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.


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).

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.

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.

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.

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.

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.

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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