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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so far RECON says LLCOC is further SW than the 11pm advisory location
Member Since: June 13, 2009 Posts: 2 Comments: 10902
Recon is at 15.7N and they still haven't reached the center. The center position at 11pm was at 15.8N.
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Quoting serialteg:


and once again reality proves itself


Not that scary.
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2848. Skyepony (Mod)
OFCL hit the lid with Isaac leading after it's first 24hr in the models race. Average model error for Isacc.
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Quoting RussianWinter:
Am I seeing 2 vortexes or what?
Yes.
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Time: 05:32:00Z
Coordinates: 15.7667N 64.1833W
Acft. Static Air Press: 843.3 mb (~ 24.90 inHg)
Acft. Geopotential Hgt: 1,540 meters (~ 5,052 feet)
Extrap. Sfc. Press: 1004.9 mb (~ 29.67 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 62° at 18 knots (From the ENE at ~ 20.7 mph)
Air Temp: 20.0°C (~ 68.0°F)
Dew Pt: 14.7°C (~ 58.5°F)
Peak (10s) Flt. Lvl. Wind: 19 knots (~ 21.8 mph)
SFMR Peak (10s) Sfc. Wind: 9 knots (~ 10.3 mph)
SFMR Rain Rate: 4 mm/hr (~ 0.16 in/hr)
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Quoting GTcooliebai:
That's a scary realization.


and once again reality proves itself
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Center is most definitely south of where the NHC has it.
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Living on the coast of jersey my whole life ive never really seen or remember a bad hurricane here..irene was nothing in atlantic city...i have a 7am flight to orlando in the morning like 5 hours from now im staying till tuesday night red eye flight back home...as a hurricane junkie like i am always been wanting to chase one..lets just say the chase is on..my cuz gets married friday, partying into saturday and then the im gonna hit the road for all day monday and possibly into tuesday..im bringing my computer down so if it gets crazy ill try to post here...

Also..im leaving tuesday night getting into jerz around midnight..so if it follows the GFS track i will be able to get the full blow from it in FL then beat it to NJ and get it again here...crazy senerio could happen..ofcorse i dont want damage and deaths and what as i will be there and cuz getting married will be..but hell with my plans of hitting south beach and partying my @#$ off ill chase a hurricane haha
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Quoting GTcooliebai:
The 00z GFS was west of the 18z GFS in the longer ranger with a landfall more towards Apalachicola as opposed to the Big Bend.
Yes. And the odds don't change on coast of Georgia landfalls or those landfalls east of Apalachicola but north of Tampa. So, Panama City or west is more likely.
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I posted this up earlier...but its still my current idea for the track
the white shade inside the cone is my best bet...

add or take 5 mph from the wind intensities shown...

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With the stronger convection getting sucked into the MLC further south its only a matter of time before that one becomes dominant IMO....we will have to see what recon finds
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Quoting MississippiWx:
Recon is still finding weak winds on the northern side.

Recon is still in it? Whoa. Hope they were far away from that beyond-extreme convection burst.
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2838. Grothar
Quoting Bielle:


Where does one store another 500 gallons of water? Just curious . . .


Usually in a 500 gallon tank, Unless they have a very big bathtub.
Member Since: July 17, 2009 Posts: 69 Comments: 25291
2837. Grothar
Quoting HoustonTxGal:


That is Lynn Anderson not Loretta Lynn :o)


Who do you think did it first? I was around when she first sang it. I know my country music.
Member Since: July 17, 2009 Posts: 69 Comments: 25291
Am I seeing 2 vortexes or what?
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Recon is still finding weak winds on the northern side.
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Quoting scott39:
I think its putting on a pretty show tonight, and will show its true colors in the daytime.


Yeah I have a feeling that this storm is going to keep us guessing.. It just looks like it would do a rapid intensification pretty soon b/c only on here and the way its going.. The water is getting warmer and warmer.. Thats no Bueno!!!
Member Since: August 15, 2008 Posts: 0 Comments: 2133
2833. emguy
A track that has Isaac moving along the west coast of Florida, gradually bending north along the way, is a track I would favor.

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Quoting Levi32:


I still think a slight eastward adjustment is going to be necessary. 96-120 hours is a very long time. Rarely do the models actually have it perfectly nailed that far out. Usually shifts still occur until the final 48 hours or less. Lots of wiggle room left with this one. Until it crosses the mountains of the greater Antilles we won't know anything for certain either, because those mountains are notorious for doing unpredictable things to these types of storms. The track could potentially change on a dime at that point, with almost no way to predict it in advance.


True. The mountains tend to tear up some storms, don't they? Especially ones that aren't pulled together too well anyway.

Well, it gave me great excuse to take down a dead Black Jack Oak today, so I'm good.
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Quoting Levi32:


I still think a slight eastward adjustment is going to be necessary. 96-120 hours is a very long time. Rarely do the models actually have it perfectly nailed that far out. Usually shifts still occur until the final 48 hours or less. Lots of wiggle room left with this one. Until it crosses the mountains of the greater Antilles we won't know anything for certain either, because those mountains are notorious for doing unpredictable things to these types of storms. The track could potentially change on a dime at that point, with almost no way to predict it in advance.
Except the ECMWF is farther west and that is the top model.
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Time: 05:22:00Z
Coordinates: 16.2N 64.5333W
Acft. Static Air Press: 843.3 mb (~ 24.90 inHg)
Acft. Geopotential Hgt: 1,549 meters (~ 5,082 feet)
Extrap. Sfc. Press: 1006.5 mb (~ 29.72 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 62° at 19 knots (From the ENE at ~ 21.8 mph)
Air Temp: 18.5°C (~ 65.3°F)
Dew Pt: 17.3°C (~ 63.1°F)
Peak (10s) Flt. Lvl. Wind: 19 knots (~ 21.8 mph)
SFMR Peak (10s) Sfc. Wind: 5 knots (~ 5.8 mph)
SFMR Rain Rate: 4 mm/hr (~ 0.16 in/hr)
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2829. Bielle
Quoting Grothar:


Yeah, I've notice that, too! I guess I may have to get another 500 gallons of water.


