Isaac reorganizing as it blows through the Lesser Antilles

By: Dr. Jeff Masters , 8:34 PM GMT on August 22, 2012

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Tropical Storm Isaac continues to maintain top winds of just 45 mph as its center prepares to move through the Lesser Antilles islands late this afternoon and early this evening. The entire Lesser Antilles chain of islands is receiving heavy rains from Isaac, with Martinique picking up 1.46" of rain as of 2 pm EDT, and St. Lucia receiving 1.49". However, Isaac is not yet generating much in the way of tropical storm-force winds, and none of the islands had received winds in excess of 30 mph as of 4 pm EDT. During their storm penetration to obtain their 2 pm EDT center fix, an Air Force Reserve aircraft measured top surface winds of just 40 mph, and a central pressure of 1004 mb. Top winds at their 5000 foot flight altitude were 49 mph. Isaac is undergoing significant changes to its structure. The plane found the center had become a broad, elongated oval that extended 40 miles from NW to SE. The old center, fixed at 2 pm near 16.1°N, and closer to the dry air to Isaac's north, is being challenged for dominance by a new center that is attempting to form near a burst of heavy thunderstorms at 15.5°N. The Hurricane Hunters' latest fix at 3:50 pm EDT put the center near 15.9°N, a southwards shift of about 17 miles. The resulting battle between centers is giving Isaac a rather odd spiral rectangular shape, as seen on visible satellite loops. The Hurricane Hunters did not observe an eyewall trying to form, and recent microwave satellite images also show no signs of an eyewall forming. A large area of dry air to the north of the storm, as seen on water vapor satellite loops, continues to interfere with development, and Isaac will not be able to begin strengthening until it resolves the battle between it two competing centers, and casts out the dry air infiltrating it. This is going to take at least a day, since Isaac is a very large storm, and it takes more time to spin up a big chunk of the atmosphere. Radar imagery from Barbados and Martinique show plenty of heavy rain showers, mostly on the south side of Isaac where it is moister. There has been a modest increase in spiral banding since this morning.


Figure 1. Afternoon satellite image of Isaac, showing its odd spiral rectangle shape.

Latest model runs for Isaac
The latest set of 12Z (8 am EDT) model runs are very similar to the previous set of runs, which I discussed in detail in this morning's post. The models show a westward track to a point on the south coast of Hispaniola. Isaac's center shift to the south may require some modest adjustments to the south and west for the models. This would result in the storm spending a few hours less time over Hispaniola, and more time over or just south of Cuba. This would slightly decrease the risk to the Dominican Republic, the east coast of Florida, and the Bahamas, but increase the risk to the west coast of Florida. The ECMWF--our best performing model over the past two years--continues to be an outlier among the models. It predicts that Isaac will track just south of Cuba, cross the western tip of Cuba on Monday, then head north towards an eventual landfall in Louisiana. However, this model is keeping Isaac weaker than the other models, and thus predicts the storm will have a weaker response to the trough of low pressure over the Southeast U.S. If the official NHC intensity forecast is right and Isaac becomes a hurricane on Thursday, the more southerly track of the ECMWF is not going to verify, and Isaac will spend considerable time over Cuba on Saturday and Sunday. Where Isaac pops off the coast of Cuba will be critical in determining its future path and intensity, and at this point, we don't know it its more likely that Isaac will go up the east coast of Florida, the west coast, or straight up the peninsula over land. At this point, I'd put the odds at:

40% chance of a track through the Gulf of Mexico, west of Florida
25% chance of a landfall in South Florida, and a track mostly over the Florida Peninsula
35% chance of a track along the east coast of Florida

