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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2601. JLPR2
Quoting ProgressivePulse:


No need to be shy, all satellite data points to that fact.


I seen a few get clobbered tonight for speaking of such blasphemy.
LOL!
Member Since: September 4, 2007 Posts: 7 Comments: 8747
2600. Grothar
Quoting scott39:
Wasnt thier a hurricane that was moving over 20mph, that hit the East Coast years ago?


The hurricane of 1938 (The Long Island Express) was moving at over 60 mph when it hit.
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How Grothar sneaks into the assisted living computer room every night to blog I'll never know. :) May we all live as full a life as you have Grothar. You putting your exploits into a book? If not, please do, Gro.
Member Since: April 18, 2012 Posts: 0 Comments: 2437
2598. will40
Quoting scott39:
Wasnt thier a hurricane that was moving over 20mph, that hit the East Coast years ago?



Hazel accelerated to over 77 km/h (48 mph) upon landfall

hurricane Hazel 1954
Member Since: September 19, 2005 Posts: 2 Comments: 4266
2597. bdm2225
Bonjou from Port-au-Prince. How bad is it for us??
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Thanks Grothar for the tip. Kinda sorta worked but not for all hours. Huffman's 850/300mb shear maps 850 vort and 500 mb pmsl almost all say file not found. Especially the higher hour maps.

No white fly yet btw! hope yours have dissapeared.
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2595. JLPR2
In other news the barometer in my room (Carolina,PR) went down another millibar in the past hour, now down to 1011mb with Isaac still hundred of miles away.
Member Since: September 4, 2007 Posts: 7 Comments: 8747
2594. GetReal
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Still looks like a slight bubble of upper level ridging develops between Isaac and Joyce on the 500mb map from the GFS.

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Quoting JLPR2:


Actually the GFS hinted at this in the 3-9hrs frames.


No need to be shy, all satellite data points to that fact.
Member Since: August 19, 2005 Posts: 5 Comments: 5452
2591. Grothar
Quoting hydrus:
I was wondering if anyone on the blog is having trouble with the blos being squeeed to the point one can barely read it.?


Did you mean blobs or blogs??
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Quoting Levi32:
I'm becoming convinced the GFS is seeing Isaac hanging up on the Cuban mountains due to frictional effects, and that's why it gets it on the west side of Florida. Running the 500mb 0z loop makes no sense to me unless you consider Cuba to be "holding on" to the storm.


Speaking of which, Levi, do you think the GFS accounting for the dry air that Isaac will be pulling off the FL peninsula should that track verify? Seems to me its spinning up to fast in the WGOM.
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Quoting Tribucanes:
Levi, or anyone else, what's stopping Isaac from intensifying quickly before he reaches the mountainous regions of Cuba? Isaac looks to be organizing well and I would not be surprised at all to see Isaac reach hurricane status tomorrow. Please and thank you.
Maybe even earlier H status. Getting really exited now. Earlier someone compared Felix 2007 with Isaac and at the current location begun 2007 one of the fastest RI ever.
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2588. Grothar
Quoting Tribucanes:
Levi, or anyone else, what's stopping Isaac from intensifying quickly before he reaches the mountainous regions of Cuba? Isaac looks to be organizing well and I would not be surprised at all to see Isaac reach hurricane status tomorrow. Please and thank you.


Nothing.

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2587. hydrus
I was wondering if anyone on the blog is having trouble with the blos being squeeed to the point one can barely read it.?
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Quoting JLPR2:
It seems that the LLC could have done a little jump to the south, possibly aligning with the MLC along 14.8n, 64w. IMO Now don't kill me for saying this. XD



We are going to get that big wet tail.
Member Since: April 29, 2009 Posts: 75 Comments: 14571
2572. The answer is Hugo. I lived in Charlotte. It got up on us in a hurry 175 miles inland still at hurricane strength. Made an enormous mess that people still talk about there. I was without power for 3 weeks.
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Quoting StormJunkie:


Banding is much more impressive now than earlier. Seems that LLC and MLC may have finally worked out some of their differences after lots of yelling, lawyers fees, and maybe a couple of affairs...


Nah...just picked up discarded cases of booze. One got the other one drunk then ate him! ;-)
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Just a little eerie that Isaac may make landfall on August 29
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2582. pcola57
Quoting KEEPEROFTHEGATE:


Starting to get my undevided attention Keep..(goes to pull cover off generator and checks cords...again)
And thanks for posting all of them.. :)
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2581. JLPR2
Quoting JLPR2:
It seems that the LLC could have done a little jump to the south, possibly aligning with the MLC along 14.8n, 64w. IMO Now don't kill me for saying this. XD



Actually the GFS hinted at this in the 3-9hrs frames.
Member Since: September 4, 2007 Posts: 7 Comments: 8747
Quoting JLPR2:
It seems that the LLC could have done a little jump to the south, possibly aligning with the MLC along 14.8n, 64w. IMO Now don't kill me for saying this. XD



Not too far fetched. In fact, the deepest convection appears to be migrating southward, if anything. Really not sure. Recon would be nice!
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Well I was expecting to see a further shift to the West with this GFS update, mainly because of the low strength. However it did not change much and the East coast is still in the error track. So I guess that the shutters that need a ladder will start going up in the morning. Nite all.
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GFS still holding onto a trough that will turn this due north and eventually northeast, this track will be about timing.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
2577. GetReal
well this run of GFS is now calling for a lanndfall near Panama City Beach....The trending WEST of the GFS continues slowly as it comes more in line with the EURO.
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Bout how strong is that cane showing at 139
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Quoting KEEPEROFTHEGATE:
HR 144 FINAL



The only difference is a shift further to the west in the gulf...hits around the central Panhandle instead of the big bend area
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Quoting KoritheMan:
0z upper air data:

Jacksonville:



Orlando:



Miami:



Nassau:



The trough is still lingering along the east coast upstream of south Florida, as denoted by the uniform southwesterly flow at Jacksonville. However, the maps for areas downstream aren't too dissimilar to what they were at 12z. If anything, the ridge may have strengthened in those areas.

