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 KoritheMan:


The truth lies somewhere in between.

Seriously, I don't know anymore.


I am starting to think that this may go south of the Greater Antilles if current trends continue
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I hope this storm stays away from the panhandle of FL.. Ive been through these storms 3 times with hurricanes and 5 tropical storms.. I know what they can do and and sometimes the end result isn't too nice.. I wish this on nobody and isn't fun at all, especially when lives are at stake if your caught in the middle of these storms that last for awhile.. If this storm or any other storm comes near you, take this seriously and get to a safest place for you and your family..
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Quoting KoritheMan:


The truth lies somewhere in between.

Seriously, I don't know anymore.
but it looks like the gap is starting to close though where the spread just a few days ago had anywhere from the OBX to Texas. Looks like it's now anywhere from South FL. to the Panhandle of FL. of course not ruling out any other areas, but just going by what the models are showing now.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
Night all, can't stay up for this time consuming HH flight. Will check it out in the AM when we have rapid scan close up Vis imagery.
Member Since: August 17, 2005 Posts: 26 Comments: 16874
Quoting GTcooliebai:
So the question is does it hug the south coast of Cuba, or go over Haiti and hug the north coast of Cuba like the GFS?
@ 96 hrs the Euro goes over the center of Cuba in a NW direction
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Quoting KoritheMan:


If anything I think it will just shift the first two or so days south, but the track should more or less blend toward the panhandle after that.


We will see the first model runs with the new center initialized should be interesting...I think the panhandle looks okay for now but I think anyone from Grand Isle, La to the west coast of Florida should pay attention...if it goes far enough south and stays weak it could sneak further westward...we will see
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Quoting cirrocumulus:
Outlier, but overall best model for predicting paths of storms.


Yeah but so far with TS Isaac folks have called it the outlier. I think it may be a little closer to what is happening though. I'm no expert and I don't know anything about forecasting. But I think this may be MS,AL,FL panhandle lanfall. Just call it a gut feeling.
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Quoting GTcooliebai:
So the question is does it hug the south coast of Cuba, or go over Haiti and hug the north coast of Cuba like the GFS?


The truth lies somewhere in between.

Seriously, I don't know anymore.
Member Since: March 7, 2007 Posts: 602 Comments: 21231
I hope this doesn't freak anyone out.

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Quoting KoritheMan:


Much more reasonable track, IMO.
So the question is does it hug the south coast of Cuba, or go over Haiti and hug the north coast of Cuba like the GFS?
Member Since: August 31, 2009 Posts: 0 Comments: 5628
Quoting wunderkidcayman:

I think that line that showed it passing lands of Haiti and Cuba could now be S of these land masses which could mean big trouble for Jamaica


I think we could see a watches up for Jamaica soon if it continues this track and possible strengthen more b4 it gets closer..
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Quoting louisianaboy444:


Seems reasonable the consensus looks to be on the Florida Panhandle right now...let's see what the center reformation does to the models...it shouldn't change them too much though


If anything I think it will just shift the first two or so days south, but the track should more or less blend toward the panhandle after that.
Member Since: March 7, 2007 Posts: 602 Comments: 21231
Quoting KoritheMan:


It's showing a landfall into the western Florida panhandle. At least, that's what I'm getting from extrapolating the run before it's finished.


Seems reasonable the consensus looks to be on the Florida Panhandle right now...let's see what the center reformation does to the models...it shouldn't change them too much though
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This is definitely a timing track the trough is actually hung back some on the latest Euro but is right of the 18z run which had the trough advancing more quickly.
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Quoting scott39:
Anyone have a link to the EURO?
Link
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Quoting louisianaboy444:


Those links dont work for me


It's showing a landfall into the western Florida panhandle. At least, that's what I'm getting from extrapolating the run before it's finished.
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Quoting FLPandhandleJG:
man if this storm gets its act together.. and thats where the center is at right now.. wouldn't u think Jamaica might be in the path if keeps going west.. B/c we notice it wobble wsw for a bit even though it does make a slight wnw turn it would still get some of Jamaica right or ?

