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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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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2800. scott39
Quoting Grothar:


We don't do Loretta Lynn at this hour.
I wouldnt do Loretta lynn at any hour.
Member Since: June 13, 2009 Posts: 0 Comments: 6863


humph...
Member Since: July 12, 2007 Posts: 0 Comments: 6049
2798. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 174 Comments: 54432
2797. Grothar
Quoting MississippiWx:


GFS has been out for an hour. Lol. It showed a final landfall farther west in the Panhandle.


That's why I posted the first one over an hour ago. LOLOLOL
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Still not gaining much/any lat. Anyone think the folks in Haiti will catch a break?

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2795. scott39
Ok, Its time to lay our bets on the table. Lol
Member Since: June 13, 2009 Posts: 0 Comments: 6863
Quoting GTcooliebai:
You think wind shear is causing the convection to blow off to the south or is it speed shear? And has it seemed to slow down to you?


Hmm, speed sheer. Don't know enough about it. Maybe!
Member Since: August 17, 2005 Posts: 26 Comments: 16873
2793. Dunkman
GFS hits Apalachicola at about 150h and is substantially stronger than 18z...not sure why Allan's site is slow tonight.
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2792. Grothar
Quoting scott39:
I didnt promise you
a rose garden


We don't do Loretta Lynn at this hour.
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Quoting Grothar:
This is strange because it looks like the GFS is moving a little back to the right.



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


Ok. I'm thinking of how the track is trending west though.
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.
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Quoting StormJunkie:


I'm having a hard time viewing in 2d...The colors/shades and add in that crazy scud alien blob...Well my eyes start seeing it in 3d; like one of those pictures you were supposed to stair at and see a 3d image.


Uh. Too much coffee, maybe? lol It did look weird though. What would cause it do that?
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Quoting Storm Junkie:

Pow, bash and wham! Did something just take over that wheel or what?

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Time: 05:12:00Z
Coordinates: 16.6N 64.9167W
Acft. Static Air Press: 843.4 mb (~ 24.91 inHg)
Acft. Geopotential Hgt: 1,552 meters (~ 5,092 feet)
Extrap. Sfc. Press: 1007.1 mb (~ 29.74 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 50° at 23 knots (From the NE at ~ 26.4 mph)
Air Temp: 18.0°C* (~ 64.4°F*)
Dew Pt: 18.0°C* (~ 64.4°F*)
Peak (10s) Flt. Lvl. Wind: 25 knots (~ 28.7 mph)
SFMR Peak (10s) Sfc. Wind: 24 knots (~ 27.6 mph)
SFMR Rain Rate: 3 mm/hr (~ 0.12 in/hr)
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Quoting Grothar:
GFS is coming in slowly. 84 hours.




GFS has been out for an hour. Lol. It showed a final landfall farther west in the Panhandle.
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Quoting Levi32:


Indeed. Good to see you as well.


Thank you. I just checked out your blog. You still think Issac is going to track up the east coast?
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Quoting lottotexas:
GFS ensemble mean has been on same track since 00z 8/20


Ok. I'm thinking of how the track is trending west though.
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Quoting 1900hurricane:
I'm going to be completely honest: I don't have any idea where the center is under there at the moment.

If you stare at that green long enough you will go blind.
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Quoting weatherwart:


Dang! That looked alien. Ha.


I'm having a hard time viewing in 2d...The colors/shades and add in that crazy scud alien blob...Well my eyes start seeing it in 3d; like one of those pictures you were supposed to stair at and see a 3d image.
Member Since: August 17, 2005 Posts: 26 Comments: 16873
Just about an hour or two away from one nice ball of convection with Isaac, the night hours have been very good to him. Center should be quite easy to pick out by then. Nothing really inhibiting Isaac from reaching hurricane strength by tomorrow afternoon. The question then will be how strong he can become before making landfall over mountainous Eastern Cuba. Isaac shaping up to be the storm of the year. Not saying he's going to be historical in nature. Not saying he not going to be either.
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2780. Grothar
This is strange because it looks like the GFS is moving a little back to the right.

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Quoting 1900hurricane:
I'm going to be completely honest: I don't have any idea where the center is under there at the moment.



That's what recon is for.
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Member Since: July 12, 2007 Posts: 0 Comments: 6049
I'm going to be completely honest: I don't have any idea where the center is under there at the moment.

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2776. Grothar
GFS is coming in slowly. 84 hours.


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2775. scott39
Quoting Grothar:


I beg your pardon.
I didnt promise you
a rose garden
Member Since: June 13, 2009 Posts: 0 Comments: 6863
2774. Levi32
Quoting weatherwart:


Wow. I remember when you were just starting your first semester. Good to see you! Hope all is going well with you.


