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 Levi32:


You just quoted one of the reasons. The upper pattern just doesn't look quite right to me, which is why I think a slight nudge to the right will be necessary in the end.
Will Joyce situated to the NE of Isaac @ 96 hrs have any influence on tracks of either storm ?
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Uh-oh. So after the original southern Florida landfall, it's headed towards the panhandle.

129 hours:

Member Since: September 2, 2009 Posts: 130 Comments: 21194
2547. Patrap
Foul Language is a explicit Violation of the Terms of service
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2546. ch2os
Quoting Levi32:


Florida scrape and Georgia full landfall?


I can verify that Ft. Pierce, FL got a direct hit from Hurricane David. My husband and I went out during the lull of the eye to inform our parents of the exciting news that I was pregnant. Of course we got chewed out from everyone for being so dumb to venture out during the eye.
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2545. KEEPEROFTHEGATE (Mod)
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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.
What are the odd's now? 50% me thinks.
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2543. KEEPEROFTHEGATE (Mod)
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2542. scott39
Quoting KEEPEROFTHEGATE:
I dont see any difference
Member Since: June 13, 2009 Posts: 0 Comments: 6857
2541. 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.
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2540. Grothar
Quoting will40:



yup at 60hrs


They are trying to load. They are having problems with their server.
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2539. will40
2512. StormJunkie


ty SJ
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Quoting chevycanes:

that was the p3 returning to barbados, not the c130 hurricane hunter.

the c130 should have just taken off.


Ahhhh. Crap. I guess it would help if I read. Lol. Thanks.
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Not too different then what GFS has been showing, West-FL coast brush or near impact.
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Quoting KEEPEROFTHEGATE:


Basically will mirror the 18Z...still think it initialized this system too far north to begin with but that is just my opinion...I am not biased because I find the Euro did the same exact thing....would like to see them get a definite center fix
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2535. snowboy
there is no sign of Isaac moving toward Hispaniola - it's looking like he'll pass by to the south..
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This is like sitting around waiting for a train wreck.. when will it happen, where will it happen... But one thing is for sure... it WILL happen
Member Since: September 18, 2007 Posts: 0 Comments: 1074
Ridge of high pressure over TX should move to the northeast in a few days which may slow Isaac down in the gulf of Mexico, depending on where the high goes after that, determines the forecast track. If the high remains over the east, then this will probably head to the western gulf. If the high does not remain over the east, then the FL panhandle is at risk. Right now EVERYONE along the gulf coast should watch Isaac.
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2532. Grothar
Quoting 954FtLCane:


I've been having that prob for a few days now with Huffman's site.


Just hit CTRL & F5 button to release the old information
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2531. KEEPEROFTHEGATE (Mod)
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2530. scott39
Quoting scott39:
<>em
Member Since: June 13, 2009 Posts: 0 Comments: 6857
2529. KEEPEROFTHEGATE (Mod)
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Quoting galvestonhurricane:


smh
?
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Quoting GetReal:
Please note the location of the 12 hour GFS forecat position.







Isaac better slam on the brakes because he is already approaching where he is forecated to be 12 hours from now. If this run is already off with the position of Isaac, how far off is it further west into this hypothetical run?


This storm thinks it is in the Pacific Ocean!
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2525. KEEPEROFTHEGATE (Mod)
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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.



Probably because there is a trough coming down from the north eroding that ridge.
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Quoting MississippiWx:


Google Earth isn't updating for some reason. I think it has something to do with them leaving earlier than scheduled. Look at the actual live decoder.

that was the p3 returning to barbados, not the c130 hurricane hunter.

the c130 should have just taken off.
Member Since: August 6, 2007 Posts: 0 Comments: 693
Quoting Joshsen18:
my wife and I are traveling to cancun Mexico area for our honeymoon......are we safe? She is soooo worried figured maybe someone can give us advice? Really don't want to cancell but I want to be safe?....?