Which model should you trust?
Wunderground provides a web page with computer model forecasts for many of the best-performing models used to predict hurricane tracks. So which is the best? The best forecasts are made by combining the forecasts from three or more models into a "consensus" forecast. Over the past decade, NHC has greatly improved their forecasts by relying on consensus forecast models made using various combinations of the GFS, GFDL, NOGAPS, UKMET, HWRF, and ECMWF models. If you average together the track forecasts from these models, the NHC official forecast will rarely depart much from it, and the NHC forecast has been hard to beat over the past few years. The single best-performing model over the past two years has been the ECMWF (European Center model). The GFS model has been a close second. You can view 7-day ECMWF and 16-day GFS forecasts on our wundermap with the model layer turned on. Ten-day ECMWF forecasts are available from the ECMWF web site. The European Center does not permit public display of tropical storm positions from their hurricane tracking module of their model, so we are unable to put ECMWF forecasts on our computer model forecast page that plots positions from the other major models. As seen in Figure 2, the HWRF and GFDL were well behind the ECMWF and GFS in forecast accuracy in 2011, but were still respectable. The BAMM model did very well at 4 and 5-day forecasts. The UKMET, NOGAPS, and CMC models did quite poorly compared to the ECMWF, GFS, GFDL, and HWRF. For those interested in learning more about the models, NOAA has a great training video (updated for 2011.)


Figure 2. Skill of computer model forecasts of Atlantic named storms 2011, compared to a "no skill" model called "CLIPER5" that uses just climatology and persistence to make a hurricane track forecast (persistence=a storm will tend to keep going in the direction it's current going.) OFCL=Official NHC forecast; GFS=Global Forecast System model; GFDL=Geophysical Fluid Dynamic Laboratory model; HWRF=Hurricane Weather Research Forecasting model; NOGAPS=Navy Operational Global Prediction System model; UKMET=United Kingdom Met Office model; ECMWF=European Center for Medium Range Weather Forecasting model; TVCA=one of the consensus models that lends together several of the above models; CMC=Canadian Meteorological Center (GEM) model; BAMM=Beta Advection Model (Medium depth.) Image credit: National Hurricane Center 2011 verification report.

I'll have a new post in the morning.

Jeff Masters

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Quoting cntrclckwiseSpenn:


it will definitely develop in the GOM. Just if its a stronger storm crossing, i think it will take longer to redevelop if it was knocked down to a weak storm, than a weak storm that crosses and then has a broad circulation still intact allowing it to regenerate more quickly then one the has its circulation displaced.


if it never develops down in the carib it will never make it to the gulf, it will run into yucatan
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Quoting uncwhurricane85:
exactly if it ever develops!


it will definitely develop in the GOM. Just if its a stronger storm crossing, i think it will take longer to redevelop if it was knocked down to a weak storm, than a weak storm that crosses and then has a broad circulation still intact allowing it to regenerate more quickly then one the has its circulation displaced.
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Was just watching TWC and they still have the center at 16N.
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Quoting emguy:
This time last night...I posted the following forecast thinking. The red lines were "my cone of thinking". I also had an orange line, and expresssed on the blog that this was an area that was "out of bounds" for my forecast cone, and not part of my forecast, but there to denote some respect for the Euro.

Last Night:



With tonight's likely relocation of the center, everything in that previous map may now be out of bounds...These are the adjustments...Florida is still the heart of it...but the look is different. The map is based on everything I can consider this evening (which is a lot of info):



Anything on intensity emguy?
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Quoting cntrclckwiseSpenn:


depends where it crosses and how strong it is when it crosses. I actually tend to believe that weaker tropical storms and cat one hurricanes have an easier time crossing the islands and reorganizing, but that's just my to cents.
exactly if it ever develops!
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Quoting Drakoen:
From the 00z models, the NHC continues to have a good track with the model clustering tightening.


Agreed.
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Quoting uncwhurricane85:
yall are talking about a stronger storm crossing cuba, which i understand, but really? the circulation has gotten much weaker today, although thunderstorm activity is up, the storm still is so poorly developed. its just to large and moving to fast.


depends where it crosses and how strong it is when it crosses. I actually tend to believe that weaker tropical storms and cat one hurricanes have an easier time crossing the islands and reorganizing, but that's just my to cents.
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Quoting louisianaboy444:


Lol maybe so I realized that everyone will see what they want to see anyway ;p


I see what the models see. :P
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Quoting KoritheMan:


It will.