Indeed, time flies lol. Good to see you as well.
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Quoting Zeec94:
Good early morning everyone. Isaac still trying to get that tightening together. He's looking better, but still not the storm we will see in a few days time. What's everyone's overall thoughts right now on Isaac. Have you gotten your forecasts correct so far?


Yo man, hows it goin
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Quoting KEEPEROFTHEGATE:



a strait in the Caribbean Sea, between the islands of Cuba and Hispaniola



LOL. I knew that. Thanks. Hence the :P on the post.
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Quoting Grothar:
The 66 hours GFS. Look what's trying to creep up on the lower right.

And look what's coming down from Saskatchewan and Manitoba.
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2770. Zeec94
Good early morning everyone. Isaac still trying to get that tightening together. He's looking better, but still not the storm we will see in a few days time. What's everyone's overall thoughts right now on Isaac. Have you gotten your forecasts correct so far?
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2769. KEEPEROFTHEGATE (Mod)
Quoting AllStar17:


No. Never. :P



a strait in the Caribbean Sea, between the islands of Cuba and Hispaniola

Member Since: July 15, 2006 Posts: 174 Comments: 54432
2768. Grothar
Quoting redwagon:

Where do you think the center line of your cone would wind up since it's out of the picture?


I beg your pardon.
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Time: 05:02:00Z
Coordinates: 17.15N 64.8667W
Acft. Static Air Press: 843.6 mb (~ 24.91 inHg)
Acft. Geopotential Hgt: 1,558 meters (~ 5,112 feet)
Extrap. Sfc. Press: 1008.0 mb (~ 29.77 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 63° at 34 knots (From the ENE at ~ 39.1 mph)
Air Temp: 18.8°C (~ 65.8°F)
Dew Pt: 16.0°C (~ 60.8°F)
Peak (10s) Flt. Lvl. Wind: 35 knots (~ 40.2 mph)
SFMR Peak (10s) Sfc. Wind: 26 knots (~ 29.9 mph)
SFMR Rain Rate: 3 mm/hr (~ 0.12 in/hr)
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actually i think if the storm were to hit land it might actually help it...not stay over land but just long enough for the MLLC and LLC to alighn then pop off the coast it would probably develop faster than over water at this point...its happened before
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2765. Grothar
The 66 hours GFS. Look what's trying to creep up on the lower right.

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2764. scott39
I cant remember when the Eastern Caribbean was a good area for a TC to develope.
Member Since: June 13, 2009 Posts: 0 Comments: 6863
Quoting sullivanweather:


Don't just follow the convective, cold cloud tops. Use the channel2 IR and pay attention to the low-level circulation. Looking at the other channels which won't pick up the low-level clouds well gives the impression the storm is zipping along to the west even faster than it actually is.
Yes, except the center already redeveloped farther south once today and so one has to notice the tendencies of various circulations embedded within the storm.
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Quoting StormJunkie:


Thats the attacking scud blob,,,What the hell was that?

****** came out of nowhere! lol well it was a stress laugh
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Quoting StormJunkie:


Thats the attacking scud blob,,,What the hell was that?
You think wind shear is causing the convection to blow off to the south or is it speed shear? And has it seemed to slow down to you?
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Quoting Levi32:


Yes, starting my 3rd year at UAF.


Wow. I remember when you were just starting your first semester. Good to see you! Hope all is going well with you.
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Well goodnight all I've got to get some sleep to. Maybe a little busy in a couple days.
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Hopefully recon can pin down a more defined center this mission.
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Quoting MississippiWx:
Isaac doesn't seem to have a true CDO. Just looks like a large area convection racing around a mid-level center. It doesn't seem to be constant bursting over the center, so I wouldn't expect much of a difference in the strength. Just a hunch.

Yeah, I'm not sure either, which is one of the reasons I would love a microwave pass. It looked like this a few hours ago, which is definitely not a CDO:

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Quoting StormJunkie:
If you run this for 20 frames and then speed it up, well it looks like one blob on the Ease side got really pissed and high tailed it at breakneck speeds to a blob on the west side, That was a little strange...



That would have been impressive to watch in real time from space. All those clouds falling and rising that quickly.
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2755. Levi32
Quoting weatherwart:


There's a familiar name. Hi Levi. It's been a while since I've been here. Are you still in school?


Yes, starting my 3rd year at UAF.
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Quoting Grothar:

Where do you think the center line of your cone would wind up since it's out of the picture?
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Quoting Grothar:


Hey, Allstar, have you ever heard of the Windward Passage? :)


No. Never. :P
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Quoting StormJunkie:


Thats the attacking scud blob,,,What the hell was that?


Dang! That looked alien. Ha.
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2751. scott39
EURO picked up on Isaac not developing as fast as the GFS.
Member Since: June 13, 2009 Posts: 0 Comments: 6863

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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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