical waves in special feature section and paragraph P7 has locally inflated this upper ridge into cells of upper anticyclonic outflow. Upper vortex SW of Bermuda...once embedded in this upper ridge...has weakened into an upper trough and could soon merge with longwave upper trough regime in paragraph P1. Upper vorticity west of the upper ridge...located in the south Gulf of Mexico...is weakening to the south of the SW US upper ridge in paragraph P1.

P6...Tropical wave moving across central America is entering the eastern Pacific...and therefore this is the last statement on this tropical wave in this blog. After being enhanced by upper outflow of south-central Caribbean upper ridge in paragraph P5...its t-storm activity has reduced as it becomes suppressed by westerly shear south of the upper vorticity in paragraph P3.

P7...Tropical wave in the eastern Caribbean is pushing into the central Caribbean. Due to widespread vigorous t-storm activity that had overspread the Lesser Antilles...Puerto Rico...and Hispaniola...it was introduced into the National Hurricane Center outlook in last 24 hrs. Its vigorous activity has been supported by one of the cells of anticyclonic upper outflow described in paragraph P5. However...it is quickly heading into a much less favorable environment of westerly vertical shear associated with the upper vorticity in paragraph P3...so no tropical cyclone formation is expected here.

P8...Tropical wave WSW of the Cape Verde Islands...designated as Invest 99-L...has its own special feature section above. See special feature section for further details on this system.

P9...Tropical wave rolling off of Africa in the previous discussion has been added to NHC TAFB maps...at a location south of the Cape Verde Islands. Upper winds have become more favorable for this tropical wave as the northerly shear has reduced...thanks to anticyclonic outflow of Invest 99-L moving away from this system. This system has impressively developed a low pressure swirl also marked in TAFB maps. Despite better upper winds and the development of this swirl...there is little t-storm activity as of this writing...as if the dry air metioned in paragraph P4 is suppressing this tropical wave.

Updated: 9:11 AM GMT on August 01, 2012

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