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 GetReal:


It looks like young Isaac is sticking to the extreme left side of the cone....

The image you posted is Typhoon Bolaven, which is forecast to affect South Korea as a large typhoon.
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3398. LargoFl
Quoting 900MB:


NHC says weak Cat 1. Pretty big divergence here. I think Issac now has its moment to blow up. Looks South of NHC forecast points right now. Or, am I seeing things?
this slow down means something, maybe this is what the GDFL was predicting, and the nam and nogaps
Member Since: August 6, 2011 Posts: 4 Comments: 37077
3396. Relix
Quoting kmanhurricaneman:
thanks Chiggy been trying to tell some people on here


Huh?


The gap is easily seen here.
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Quoting tkdaime:
Gfdl model says cat 4 for south florida


That reminds me of another Cat 4 (upgraded to 5) 20 years ago during an El Nino year.............
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Quoting AussieStorm:


it says, Aug 23 10:06UTC
it's now Aug 23 11:25UTC

The numbers in purple is when the ASCAT passed through this region. Yesterday at 16:25 UTC.
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3393. LargoFl
Quoting GetReal:


It looks like young Isaac is sticking to the extreme left side of the cone....
see that right hand track..IF issac goes that way..we are in big trouble
Member Since: August 6, 2011 Posts: 4 Comments: 37077
Arctic Sea Ice Anomaly has even managed to grow more in the past few days.



Area Death Spiral


Extent Death Spiral


Volume Death Spiral
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3391. 900MB
Quoting tkdaime:
Gfdl model says cat 4 for south florida


NHC says weak Cat 1. Pretty big divergence here. I think Issac now has its moment to blow up. Looks South of NHC forecast points right now. Or, am I seeing things?
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Quoting SFLWeatherman:
Wow now it moving at 12MPH!!
More time to strengthen past the island and then pulled out to sea!
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Quoting 10Speed:


Well, of course they have. Just the forecast of a couple of clouds and a light sprinkle in the Gulf is good enough excuse for rising gas prices anymore.

Oh really, that quick. Did they go up when Ernesto was around?
Member Since: September 30, 2007 Posts: 9 Comments: 15932
3388. GetReal

It looks like young Isaac is sticking to the extreme left side of the cone....
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3387. LargoFl
Quoting LargoFl:
man that GDFL model scares me..a direct hit on hmm maybe miami area, then up thru florida, coming out somewhere near tampa..........omg plse be wrong
Member Since: August 6, 2011 Posts: 4 Comments: 37077
Quoting Chiggy:


Steering clearly shows W if not WSW... look at the pic again Sir..
thanks Chiggy been trying to tell some people on here
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3385. Chiggy
Now that the vis pictures are coming in satellite loops can someone pelase verify that there is aleast one center at 14N between 64-65W...!? thx
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Quoting TropicalAnalystwx13:

Over and east of the Leeward Islands since that image is from yesterday afternoon.


it says, Aug 23 10:06UTC
it's now Aug 23 11:25UTC
Member Since: September 30, 2007 Posts: 9 Comments: 15932
Isaac can't delay intensification much longer. Wind shear is no longer a problem it appears, as outflow has spread out in the northern semicircle of the storm. Deep convection has been persistent over the center, and Isaac is moving over 28-29C SSTs with decent OHC values. There are no signs of dry air intrusion.

That being said, because Isaac has taken so long to organize, it is going to go on a farther south path; one that does not take it over the highly mountainous terrain of Hispaniola anymore. Jamaica will be affected a lot more than originally anticipated, and this storm will enter the East Gulf...almost undoubtedly. As for my forecast, if this follows the NHC track, this could easily become a strong Category 1 to minimal Category 2 hurricane before landfall in the Florida Panhandle. If it's just a little farther west, it could be much stronger like the ECMWF shows.



Meanwhile, Tropical Depression Ten has managed to fire deep convection that completely covers the center. It has been organizing slowly for most of the night, and the satellite appearance is one of a minimal tropical storm. I'd look for an upgrade this morning. It won't be able to strengthen much more due to dry air and moderate to high vertical wind shear, but this could be a threat to Bermuda in the long range.

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3382. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 37077
Where FL lol one 00Z ENSEMBLE
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hey IKE whats shaking long time no hear. glad to hear you ok.
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3377. Chiggy
Quoting GTcooliebai:
Water Vapor Loop shows a wnw flow ahead of Isaac:



Steering Supports this too, so aside from all the relocation should resume the heading that the NHC put out.




