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 Tazmanian:
hey look what i found on TD 10


AL, 10, 2012082312, , BEST, 0, 149N, 416W, 35, 1006, TS,
Looks like Joyce, which would put us at 10 named storms for the season, Taz.
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Quoting ProgressivePulse:
Morning All.

Chances increasing for an EC FL strike.


Really? I thought models were trending west.
Member Since: July 11, 2006 Posts: 14 Comments: 11206
Quoting rmbjoe1954:


Anywhere within the cone is a possible target. The consensus has it going wetsward so why do you think east central FL?


I didn't say that is what I think. I said the chances are increasing this morning as a bulk of the models on the map I posted are now heading there, unlike yesterday...
Member Since: August 19, 2005 Posts: 5 Comments: 5043
3648. Brock31
If Isaac were to undego a substantial strengthening, would that pull him north more quickly even with the ridge situated as it is?

I know stronger storms tend to move poleward, I'm just wondering how strong Isaac needs to be to make a more northerly turn or if the ridge is too strong and would prevent such movement.
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Latest ADT for Isaac:
2012AUG23 104500 3.7 992.7 59.0 3.2 3.6 4.0

If you look only at the satellite picture, Isaac is a hurricane LOL. But in contrast to ADT, it's just a minimal tropical storm as of now.
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Joyce?

AL, 10, 2012082312, , BEST, 0, 149N, 416W, 35, 1006, TS, 34, NEQ, 40, 30, 0, 40, 1013, 225, 40, 0, 0, L, 0, , 0, 0, TEN, M,
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hey look what i found on TD 10


AL, 10, 2012082312, , BEST, 0, 149N, 416W, 35, 1006, TS,
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Quoting Clearwater1:
Since the Euro has moved further east, I also notice the 06 gfs also moved a tad further east, in fact, back to it's position up the spine of FL, as in a few days back. I would think, with that in minds and the NHC averaging mostly the gfs and Euro the official cone and track to be nudged to the east as well. Maybe at 11 am.


6z still looks SW FL coastal path to me, but yes it moved slightly east. Only one run. Overall convergence of various respected models finally looks to be taking place in last few runs.
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The panhandle cannot handle anymore water. We still have standing water everywhere from the past two months of raining almost every day.
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Quoting ProgressivePulse:


East Coast Florida Landfall (EC FL)


Anywhere within the cone is a possible target. The consensus has it going westward so why do you think east central FL?
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3641. LargoFl
Quoting ILwthrfan:
Isaac still trying to tighten up this morning I see, although he still looks much improved from last evening. I wouldn't expect any significant strengthening until he organizes his CDO, which still has the looks of two brothers fighting. Been a very interesting storm. I have not yet seen two disturbances combined into one, at least Atlantic speaking, and that is pretty much what happened with Issac.

I have also noticed that Issac is south of guidance, by a rather significant amount. NHC still waiting for the dominant center to finish forming before adjust tracks further I guess. I still think at most Issac scrapes Hispaniola to the western peninsula. He may miss it altogether.

.see the GDFL models isnt budging..right up thru florida..im preparing, regardless..luckily so far we got days to do that
Member Since: August 6, 2011 Posts: 4 Comments: 37101
Quoting louisianaweatherguy:
folks... as long as Isaac keeps a W / WSW motion going on, the models will CONTINUE to shift westward... I'm thinking more of a panhandle landfall with this one...


oh god dont devistate the south floridians
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Quoting Clearwater1:
Since the Euro has moved further east, I also notice the 06 gfs also moved a tad further east, in fact, back to it's position up the spine of FL, as in a few days back. I would think, with that in minds and the NHC averaging mostly the gfs and Euro the official cone and track to be nudged to the east as well. Maybe at 11 am.


Euro moved east??? I don't think so...Pretty much ALL models have shifted westward..unless I'm missing something..
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Quoting LargoFl:
This will be wrong!
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Quoting ProgressivePulse:
IMO the center is the smaller red ball to the NE. Large convective mass should shoot around it today should intensification commence.

You're right that is exactly where the center is located, what everybody else is talking about is the MLC, is it possible for the LLC to get pulled in by the MLC, I guess so, but I don't see it happening.
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I see Issac is still a hot mess this morning, and hasn't stacked his LLC and MLC yet.
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Quoting ProgressivePulse:
Morning All.