Lol maybe so I realized that everyone will see what they want to see anyway ;p
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Quoting emguy:


Excellent question. A storm rolling over the mountains of Hispanola and Cuba would be weaker and experience a more westerly component due to that weakness plus the geagraphical dynamics.

If Isaac stays south of all these mountains and move toward central Cuba as it may be looking to do...The system becomes much stronger, and more vertically stacked, feeling a stronger pull into the weakness. The track I show tonight demonstrates a non dramatic, yet gradual bend into that weakness due to the large overall size of the cyclone.


but I think you can tell the high to the north of it is fairly strong with the dry air intrusion which has allowed to stay weaker than forecasted. Remember, it was supposed to be a cat two by this time and recurving up the coast. could be wrong, but I think it will remain more westward until it starts hitting the really warm waters of off cuba, and then become more poleward as it gets stronger. the nhc had it at a cat one by the time it makes landfall with haiti and it might even graze that as just a tropical storm.
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yall are talking about a stronger storm crossing cuba, which i understand, but really? the circulation has gotten much weaker today, although thunderstorm activity is up, the storm still is so poorly developed. its just to large and moving to fast.
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Quoting louisianaboy444:


If it were up to you it would be at 16N right now on a crash course into the DR...its at approximately 14.7 heading a 260 degrees..it better start climbing


It will.
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3139. emguy
Quoting HurricaneHunterJoe:


West Coaster emguy? Anny thoughts on intensification once Isaac comes off Cuba?


Boy oh boy Joe, that is a tough call...but the waters around Jamaica and south of Central Cuba are hot enough to intensify a Hurricane at a good clip.
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3138. Drakoen
From the 00z models, the NHC continues to have a good track with the model clustering tightening.
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3137. emguy
Quoting KoritheMan:


I do have one question: I see some people saying that the center relocation will actually denote a greater poleward component of motion at longer ranges. Why is that? To my mind it would be the opposite. For the record I'm nowhere near Mobile or points further west, but I'm not buying a Charley-like recurve into the western peninsula. Seems climatologically unlikely.


Excellent question. A storm rolling over the mountains of Hispanola and Cuba would be weaker and experience a more westerly component due to that weakness plus the geagraphical dynamics.

If Isaac stays south of all these mountains and move toward central Cuba as it may be looking to do...The system becomes much stronger, and more vertically stacked, feeling a stronger pull into the weakness. The track I show tonight demonstrates a non dramatic, yet gradual bend into that weakness due to the large overall size of the cyclone.
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Out of ideas....this'll be my last....SEEEEEEYAA!!!

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3135. wn1995


my latest thinking with Isaac.

More westward once again with a cat 2 hurricane making landfall in the fl panhandle in 7 -8 days.

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Good morning everyone. finally got off work. does anyone think this may happen?
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also stronger storms have the dynamic effect of wanting to turn right quicker than weaker slower storms, it's sort of akin to the motion of a spinning top.
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HH aircraft NOAA-42 "Kermit" has just taken off from Barbados. Heading to Issac now from the south-east.
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Quoting KoritheMan:


That's because significant surprises rarely happen. lol


If it were up to you it would be at 16N right now on a crash course into the DR...its at approximately 14.7 heading a 260 degrees..it better start climbing
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Quoting Dunkman:
Euro hits Pensacola at 168h. It certainly seems that the Euro and GFS are coming together.