In short, Isaac is not going poleward in an immediate fashion.

Nice use of the soundings. I always try to use then whenever available. It's a fantastic way to look through the deep layer of the atmosphere that we can't get otherwise.
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Levi, or anyone else, what's stopping Isaac from intensifying quickly before he reaches the mountainous regions of Cuba? Isaac looks to be organizing well and I would not be surprised at all to see Isaac reach hurricane status tomorrow. Please and thank you.
Member Since: April 18, 2012 Posts: 0 Comments: 2437
2572. scott39
Wasnt thier a hurricane that was moving over 20mph, that hit the East Coast years ago?
Member Since: June 13, 2009 Posts: 0 Comments: 6890
2571. JLPR2
It seems that the LLC could have done a little jump to the south, possibly aligning with the MLC along 14.8n, 64w. IMO Now don't kill me for saying this. XD

Member Since: September 4, 2007 Posts: 7 Comments: 8747
Quoting Levi32:
See this. The 700mb high is centered over Houston, and the winds are NNE across the entire north gulf coast. How is a storm on the coast of Cuba supposed to move northwest into that lol. Movement should be more NNW or north here.



And I believe that is one of the main differences between the Euro's previous runs and the GFS -- the position of that high.
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Is it safe to say that Isaac is larger than most tropical storms?
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2568. KEEPEROFTHEGATE (Mod)
HR 144 FINAL

Member Since: July 15, 2006 Posts: 175 Comments: 54858
2567. Grothar
Quoting 954FtLCane:


shoot tried that and it still doesn't work.


They are still loading the information in increments. Patience.
When you go to the bottom for the dynamic model runs, you often have to do that to clear the screen.
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Quoting TomballTXPride:
Tampa Tampa Tampa. And more Tampa Tampa Tampa. The GFS is RELENTLESS with this. Betcha I know who it's voting for come Nov.


It's gonna have to register as a lobbyist, lobbying for additional funding for FEMA and the NHC.
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2565. Levi32
Quoting KoritheMan:
0z upper air data:

Jacksonville:



Orlando:



Miami:



Nassau:



The trough is still lingering along the east coast upstream of south Florida, as denoted by the uniform southwesterly flow at Jacksonville. However, the maps for areas downstream aren't too dissimilar to what they were at 12z. If anything, the ridge may have strengthened in those areas.

In short, Isaac is not going poleward in an immediate fashion.


Today I remembered how wonderful soundings are. I have a whiteboard up on my desk keeping track of the height values at a few locations. Gotta love real data.
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2564. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 175 Comments: 54858
48H GEM regional (15 km) 00Z integration shows pretty flat heights from FL and along the northern gulf coast, a def zone NE-SW thru the Keys (an outer band?) and really nothing blocking a more western track...

Link
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Just got back for the evening. Dang, I don't think I've ever seen convection move west (or any direction) that fast before!

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2560. scott39
Quoting KEEPEROFTHEGATE:
hmm
Member Since: June 13, 2009 Posts: 0 Comments: 6890
2559. Grothar
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2558. gator23
Quoting TomballTXPride:

Probably not Texas. I think we're good. We'll be happy to take in evacuees though!


You dont want Floridians though... Well complain the food isnt as good as it is in Miami and then complain that your bars close at 2:00am
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Quoting Gumbogator:
The cone of the unknown.........boy ain't dat the truth!! Intensity and track is a big guess. It is so typical of a large envelope system to take it's time to get out of the dust bowl E Caribbean**** If Sir Isaac gets wound up and can avoid the mouse traps(landmasses) look out. Cantore shut down Norcross on this battle of Westcaster VS Eastcaster!! Geeez the track is hardly ever down the middle on a slow developer/huge envelope system.


Is it ever. Im left wondering if i'll ever see anything from him. Already stqrting to feel stood up.
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2556. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 175 Comments: 54858
0z upper air data:

Jacksonville:



Orlando:



Miami:



Nassau:



The trough is still lingering along the east coast upstream of south Florida, as denoted by the uniform southwesterly flow at Jacksonville. However, the maps for areas downstream aren't too dissimilar to what they were at 12z. If anything, the ridge may have strengthened in those areas.

In short, Isaac is not going poleward in an immediate fashion.
Member Since: March 7, 2007 Posts: 597 Comments: 21112
2554. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 175 Comments: 54858
2553. Levi32
Quoting lottotexas:
Will Joyce situated to the NE of Isaac @ 96 hrs have any influence on tracks of either storm ?


Looks too weak on the models to be a significant influence on Isaac's recurve to me. If it was a developing hurricane I think it would be a different story.
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Quoting Grothar:


Just hit CTRL & F5 button to release the old information


shoot tried that and it still doesn't work.
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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.

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