Chances increasing for an EC FL strike.



Yeah I think we can thank TD 10 (soon to be Joyce) for that. Look how the western extension of the ridge erodes on the 06Z GFS.


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Quoting SFLWeatherman:
a SE FL??? how is that EC FL


East Coast Florida Landfall (EC FL)
Member Since: August 19, 2005 Posts: 5 Comments: 5043
3633. snotly
I don't know, it keeps eating all the dry air and SAL to the northeast side, I think that's why the convection on the southwest side has been tugging the storm in a southerly direction and keeping the storm weaker.

Quoting StormTracker2K:
Expect Issac to really gain some wind speeds today. My guess is we will have a 60mph Issac at 5pm then 75mph Issac at 11pm.

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Quoting AussieStorm:

It's updated for Isaac.


All they did was update the current location. None of the forecast points were changed and usually the times match the initial time. All of the points say 2am they should say 8am. If you look at the other graphics they say 2am as well.
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Quoting RTSplayer:



The radar doesn't lie.

Just because you measure a lower pressure in a thunderstorm doesn't mean jack.

The circulation is nowhere near the official fix, and it's freaking obvious.


but where is it coordinate wise?
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folks... as long as Isaac keeps a W / WSW motion going on, the models will CONTINUE to shift westward... I'm thinking more of a panhandle landfall with this one...
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Beachy! Great to see you --- you guys certainly don't need any more water up that way. With a storm so big, even if it doesn't become a significant hurricane the water could cause all kinds of problem
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3628. LargoFl
Quoting Clearwater1:
Since the Euro has moved further east, I also notice the 06 gfs also moved a tad further east, in fact, back to it's position up the spine of FL, as in a few days back. I would think, with that in minds and the NHC averaging mostly the gfs and Euro the official cone and track to be nudged to the east as well. Maybe at 11 am.
..anyone remember charley?..im beginning to feel like that once again..it was the only storm i ever plywooded the windows for..luckily he changed course and came in way south of me That time if i remember the name of the storm right...
Member Since: August 6, 2011 Posts: 4 Comments: 37101
EC FL strike = less land & more warm water than the current track. Very muddy however till splash down what exactly Isaac will be. Will it be another Ernesto (06) or will it be Isaac (12).
Member Since: August 19, 2005 Posts: 5 Comments: 5043
Isaac still trying to tighten up this morning I see, although he still looks much improved from last evening. I wouldn't expect any significant strengthening until he organizes his CDO, which still has the looks of two brothers fighting. Been a very interesting storm. I have not yet seen two disturbances combined into one, at least Atlantic speaking, and that is pretty much what happened with Issac.

I have also noticed that Issac is south of guidance, by a rather significant amount. NHC still waiting for the dominant center to finish forming before adjusting tracks further west I guess? I still think at most Issac scrapes Hispaniola to the western peninsula. He may miss it altogether.

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3625. sar2401
Quoting weatherh98:
keep in mind the lines have nothing to do with time and everything to do with wind speed.



I don't really understand your model but it would be a good idea to remove the NOAA logo from the left top. It's going to confuse new people who might think it's some kind of official forecast.
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Quoting SFLWeatherman:
DID you see the CMC today the CMC and GFDL is going for S FL


CMC has been lousy with this storm so far. It had it north of PR already a few days back.
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Quoting Gearsts:
The lowest pressure are in the northeast side of the system and thats where the center should be.



The radar doesn't lie.

Just because you measure a lower pressure in a thunderstorm doesn't mean jack.

The circulation is nowhere near the official fix, and it's freaking obvious.
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Quoting Beachfoxx:
Hi all...

Prep time! For those on the FL Panhandle make a plan & stay alert.
In the meantime - pray for shear!


About time you showed up!
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Quoting ProgressivePulse:
IMO the center is the smaller red ball to the NE. Large convective mass should shoot around it today should intensification commence.



It has already commenced, note those cold cloud tops
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Quoting Clearwater1:
Since the Euro has moved further east, I also notice the 06 gfs also moved a tad further east, in fact, back to it's position up the spine of FL, as in a few days back. I would think, with that in minds and the NHC averaging mostly the gfs and Euro the official cone and track to be nudged to the east as well. Maybe at 11 am.