How many miles has the euro come back to the east?
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When are the next Euro runs.. What time? Tia
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Quoting KoritheMan:


You seeing a more poleward shift in the short-term, long-term, or both? I personally think the center relocation will herald a more southward initial track, but I agree with a stronger storm moving more northward after that.


yes, I think the center relocation is temporary, but has big implications in terms of the crossing (Which is a worst case scenario for the GOM side). You also must take into consideration the timing of the next trough coming down, and any weakness an intensifying storm may have with 96 L and the trough eroding part of Bermuda high. I just don't see it going as far west as the AL, but I didn't see a recurve either. Things can definitely change in a short time, but I see this being the "rare" west coast of FL storm.
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And to add to my last statement...Yes we will probably have a general area zoned out but with a Gulf storm even the most suttle track changes can have big effects as to what area gets the storm with LA/MS/AL/FL panhandle all being in pretty much close proximity...
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Quoting louisianaboy444:


Lol slow your roll...if the Euro shows the panhandle again tomorrow along with the GFS then i'd feel "a little more confident" but no way is this track "nailed down" if it were that easy the NHC wouldnt need experts...we still have had a center relocation to the south which can still affect tomorrow's runs..and with the storm being this disorganized the NHC's intensity scheme has not exactly panned out so far...My thinking is similar to yours but I am not prepared to use the strong confident language that you are. Lots of surprises still to come which I know you are not a fan of


That's because significant surprises rarely happen. lol
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Still going west and still south of forecast points.

The models all have it tracking WNW from this point. Somehow, I don't think that's going to happen.
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Quoting KoritheMan:


After I messed up terribly with Ernesto, yes. :P


Lol slow your roll...if the Euro shows the panhandle again tomorrow along with the GFS then i'd feel "a little more confident" but no way is this track "nailed down" if it were that easy the NHC wouldnt need experts...we still have had a center relocation to the south which can still affect tomorrow's runs..and with the storm being this disorganized the NHC's intensity scheme has not exactly panned out so far...My thinking is similar to yours but I am not prepared to use the strong confident language that you are. Lots of surprises still to come which I know you are not a fan of
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Quoting emguy:
This time last night...I posted the following forecast thinking. The red lines were "my cone of thinking". I also had an orange line, and expresssed on the blog that this was an area that was "out of bounds" for my forecast cone, and not part of my forecast, but there to denote some respect for the Euro.

Last Night:



With tonight's likely relocation of the center, everything in that previous map may now be out of bounds...These are the adjustments...Florida is still the heart of it...but the look is different. The map is based on everything I can consider this evening (which is a lot of info):



I do have one question: I see some people saying that the center relocation will actually denote a greater poleward component of motion at longer ranges. Why is that? To my mind it would be the opposite. For the record I'm nowhere near Mobile or points further west, but I'm not buying a Charley-like recurve into the western peninsula. Seems climatologically unlikely.
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3122. STSUCKS
Quoting dfwstormwatch:
Link

My forecast for Issac


seems reasonable but it looks like you have the storm starting wnw right away..the storm is still moving west. at least in the first 12 hrs it should be a more westerly track
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3121. emguy
This time last night...I posted the following forecast thinking. The red lines were "my cone of thinking". I also had an orange line, and expresssed on the blog that this was an area that was "out of bounds" for my forecast cone, and not part of my forecast, but there to denote some respect for the Euro.

Last Night:



With tonight's likely relocation of the center, everything in that previous map may now be out of bounds...These are the adjustments...Florida is still the heart of it...but the look is different. The map is based on everything I can consider this evening (which is a lot of info):

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Quoting AllStar17:
Take these models with a grain of salt. They aren't initialized correctly.
I agree.
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Quoting louisianaboy444:


LOl you just determined to nail a 6-7 day forecast arent you


After I messed up terribly with Ernesto, yes. :P
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Quoting dfwstormwatch:

Issac at landfall euro


Looks like Mobile.
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Quoting cntrclckwiseSpenn:


not a good trend for the west coast of FL area, as the tracks usually tend to trend more east the stronger the storm gets, the more propensity it has to want to turn northeast/east.


You seeing a more poleward shift in the short-term, long-term, or both? I personally think the center relocation will herald a more southward initial track, but I agree with a stronger storm moving more northward after that.
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Quoting KoritheMan:


All I needed was a consensus. I'm a bit more confident now that this hits somewhere along the Florida panhandle. If the Euro continues this shift at 12z, and especially 0z after the G-IV mission, then I think the final destination has been pretty much nailed down.


