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.


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.


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).

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.

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...

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.

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.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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