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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18Z GFS is ..well interesting..especially what goes on
in the atlantic while Issac is west coat fl scraping
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Here's the 18z TVCN:

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Quoting JLPR2:
Now what's Isaac doing?

That eastern convection is DEEP
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948. JLPR2
Quoting Bluestorm5:
That's supposed to be Kirk according to models... a nasty storm as well.


Seems male names are the problem this year.
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Quoting Floodman:


It is that time of year after all


Mate!!!!!
How is the shelter project in Haiti going, are they some what prepared for Isaac?
Member Since: September 30, 2007 Posts: 9 Comments: 15940
i have noted that Isaac is S of his forcast points if he dos not start moveing more N he will likey miss PR
Member Since: May 21, 2006 Posts: 5091 Comments: 115255
Quoting KEEPEROFTHEGATE:


well well well
look what the cat dragged in

we had you marked as MIA


I AM MIA...**smiles cryptically** "You don't know me, you never knew me...I was never here"
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944. JLPR2
Now what's Isaac doing?

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Tembin's eye is reappearing..just in time for landfall tomorrow. :/

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Member Since: August 6, 2011 Posts: 4 Comments: 39711
I think this storm will make the GOM. If and when it does, things could very well get quite interesting.
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Quoting PRweathercenter:
Blog Update, New Information About The Center Of Tropical Storm Isaac




Link


Yup just read your blog Excellent short and to the point. COC is under Extremely intense convection We could see a Hurricane by 11am tomorrow. Thanks for clarification and stay safe man!
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The large size of this storm reminds me of Gilbert in its formative stages. I believe someone else on this blog mentioned that.
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Quoting MiamiHurricanes09:
Speechless. That poleward outflow is immaculate.

I have a hard time believing that this is just a 90kt cyclone.

West-Pac typhoons are truly beautiful works of...disastrous art.


There's a WU blogger in Korea,Bobby, watching that one as we are Isaac.
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Felix, 2007:



Isaac, 2012:

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Quoting KeyWestwx:
It could- that would put me on the 'soft' side of the storm.
yes, with your water temps at 90, you dont want this near you
Member Since: August 6, 2011 Posts: 4 Comments: 39711
When do the new models runs come out? The newest ones I see are from 2pm still...
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Quoting Grothar:
..its looking like south florida should be preparing soon, would not be surprised to see warnings up as soon as thursday night there
Member Since: August 6, 2011 Posts: 4 Comments: 39711
looks like Isaac may miss PR
Member Since: May 21, 2006 Posts: 5091 Comments: 115255
Won't be to long now before we hear reports of flooding coming from out the islands..
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Quoting LargoFl:
yep i noticed the GDFL in that run hits Miami AND tampa..hey it could happen huh
It could- that would put me on the 'soft' side of the storm.
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Has anyone posted on oil rig evacuations yet?
I am sure it is being considered.
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Quoting PRweathercenter:
Tropical Storm Isaac To Pass Thourgh The Local Area Tommorrow
08/22/20120 Comments
 
Good evening,

The big story tonight continues to be tropical storm Isaac as it approaches the island of Guadalupe. Apparently this system has been reorganizing during the entire course of the day with the new low-level circulation forming right near the island of Guadalupe. We can notice this on the latest imagery from the martinique radar.


If we also noticed the infrared imagery of the system there's very cold cloudtops formming over the new center of circulation. It appears that now as this system enters the Caribbean has a chance to rapidly intensified as it passes very near Puerto Rico and the US Virgin Islands sometime tomorrow. We can expect very gusty winds in portions of Puerto Rico during the early morning hours Thursday with conditions deteriorating in the afternoon with hurricane conditions possible.


I would not rule out the idea of the system coming over the southwestern tip of Puerto Rico sometime Thursday evening near the towns of Waneka and Cabo Rojo. Regardless of whether the system directly touches Puerto Rico or not we can definitely expect to be on the worst side of the system, the north and eastern quadrant of the circulation. In this quadrant is where the heavyiest winds and rains are located also with possible tornadoes embedded in the system.If you have not made your current preparations please do so now as tomorrow will be too late! Please be safe and stay tuned to Puerto Rico weather center for all your local updates and warnings.

Dave Goodspeed




GUANICA NO WANEKA!
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Quoting AussieStorm:
What is the ECMWF(Euro) seeing what the GFS isn't???

ECMWF 240hrs


ECMWF 192hrs...



GFS 135hrs....


Yeah, that's the 12z Euro run, right? It moved east from the earlier 00z run. Let's see what happens at the next run, 00z Euro. It may move even further east.

The 18 gfs and the 12z GFS basically stayed the same. A better comparison will be the 00z Euro. to the 00z gfs. imo
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The new wave is pretty good sized...

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T.C.F.W.
09L/TS/I/CX
MARK
15.51N/62.33W
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Quoting bayoubug:
A weeker storm should follow the bottom of the track right....


Or fal out of the track entirely. Bigger the storm, to a piint, the more highs and lows effects are felt
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Quoting PRweathercenter:
Tropical Storm Isaac To Pass Thourgh The Local Area Tommorrow
08/22/20120 Comments
 
Good evening,

The big story tonight continues to be tropical storm Isaac as it approaches the island of Guadalupe. Apparently this system has been reorganizing during the entire course of the day with the new low-level circulation forming right near the island of Guadalupe. We can notice this on the latest imagery from the martinique radar.