Steering clearly shows W if not WSW... look at the pic again Sir..
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3376. LargoFl
Quoting 10Speed:


Well, of course they have. Just the forecast of a couple of clouds and a light sprinkle in the Gulf is good enough excuse for rising gas prices anymore.
..and..labor day is fast approaching
Member Since: August 6, 2011 Posts: 4 Comments: 37077
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3372. WxLogic
Quoting IKE:
Doing fine everyone.


Wow... long time. Glad you're alive. :)
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Quoting TampaCat5:
I think one thing is clear from the models I've seen and from satellite so far. Isaac is just too big to ever really get his act together. Too much of the atmosphere to pull together. He is actually suffering from his large size and I'm doubting he will ever consolidate very well. (I do have some crow handy though)
Definately agree the large size is impending development. I think that was also due to the speed of the system. A monsoonal system racing at 20 mph is hard to consolidate. I know I keep saying it but once it slowed down this morning I think we are in for a show this on might wrap up nicely.
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3369. 10Speed
Quoting AussieStorm:

Has gas prices gone up yet???


Well, of course they have. Just the forecast of a couple of clouds and a light sprinkle in the Gulf is good enough excuse for rising gas prices anymore.
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3368. beell
Quoting IKE:
isaac looking a little better this morning vs. last night. Good morning everyone!


Hello, Ike. How was your nap?
:)
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Opps.....20-30ft surface wave heights....

Member Since: September 30, 2007 Posts: 9 Comments: 15932
Quoting presslord:
This is gonna be a Florida panhandle event



Right now I'm calling somewhere between Mobile Bay and Tallahassee, and that's assuming it doesn't do much more jogging to the S.

Right now it's still moving about 0.1 degree to the S every 2 hours or so...
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Quoting IKE:
Been doing ok. Didn't want to hog the blog during Isaac. Large system.


Welcome Back Old Friend.
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3363. Relix
Quoting serialteg:


strongly doubt it relix man

Hortense had a similar case where it was a disorganized mess, and somehow it flew through the Mona passage. You should listen to WKAQ XD! They pulled the Hortense card and I jumped to the PC
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Nice to see you back Ike!
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3361. K8eCane
Be careful Ike. This one might be coming to see ya.
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Could not help myself had to make a rare post for Ike.Good to see you and God Bless ike.
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Wow now it moving at 12MPH!!
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Quoting presslord:
This is gonna be a Florida panhandle event


Agree with you presslord.

It could also be a west coast Fla event (prior) if Isaac hugs the coast at a Cat 1 or better.

Tampa would be in the NW quadrant and will get some surge from what appears to be a large storm. It only takes a Cat 1 to cause evacuation of the convention center area.
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3357. LargoFl
..SEE..the Nam has it HERE on sunday..NOT in the gulf...maybe the GDFL is on to something here..and..this IS in the area Levi said it would be yesterday..all eyes ON this storm the next few days
Member Since: August 6, 2011 Posts: 4 Comments: 37077
I think one thing is clear from the models I've seen and from satellite so far. Isaac is just too big to ever really get his act together. Too much of the atmosphere to pull together. He is actually suffering from his large size and I'm doubting he will ever consolidate very well. (I do have some crow handy though)
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3355. K8eCane
IKE!!!!Good to see ya. Surprise for me this morning
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Good Morning. Just reading the NHC 5:00 discussion, seems they are confident in their track forecast based on the timing of the trof (Florida-Eastern Gulf). The short-term wild card is how strong Issac will get over the next 48 hours and any tweaking based on that. He looks much better this morning (a "one" blob system) than last evening with the elongated COC and does appear to be headed towards that NW jog they discussed at 5:00. Remember he is not currently forecast to reach hurricane strength until his closer approach to Hispanola.

Gonna have a busy day so I will check in around lunch. Have a great morning everyone.
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Quoting AussieStorm:
Where is Isaac??????


Over and east of the Leeward Islands since that image is from yesterday afternoon.
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Quoting Relix:
What is happening? Woke up this morning... saw Isaac was pretty far. Still got out of bed and I hear from the radio that Isaac could be pulling a Hortense (1996)? What?


strongly doubt it relix man

Member Since: August 22, 2006 Posts: 2 Comments: 1975
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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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