LOl you just determined to nail a 6-7 day forecast arent you
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Quoting HurricaneHunterJoe:


West Coaster emguy? Anny thoughts on intensification once Isaac comes off Cuba?


definitely intensify, unless it gets caught up over the mountains and eastern cuba. but as were seeing tonight, and as another poster alluded, it would have to gain pretty good lat. to hit the mountains.
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But you are right in the sense that the overall trend has been eastward with the Euro but I would love to see the Euro strike the Panhandle again at the 12Z to rule out run to run changes
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Quoting emguy:
The only issue I may have had with the last run of the GFS is in the very end of run. By then, I would expect that Isaac will be moving in more of a due north direction. In fact, I would go as far as to say we probably won't see this go any farther west than Apalachicola, FL.

Other than that, really not much change in my thinking tonight from last night and I think the official track is a very good one.

I do agree with Levi that there may be some influence from land over Hispanola/Eastern Cuba with regards to track. This is something that has occurred many more times than just David in 1979. It all has to do with wind dynamics around the circulation of a storm and geography.

Being that Isaac has a large envelope, it is hard to argue with history so this has to be a valid forecasting consideration. On that note, the forecast and the models are looking pretty good and I'm not expecting there will be much change.


West Coaster emguy? Anny thoughts on intensification once Isaac comes off Cuba?
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Quoting KoritheMan:


The Euro is now near the western panhandle, while the GFS remains aggressive on an eastern panhandle solution. Blending that leads to a definitive eastward shift.


not a good trend for the west coast of FL area, as the tracks usually tend to trend more east the stronger the storm gets, the more propensity it has to want to turn northeast/east.
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Quoting louisianaboy444:


NO It dont the Euro shifted a tad east and the GFS shifted a tad west and yeah you are right they are a bit closer now but both models have had suttle run to run changes


All I needed was a consensus. I'm a bit more confident now that this hits somewhere along the Florida panhandle. If the Euro continues this shift at 12z, and especially 0z after the G-IV mission, then I think the final destination has been pretty much nailed down.
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RECON did not find anythink N oving back WSW-SW
Member Since: June 13, 2009 Posts: 2 Comments: 11704
Quoting KoritheMan:


The Euro is now near the western panhandle, while the GFS remains aggressive on an eastern panhandle solution. Blending that leads to a definitive eastward shift.


NO It dont the Euro shifted a tad east and the GFS shifted a tad west and yeah you are right they are a bit closer now but both models have had suttle run to run changes
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Quoting louisianaboy444:


80 miles can be significant when you're talking the "eye of the storm"
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Quoting louisianaboy444:


The Euro is now near the western panhandle, while the GFS remains aggressive on an eastern panhandle solution. Blending that leads to a definitive eastward shift.
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Quoting KoritheMan:
I wouldn't expect a significant shift from the NHC in either direction, although they will probably be a little more confident with the shift in the Euro.
I really wouldn't call the Euro a "Shift" I mean the distance from LA/MS border to AL/FL border is like what 80 miles? More like run to run tweaks similar to the GFS..the overall trend is further west with the Euro compared to the others...next run could easily shift back 80 miles westward
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Issac at landfall euro
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I have Dr. Masters blog and a Linux Mint Oracle VM install of Vista to keep me company. I'm resting easier as these models are all continuing to trend toward the west I'll tell you what.
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Quoting Kowaliga:


Yes I am joking.......I'm also on my 3rd (4th?) glass of Wild Turkey 101 :-D :-D :-D

LOL ok
Member Since: June 13, 2009 Posts: 2 Comments: 11704
Of course all the ensemble members showing it further south go where? You guessed it..Northern Gulf coast...tomorrow will be interesting indeed
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Quoting dfwstormwatch:
Link

My forecast for Issac


Seaside, FL....the rich folks'll luv ta for that!!!
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.