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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.
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.
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