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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3451. Chiggy
Issac's stacking at mid levels is a BIG problem:
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3450. emcf30
Quoting IKE:
isaac looking a little better this morning vs. last night. Good morning everyone!


Good to see you IKE. Lots of peeps have missed you
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Quoting CajunCrawfishhunter:
Guys and Ladies. How close do we need to watch the storm over here in Louisiana?


I'm keeping my eyes PEELED.

With the models moving west and the storm staying weeker than we thought it would the chances are only increasing
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This has 10 minute frames so you can see some of the SW quadrant.

The CoC may be visible on this later, as it is far south of where it is supposed to have been according to yesterday's models.

The LL grid boxes are 3 degrees increments, so don't get confused.

Long Range Radar Loop Venezuela

Member Since: January 25, 2012 Posts: 33 Comments: 1520
Quoting Chiggy:

Nope... the steering in the immediate vicinity of Issac is created by the storm itself... steering ahead of W-WSW hasn't changed for a few days
So you are agreeing that the storm is creating its own weakness?

Look at how the 1012 mb. lifts to the wnw and like I said, the CIMSS steering maps are updated every 3 hrs.

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3446. GetReal
Quoting CajunCrawfishhunter:
Guys and Ladies. How close do we need to watch the storm over here in Louisiana?



Just as much as everyone else along the N Gulf coast and Florida at this point in time.
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3445. Chiggy
Issac still barely a TS...
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Quoting LargoFl:
yes we are not out of the woods yet


Prepare ahead!
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3443. 900MB
Quoting FOREX:


On TWC, they keep showing only a CAT 1 approaching the Panhandle. Is this because they are not sure how weak the storm will be when it re-emerges onto water after crossing Cuba? I don't think they have been talking enough about the potential strength while in the Gulf.


I think the theory by NHC is that it will cross the Dominican and the mountains will disrupt. I think the consensus on this board is that we are on the left side of the cone, more likely to see a Cuban crossing, hence a stronger storm.

Right now I am guessing a borderline Cat 2, South Florida, not sure if it will be East, West, or Center.
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Quoting TropicalAnalystwx13:
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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3441. GetReal
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Guys and Ladies. How close do we need to watch the storm over here in Louisiana?
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"Really wish they would get a handle on the models, i live on westbank of new orleans , and we dont need a storm , everyone is so poor right now , it would be like another katrina episode, gas cost to much to go anywhere . But i dont wish it on anyone else>
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Quoting Chiggy:
UKMET has now moved WEST, traveling towards NOLA from the FL straights...
NOGAPS is now on the West side of FL.. CMC is the only outlier showing EAST of FL..

UKMET:


-___-
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3437. LargoFl
Quoting tkdaime:
Nogaps model hits s Florida too
yes we are not out of the woods yet
Member Since: August 6, 2011 Posts: 4 Comments: 38436
Well since they never go down the line, i may get effected judging by the models shifting slowly
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3435. HCW
LIX NWS


THE OFFICIAL FORECAST TRACK BRINGS ISAAC
INTO THE SOUTHEAST GULF OF MEXICO LATE SUNDAY NIGHT AS A CATEGORY
ONE HURRICANE LATE SUNDAY NIGHT. THE MODELS HAVE LESS SPREAD TODAY
WITH THE 00Z ECMWF HAVING SHIFTED EAST TOWARDS MOB/PNS.
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3434. Chiggy
UKMET has now moved WEST, traveling towards NOLA from the FL straights...
NOGAPS is now on the West side of FL.. CMC is the only outlier showing EAST of FL..

UKMET:
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3433. rxse7en
Quoting SFLWeatherman:
No matter where he goes, looks like it will be a major rain event for Florida again. Getting soaked this year. :(
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A new center may be trying to form NW of the (ill defined) last fix. This may go on for a while.
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3431. LargoFl
Quoting FOREX:


On TWC, they keep showing only a CAT 1 approaching the Panhandle. Is this because they are not sure how weak the storm will be when it re-emerges onto water after crossing Cuba? I don't think they have been talking enough about the potential strength while in the Gulf.
yes, so far everyone is unsure, which is why everyone needs to watch this storm closely the next few days
Member Since: August 6, 2011 Posts: 4 Comments: 38436
Quoting LargoFl:
wow does not matter if it goes up the east coast or west coast of florida..everyone gets a piece with a storm that wide