No offense, but please stop posting this question...we all see it. We've been seeing it each time you post it, and it's been answered several times since yesterday. You do know how this blog works, right?
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2521. Levi32
Quoting chrisdscane:



quick question before i go to bed why is ur cone east of models TY


You just quoted one of the reasons. The upper pattern just doesn't look quite right to me, which is why I think a slight nudge to the right will be necessary in the end.
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2520. scott39
Anyone who disregards West trends of the models and the EURO has some balls.
Member Since: June 13, 2009 Posts: 0 Comments: 6857
2519. 7544
right over south fl again peeeps ;lol
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Monday, August 27, 2012 at 8a.m EDT:

Member Since: September 2, 2009 Posts: 130 Comments: 21194
2517. angiest
Quoting OracleDeAtlantis:
More like Price Line ...


At least someone got it.
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2515. KEEPEROFTHEGATE (Mod)
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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.

Levi too be honest with you, anything is a wild card right now. the best thing to do is wait and see what the storm does because lots of things dont make sense yet they are showing up.
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Quoting will40:



yup at 60hrs


I've been having that prob for a few days now with Huffman's site.
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Quoting will40:



yup at 60hrs


Still good here.
Member Since: August 17, 2005 Posts: 26 Comments: 16823
Quoting drg0dOwnCountry:
Looks annular.


smh
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2510. KEEPEROFTHEGATE (Mod)
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2509. duttlyn
Quoting CosmicEvents:
I have advice to your wife-to-be after seeing this post 100 times. Call it off and run!!!


That's just mean. Maybe he didn't get a sufficient answer or maybe he is REALLY nervous about this really big event going on in his life! Give the guy a break... if it was MY honeymoon/wedding I'd be in full panic mode, too.
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Quoting TomballTXPride:
2494:

Track wreck of a system. Just call it an open wave and be done with it.


Yeah....that's a good idea. I'm glad you aren't at the NHC.
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Alright Levi, looking at the UW-CIMSS steering layers now, and the ridge does appear to have weakened somewhat. Not drastically, but enough for me to notice.
Member Since: March 7, 2007 Posts: 582 Comments: 20771
2506. KEEPEROFTHEGATE (Mod)
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2505. will40
Quoting TomballTXPride:
GFS Quit running on Huffman's site...

Link



yup at 60hrs
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Quoting AllStar17:


It appears they are returning to Barbados now, though. When's the next one?


Google Earth isn't updating for some reason. I think it has something to do with them leaving earlier than scheduled. Look at the actual live decoder.
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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.

Could Joyce have something to do with it, such as blocking it from going more to the NNW or North?
Member Since: August 31, 2009 Posts: 0 Comments: 5628
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.




quick question before i go to bed why is ur cone east of models TY
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2501. angiest
WFO Houston had this to say earlier today:

THE EXTENDED FORECAST IS STILL RATHER MUDDLED. OVERALL MODELS
SHOW UPPER LEVEL RIDGE BUILDING OVER TX SUN/MON. THERE STILL MAY
BE ENOUGH MOISTURE TO SUPPORT AN ISO STORM OR TWO WITH THE SEA
BREEZE SO WILL AT LEAST KEEP THE TREND OF 20 POPS IN THE FORECAST.
MODELS ARE HAVING A TOUGH TIME WITH THE EVOLUTION OF THE RIDGE
OVER THE ROCKIES AND SUB-TROPICAL RIDGE IN THE ATLANTIC WHICH
WILL PRIMARILY PROVIDE STEERING FOR TS ISAAC. GFS LOOKS WEAKER
WITH BOTH RIDGES WHILE THE ECMWF IS STRONGER WITH EACH RIDGE THUS
HAVING A MORE WESTWARD TRACK FOR ISAAC. STILL WAY TOO FAR OUT IN
FORECAST TIME TO DETERMINE THE IMPACTS FROM ISAAC IF ANY BUT
CERTAINLY SOMETHING TO KEEP TRACK OF IN THE COMING DAYS. THINK
THAT BY THE WEEKEND MODELS WILL HAVE A BETTER HANDLE ON THE
EVOLUTION OF EACH RIDGE AND TRACK OF ISAAC.
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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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