If we also noticed the infrared imagery of the system there's very cold cloudtops formming over the new center of circulation. It appears that now as this system enters the Caribbean has a chance to rapidly intensified as it passes very near Puerto Rico and the US Virgin Islands sometime tomorrow. We can expect very gusty winds in portions of Puerto Rico during the early morning hours Thursday with conditions deteriorating in the afternoon with hurricane conditions possible.


I would not rule out the idea of the system coming over the southwestern tip of Puerto Rico sometime Thursday evening near the towns of Waneka and Cabo Rojo. Regardless of whether the system directly touches Puerto Rico or not we can definitely expect to be on the worst side of the system, the north and eastern quadrant of the circulation. In this quadrant is where the heavyiest winds and rains are located also with possible tornadoes embedded in the system.If you have not made your current preparations please do so now as tomorrow will be too late! Please be safe and stay tuned to Puerto Rico weather center for all your local updates and warnings.

Dave Goodspeed





This is Jay just in case Dave is still at the office, he just posted this entry a few minutes ago, i thought it was interesting
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Quoting MAweatherboy1:
This is Bolaven in the West Pac:





It's at 90kts forecast to peak at 120kts.
Speechless. That poleward outflow is immaculate.

I have a hard time believing that this is just a 90kt cyclone.

West-Pac typhoons are truly beautiful works of...disastrous art.
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Quoting JLPR2:
Meanwhile, the next one is moving off the African coast.

That's supposed to be Kirk according to models... a nasty storm as well.
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Quoting NoNamePub:


Used to be.....in hawaii now.
ha, I moved here from Hawaii. I lived there when Ineki hit. Have a plate lunch with macaroni salad and 2 scoop rice for me!
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Member Since: August 6, 2011 Posts: 4 Comments: 39711
Tropical Storm Isaac To Pass Thourgh The Local Area Tommorrow
08/22/20120 Comments
 
Good evening,

The big story tonight continues to be tropical storm Isaac as it approaches the island of Guadalupe. Apparently this system has been reorganizing during the entire course of the day with the new low-level circulation forming right near the island of Guadalupe. We can notice this on the latest imagery from the martinique radar.


If we also noticed the infrared imagery of the system there's very cold cloudtops formming over the new center of circulation. It appears that now as this system enters the Caribbean has a chance to rapidly intensified as it passes very near Puerto Rico and the US Virgin Islands sometime tomorrow. We can expect very gusty winds in portions of Puerto Rico during the early morning hours Thursday with conditions deteriorating in the afternoon with hurricane conditions possible.


I would not rule out the idea of the system coming over the southwestern tip of Puerto Rico sometime Thursday evening near the towns of Waneka and Cabo Rojo. Regardless of whether the system directly touches Puerto Rico or not we can definitely expect to be on the worst side of the system, the north and eastern quadrant of the circulation. In this quadrant is where the heavyiest winds and rains are located also with possible tornadoes embedded in the system.If you have not made your current preparations please do so now as tomorrow will be too late! Please be safe and stay tuned to Puerto Rico weather center for all your local updates and warnings.

Dave Goodspeed


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West Pac:



Atlantic:

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Quoting HuracanTaino:
your day tomorrow should be pretty interesting

Action:
Quote
| Ignore User


Member Since: May 31, 2011 Posts: 0 Comments: 352



yeah. well. i hope. my boss said there's not going to be more than 60mph winds and i kinda scoffed. "he's from wyoming, what would he know?" should've known better.

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Quoting JLPR2:
Meanwhile, the next one is moving off the African coast.



It is that time of year after all
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Quoting JLPR2:
Meanwhile, the next one is moving off the African coast.

Big pretty one....
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hmm 8pm advisory has it at 15.7N 62.2W
last advisory had it at 16.0N 61.2W

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Quoting KeyWestwx:
Woohoo- they quoted meterologist from Weather underground 'Convention center could be under 20 feet of water in a cat4'

Did they mention that he also said there was a 3% chance that the hurricane would hit them
Member Since: June 6, 2010 Posts: 0 Comments: 4447
A weeker storm should follow the bottom of the track right....
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I think the NHC is getting fed up as well.


8:00 PM AST Wed Aug 22
Location: 15.7°N 62.2°W
Moving: W at 21 mph
Min pressure: 1004 mb
Max sustained: 45 mph


000
WTNT24 KNHC 222038
TCMAT4

TROPICAL STORM ISAAC FORECAST/ADVISORY NUMBER 7
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
2100 UTC WED AUG 22 2012


TROPICAL STORM CENTER LOCATED NEAR 16.0N 61.2W AT 22/2100Z
POSITION ACCURATE WITHIN 30 NM

PRESENT MOVEMENT TOWARD THE WEST OR 280 DEGREES AT 19 KT
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Quoting MiamiHurricanes09:
That track is in perfect agreement with what I'm currently thinking. Interested to see if it moves into the western Gulf and continues to intensify after the southern Florida crossing.
almost impossible to get to the western gulf. the texas death ridge does not approve :)
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Blog Update, New Information About The Center Of Tropical Storm Isaac




Link
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Quoting hurricanejunky:


Flood sighting! How the heck are you man? Great to see you here...


Great to be here...work is a bear, but then no one seems to be able to do it the way I want it done, right?

How are yo, brother? Ready for this?
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Quoting JLPR2:
Meanwhile, the next one is moving off the African coast.



They are stacked up like Rockettes! lol
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Euro 240 hrs
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902. JLPR2
Meanwhile, the next one is moving off the African coast.

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Quoting AussieStorm:

Isaac is doing what Ernesto did. It can't make up it's mind if it wants to strengthen or not.


conditions have not been as ideal as forecast, apparently. kinda has been a trend of the past few years
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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.