That is the overall coverage...does not mean effects will be felt 500+ miles out from the center on all side. The outer edges may be nothing more than streaming clouds banding toward the circulation.
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what time is new update
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3428. tkdaime
Nogaps model hits s Florida too
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3427. rxse7en
Quoting 900MB:
Does everyone agree that we are on the extreme left side of the cone right now?
A. Yes
B. No


C. Can't tell. Can't see the CoC, convection could just be decoupling/blowing off to the left.
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3425. GetReal
Quoting 900MB:
Does everyone agree that we are on the extreme left side of the cone right now?
A. Yes
B. No


The evidence says YES...
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3424. LargoFl
I guess one thing is for sure..unless this storm goes to texas..we can be sure to get ALOT of rain once again..gee, around me we surely do not need this, ponds, lakes and streams are full now
Member Since: August 6, 2011 Posts: 4 Comments: 38436
3423. FOREX
Quoting LargoFl:
.......hmmm now the ecmwf has issac going up thru the yucatan channel into the mid gulf..see how far apart the models are


On TWC, they keep showing only a CAT 1 approaching the Panhandle. Is this because they are not sure how weak the storm will be when it re-emerges onto water after crossing Cuba? I don't think they have been talking enough about the potential strength while in the Gulf.
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At the 850 mb level:
1004.0 mb
(~ 29.65 inHg)
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3421. Chiggy
Quoting GTcooliebai:
No you have to look at where it is at now and notice the hump that is created by the white lines and on the northern most line the flow is towards the wnw. Don't look at the steering ahead as that will change in 3 hrs. when the map is updated.

Nope... the steering in the immediate vicinity of Issac is created by the storm itself... steering ahead of W-WSW hasn't changed for a few days
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yep , it is south of puerto rico right now , has took a jog south, poor haiti is soooooooo on the wrong side of this storm
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Quoting GetReal:


That is a problem with posting from CIMSS.

I see your point.
I was just mentioning S Korea because apparently I live there.
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3418. DDR
Good morning
Heavy rain and flash flooding is wrecking havoc in parts of Trinidad as we speak.
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3417. LargoFl
Quoting SFLWeatherman:
..yep that..is the danger area alright
Member Since: August 6, 2011 Posts: 4 Comments: 38436
Yes, glad to see Ike back. Now if IndianRiverGuy shows up, it'll be nice.
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3415. LargoFl
Quoting southernbell72:
on the weather channel they said that Isaac is 1000 miles wide.
wow does not matter if it goes up the east coast or west coast of florida..everyone gets a piece with a storm that wide
Member Since: August 6, 2011 Posts: 4 Comments: 38436
Quoting TropicalAnalystwx13:

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

Sorry, I didn't have my glasses on.
Member Since: September 30, 2007 Posts: 9 Comments: 15935
Quoting 900MB:
Does everyone agree that we are on the extreme left side of the cone right now?
A. Yes
B. No


Yes
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3411. Chiggy
Quoting GetReal:



On the extreme left side of forecast track...


I am sticking to my forecast of Issac going just south of Jamaica...
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Quoting Chiggy:


Steering clearly shows W if not WSW... look at the pic again Sir..
No you have to look at where it is at now and notice the hump that is created by the white lines and on the northern most line the flow is towards the wnw. Don't look at the steering ahead as that will change in 3 hrs. when the map is updated.
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on the weather channel they said that Isaac is 1000 miles wide.
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3407. calder
Quoting AussieStorm:

Don't know how to post it but here is a link to the GFDL loop.

Here is what it says at 90hrs...

114.5kts = 131.76mph or Cat 4 Hurricane.


114kts @ 900mb...
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3406. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 38436
3405. GetReal
Quoting Bobbyweather:

The image you posted is Typhoon Bolaven, which is forecast to affect South Korea as a large typhoon.


That is a problem with posting from CIMSS.
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Quoting TomballTXPride:

Been lurking since 2004 and had a profile back then. Lost the passwords though!!
Believe you can recover lost password with your email address on here.
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Quoting 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
IDK but HH sure seems to be investigating that area thoroughly.
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3402. GetReal



On the extreme left side of forecast track...
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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.