Where does one store another 500 gallons of water? Just curious . . .
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Quoting louisianaboy444:
I'm betting they find the center further south along 15N


I think you maybe right hey maybe they may find it S of 15N but N of 14.5N but yeah I think its at 15N
Member Since: June 13, 2009 Posts: 2 Comments: 10902
Quoting CaicosRetiredSailor:

Well, that's better than nothing! What is that, about 15.4*N or so?
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Quoting Levi32:


I still think a slight eastward adjustment is going to be necessary. 96-120 hours is a very long time. Rarely do the models actually have it perfectly nailed that far out. Usually shifts still occur until the final 48 hours or less. Lots of wiggle room left with this one. Until it crosses the mountains of the greater Antilles we won't know anything for certain either, because those mountains are notorious for doing unpredictable things to these types of storms. The track could potentially change on a dime at that point, with almost no way to predict it in advance.
That's a scary realization.
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Quoting scott39:
I cant remember when the Eastern Caribbean was a good area for a TC to develope.
The ECMWF seems to take that into account!
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Quoting Grothar:


We don't do Loretta Lynn at this hour.


That is Lynn Anderson not Loretta Lynn :o)
Member Since: September 18, 2007 Posts: 0 Comments: 1073
Quoting KEEPEROFTHEGATE:


i just dont buy that trajectory no more... doesnt make sense it's been going south of the cone for a while now
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2822. scott39
Quoting Grothar:


Just don't do a Prince Harry.
Lol
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2821. Levi32
Quoting weatherwart:


Thank you. I just checked out your blog. You still think Issac is going to track up the east coast?


I still think a slight eastward adjustment is going to be necessary. 96-120 hours is a very long time. Rarely do the models actually have it perfectly nailed that far out. Usually shifts still occur until the final 48 hours or less. Lots of wiggle room left with this one. Until it crosses the mountains of the greater Antilles we won't know anything for certain either, because those mountains are notorious for doing unpredictable things to these types of storms. The track could potentially change on a dime at that point, with almost no way to predict it in advance.
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Quoting FLPandhandleJG:




I think it wants to strengthen .. anyone agree now.. lol


He sure looks better after the alien blob attack, doesn't he?
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2819. scott39
Quoting FLPandhandleJG:




I think it wants to strengthen .. anyone agree now.. lol
I think its putting on a pretty show tonight, and will show its true colors in the daytime.
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Best bet is for this to stay south of the DR but skirt Haiti and then make landfall in Cuba..question remains on Sunday where it makes landfall..eastern side or skirts the southern coastline.
For days 5-6 the next question is.. how strong the trough will be.. if it's not that strong..Issac will skirt just west of the Keys and head towards the central GOM.. if it's stronger.. it will make a landfall in the northern Keys and along the west coast of Florida. Right now, can't make any bets on anything before it hits Cuba Saturday night or Sunday.
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Quoting StormJunkie:




So what do you think caused it...They playing Red Rover, Red Rover?

That was clearly a planned attack. I've never seen anything like it.
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Quoting louisianaboy444:
I'm betting they find the center further south along 15N
Alright in that case I'll take your money, you can drop it off in my Paypal account :P
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03:19 GMT
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2814. Grothar
Quoting scott39:
What if its not money?


Just don't do a Prince Harry.
Member Since: July 17, 2009 Posts: 69 Comments: 25291
Quoting nofailsafe:


That would have been impressive to watch in real time from space. All those clouds falling and rising that quickly.




So what do you think caused it...They playing Red Rover, Red Rover?
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2812. Grothar
Quoting WXGulfBreeze:


The ensemble models as a whole seem to be moving back to the east (W. coast / central Florida) in the latest runs.


Yeah, I've notice that, too! I guess I may have to get another 500 gallons of water.
Member Since: July 17, 2009 Posts: 69 Comments: 25291




I think it wants to strengthen .. anyone agree now.. lol
Member Since: August 15, 2008 Posts: 0 Comments: 2133
Quoting CaicosRetiredSailor:


humph...

Well that's helpful...
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Quoting CaicosRetiredSailor:


humph...
I know, why did QuikSCAT have to die.
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2808. Gearsts
Quoting CaicosRetiredSailor:


humph...
Center more NE?
Member Since: August 2, 2010 Posts: 0 Comments: 1411
I'm betting they find the center further south along 15N
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Isaac sure looks like he is riding much lower than expected, but then agian, I don't know where exactly his center is. It could be in some funky location in there I would never expect.
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2805. scott39
Quoting Grothar:


5 to 1 says you can get in trouble for betting on the blog.
What if its not money?
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2804. KEEPEROFTHEGATE (Mod)
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2803. Grothar
Quoting scott39:
I wouldnt do Loretta lynn at any hour.


Ouch. :)
Member Since: July 17, 2009 Posts: 69 Comments: 25291
2802. Grothar
Quoting scott39:
Ok, Its time to lay our bets on the table. Lol


5 to 1 says you can get in trouble for betting on the blog.
Member Since: July 17, 2009 Posts: 69 Comments: 25291
2801. 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.
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About

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.