Innacurate based on incorrect COC data? Is that even possible?
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3618. Brock31
Quoting Losttsol:
Wow! Waking up this morning to see Isaac still very far south and not turning. The tracking doesn't even look right, with a huge NW turn needed in the next 6 hours.


I too was hoping to see Isaac make his way north of PR and send some much needed swell our way.
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a SE FL??? how is that EC FL
Quoting ProgressivePulse:
Morning All.

Chances increasing for an EC FL strike.

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Quoting TropicalAnalystwx13:

35 mph in 15 hours...you expect rapid intensification?


It looks ready to go if you ask me. The biggest problem for Issac has been finding one dominate center and it appears to have it now.
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IMO the center is the smaller red ball to the NE. Large convective mass should shoot around it today should intensification commence.

Member Since: August 19, 2005 Posts: 5 Comments: 5043
Quoting sar2401:
The CME model has now turned a little neutral on an eastern Gulf path. Wholesale gasoline in the overnights is holding right around $3.14, which is near the close yseterday. The oil company mets, like the rest of us, are waiting to see which models actually start to confirm before advising their clients to bet gas up. Keeping an eye on the price of wholesale gas may give us better insights than some of the other models.


Since most all rigs are well west of the forecast path, will any of those be evacuated anyway if the forecast for an eastern GOM track (as they call for now) verifies?
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The latest fix at 15.4N 64.61W seems to be agreeing with the data from buoy number 42059 which has NNW winds at 15 knts location of station is 15°3'14" N 67°28'19 pressure is also falling

img src="">
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3612. Chiggy
Quoting RTSplayer:


No, that's just wrong and ridiculous, even though that's what NHC says.

Set this to Long Range, and set animation speed to maximum, and try not to laugh at the NHC fix


HAHA I laughed sorry! Yup rotation is clearly visible more than anywhere else in that radar loop. So what's coordinates estimation then?
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Since the Euro has moved further east, I also notice the 06 gfs also moved a tad further east, in fact, back to it's position up the spine of FL, as in a few days back. I would think, with that in minds and the NHC averaging mostly the gfs and Euro the official cone and track to be nudged to the east as well. Maybe at 11 am.
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DID you see the CMC today the CMC and GFDL is going for S FL
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Quoting AussieStorm:

It's updated for Isaac.



According to the NHC track Isaac has to take an immediate jog to the north for it to be accurate.
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Unnamed 10L T2.5/2.5 23/1145Z Atlantic
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Morning All.

Chances increasing for an EC FL strike.

Member Since: August 19, 2005 Posts: 5 Comments: 5043
TD 10 is out there and looking better this morning should see an upgrade to Joyce sometime today.

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Quoting HCW:




Since when does wind speed change equal RI ? I always thought that it was a 21mb drop in less than 24 hours that equaled RI

It's applied loosely to storms. Typically a 30 mph increase In winds in 24 hours or less is RI.
Member Since: July 6, 2010 Posts: 113 Comments: 31565
Quoting Gearsts:
The lowest pressure are in the northeast side of the system and thats where the center should be.


That doesn't make sense. How many systems have you seen with the center off to the NE while all the convection is off to the SW? Normally you have the center of a weak storm with all the convection off to the N & E but this weak storm has all it's convection off to the SW of the center? I'm not buying it, I see the NHC calling for another center relocation.
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Quoting VR46L:
The Caribbean in rainbow



TD10 is looking good
Member Since: September 30, 2007 Posts: 9 Comments: 15932
3602. Jax82
Here is a zoomed in Visible loop of Isaac, its about as good as you'll get, and its rapid scan. I'm guessing he tightens up today.

Isaac Visible Loop

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3601. sar2401
Quoting StormTracker2K:
Expect Issac to really gain some wind speeds today. My guess is we will have a 60mph Issac at 5pm then 75mph Issac at 11pm.



Umm...no. Isaac is barely holding together as TS right now. There's nothing I can see that would lead to such RI over a short period of time. If it was over the Florida Straits, maybe, but not now.
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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.