I think that line that showed it passing lands of Haiti and Cuba could now be S of these land masses which could mean big trouble for Jamaica
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Quoting KoritheMan:


Much more reasonable track, IMO.


Those links dont work for me
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Quoting GTcooliebai:
Not quite it has shifted back east some on the latest run.

Link


Much more reasonable track, IMO.
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Take these models with a grain of salt. They aren't initialized correctly.
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2981. scott39
Anyone have a link to the EURO?
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Quoting HimacaneBrees:
Maybe the ECMWF is not so much of the "outlier" as it seems?
Outlier, but overall best model for predicting paths of storms.
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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.


At least you can be certain that the interaction with the mountains will not cause it to do a 180 and start heading back east!
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Isaac is a bust rain wise!!!!!!!!!!!!!!!!!!!!!!!!!!! WTF!
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Quoting HimacaneBrees:
Maybe the ECMWF is not so much of the "outlier" as it seems?
Not quite it has shifted back east some on the latest run.

Link
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Quoting KoritheMan:
The rather high central pressure reading in the new center could be because it is displaced from the deepest convection. Or it could simply elongate again.


This shows you the mess that the models have to latch on to...this track is far from certain
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Quoting stormchaser19:


GFS predicted this in 00z run,and now with this relocation COC the system will be polarized more and now will shift a little bit to the east.....what you think levi?.........And yes i´m guessing base of 00z gfs run


Link

850 mb vorticity GFS
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Quoting wunderkidcayman:
I hope this data get put into the 06Z models


No joke!
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Euro shifting to the right @96hrs. going over central Cuba
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well now I guess it make the 00Z CLIP CLP5 and LBAR look more realistic or more likely outcome than it was before before maybe
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The rather high central pressure reading in the new center could be because it is displaced from the deepest convection. Or it could simply elongate again.
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Quoting Levi32:
Ah. Bingo. 14.75N.



GFS predicted this in 00z run,and now with this relocation COC the system will be polarized more and now will shift a little bit to the east.....what you think levi?.........And yes i´m guessing base of 00z gfs run
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2969. scott39
This is not good for the GOM, or it could pull another Ernesto!
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Maybe the ECMWF is not so much of the "outlier" as it seems?
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Ok, I'm tired of trying to wrap my head around this system for the night, time for some sleep. Besides, I need to go to bed before I get kept as the 06Zs start running. :P
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Quoting KoritheMan:
So Cantore was right, then?


Yes
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2965. emguy
I would like to see the next recon pass...but shortwave generally agrees with the location of the first pass. 14.7, 63.5 may be a good bet...at least at the mid levels.

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man if this storm gets its act together.. and thats where the center is at right now.. wouldn't u think Jamaica might be in the path if keeps going west.. B/c we notice it wobble wsw for a bit even though it does make a slight wnw turn it would still get some of Jamaica right or ?
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So Cantore was right, then?
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Quoting AllStar17:
If the 14.7 N 63.5 W center is indeed the center, it would seem more likely that Isaac would miss Hispaniola just to the south.


Getting in line with my thinking
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I hope this data get put into the 06Z models
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Quoting AllStar17:
Center: 14.7 N 63.5 W

Should change the models a bit.
Probably a lot, could be a whole nother ball game.
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Is that a weakness over florida?
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Quoting StormJunkie:
With the 14.7 fix, I'm thinking this convection got pulled in towards the center. Just didn't want to get left out the action.



Yeah...that was one fishy indication earlier.
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2956. Gearsts
Expect more annoying centers north of there.
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With the 14.7 fix, I'm thinking this convection got pulled in towards the center. Just didn't want to get left out the action.

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If the 14.7 N 63.5 W center is indeed the center, it would seem more likely that Isaac would miss Hispaniola just to the south.
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well looks like I was right S of 15N but N of 14.5N
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Like the other guy, I want them to scout the area directly west of Guadeloupe/Dominica.


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Quoting Levi32:
Hey though, pressure was rising for a long while on approach to the wind center. That means the pressure center is farther north and east, so this "center" is not a well-defined anchor for the system yet. Look for fishy stuff northeast of that position.

I'm out for a while, possibly for the night. Later.

Have a good night Levi, thanks for staying up with us.
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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.