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 jeffs713:

Thank you for your insightful, alarmist trolling. It isn't terribly obvious that you are trolling, considering your recent join date, official-looking post, and alarmist post content.

I hope you enjoy your short stay here on WU.

I'm filling up my ignore list pretty fast tonight.
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Quoting sunlinepr:
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4397. Chiggy
Issac now about 0.7 degs directly south of NHC forecast point - NHC verbiage says moving WEST - hmmm, need to move almost NW if it wants to meet the next NHC point..WOW
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4396. gugi182
Southwest Puerto Rico. Just a sprinkle of rain beside that absolutely nothing concerning Issac. Not even a gust of winds absolutely nothing. Clear skies beautiful day here in this part of the island. It's all good to go grill some BBQ outside. Issac all big and mean and turned out to be a wimp!!!
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The fact that it is slowing down should suggest that it is about to take a turn. Whether it is as forecasted or to the SW it will make a significant difference as to the future tract Imo.
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Hurricane Ivan models were farther East (six days out) than they are with Isaac now.




Issac today

img src="Image and video hosting by TinyPic">
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Quoting ClwHobbyist:


I am not so sure he is right about that. When Charlie was predicted to make landfall in Pinellas County it seemed that hardly anyone around had thier windowcs covered. I think most of our neighbors in Pinellas are complacent because it has been so long since we have had hurricane conditions around here. Unless the forecase changes I will put my shutters up this weekend just to be safe, and my neighbors will shake thier heads and look at me funny like they have every other time I put my shutters up.


Maybe your neighbors have more money than they know what to do with and look forward to a little "remodeling". ;)
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NHC has Issac at 85mph pretty much right at Tampa by Tuesday morning.


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IT SHOULD BE NOTED...THOUGH...THAT NEAR THE 96
HR POINT THE GUIDANCE MODELS ARE SPREAD BETWEEN SOUTHEASTERN
FLORIDA AND WESTERN CUBA...AND AT 120 HR THEY ARE BETWEEN
NORTHEASTERN FLORIDA AND THE CENTRAL GULF OF MEXICO. THUS...
SIGNIFICANT UNCERTAINTY REMAINS ABOUT THE THREAT ISAAC POSES TO
FLORIDA.

From 11 am EDT Discussion
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Quoting WthrWonk:
trolling

Thank you for your insightful, alarmist trolling. It isn't terribly obvious that you are trolling, considering your recent join date, official-looking post, and alarmist post content.

I hope you enjoy your short stay here on WU.
Member Since: August 3, 2008 Posts: 16 Comments: 5869
4389. scott39
I dont think Isaac will be a hurricane this time on Fri. I think if he stays a moderate to strong TS, then he would go over a bigger chunk of Cuba. This would weaken Isaac before he emerges over the GOM.
Member Since: June 13, 2009 Posts: 0 Comments: 6728
4388. hydrus
Isaac looks good on satellite, but is still have trouble organizing.
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Quoting hurricanehanna:
11:00 AM AST Thu Aug 23
Location: 15.6°N 65.4°W
Moving: W at 15 mph
Min pressure: 1003 mb
Max sustained: 40 mph

track shifted a little more west
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Quoting Progster:


Its why Australia should have been discovered by the French. 11 am should contain a lot of 'splainin regarding track and steering uncertainty I expect.

We were discovered by the Dutch first. Not the English. But of course Our Island was known by the many Native tribes in the South Pacific, PNG, Indonesia, Malaysia. Maybe even African tribes that made it across the Indian Ocean.
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Quoting LargoFl:
Gov. Scott says Florida 'will be prepared' for Isaac


I am not so sure he is right about that. When Charlie was predicted to make landfall in Pinellas County it seemed that hardly anyone around had thier windowcs covered. I think most of our neighbors in Pinellas are complacent because it has been so long since we have had hurricane conditions around here. Unless the forecase changes I will put my shutters up this weekend just to be safe, and my neighbors will shake thier heads and look at me funny like they have every other time I put my shutters up.
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BULLETIN FOR ESCAMBIA COUNTY FLORIDA!

POTENTIAL FOR SEVERE TO CATOSTRAPHIC GULF STORM TO IMPACT AREA NEXT WEEK…

August 23, 2012 @ 7 a.m.

Here is my attempt to forecast the storm brewing in the Atlantic. I have read different opinions, listened to Joe Bastardi, and taken into consideration weather pattern (including the position and strength of the Bermuda High, and upper level low, etc.), history, and my own intuition. It’s sort of like a gambler when you like to make forecasts. :)

Isaac is going to become a large and dangerous hurricane, possibly very large while attaining a status of category 3-5 which would make it major to catastrophic. However, the impacts on the Republican Convention in Tampa should not be severe other than a flood threat.

Projected landfall would be between Miami westward to the TX/LA border with greatest threat being between Gulfport and Panama City. Right now I’m calling for a landfall near Pensacola. It will likely be starting to weaken when it hits and should be taking a more easterly trajectory upon landfall. This could put the greatest danger slightly to our east.

Stay tuned… LOL
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4383. Relix
There we go. NHC says west movement, west movement it is.
Member Since: August 3, 2006 Posts: 0 Comments: 2642
4382. 7544
no cone change
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...ISAAC CONTINUES MOVING WESTWARD TO THE SOUTH OF THE VIRGIN ISLANDS AND PUERTO RICO...
------------------------------------------------- -------------------------------

11:00 AM AST Thu Aug 23
Location: 15.6°N 65.4°W
Moving: W at 15 mph
Min pressure: 1003 mb
Max sustained: 40 mph

.2N .6W due west with a little northward componet
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Have to go to school will check in around lunch time
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11:00 AM AST Thu Aug 23
Location: 15.6°N 65.4°W
Moving: W at 15 mph
Min pressure: 1003 mb
Max sustained: 40 mph
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No change in track at 11am.
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EXTENDED OUTLOOK. NOTE...ERRORS FOR TRACK HAVE AVERAGED NEAR 175 NM
ON DAY 4 AND 225 NM ON DAY 5...AND FOR INTENSITY NEAR 20 KT EACH DAY

OUTLOOK VALID 27/1200Z 24.5N 81.5W...OVER WATER
MAX WIND 65 KT...GUSTS 80 KT.

OUTLOOK VALID 28/1200Z 27.5N 84.5WMAX WIND 75 KT...GUSTS 90 KT.

Issac point forecast 10 am CDT
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4376. Chiggy
Virtually no change at 11am - moving 2mph faster but still indicates a badly organized Issac... WEST he goes!
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4375. emguy
Hmmm.....Has anyone noticed how Isaac is strating to look on the last few frames of the Puerto Rico radar??? Link
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Pretty much unchanged, still 40mph and 1003mb but moving 15 mph.
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Quoting yonzabam:
Yet another storm that mysteriously fails to ramp up.

it's no mystery, If you read back you'll find out why.
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Quoting AussieStorm:

I always get those two mixed up. Why is English so hard.


Its why Australia should have been discovered by the French. 11 am should contain a lot of 'splainin regarding track and steering uncertainty I expect.
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Quoting PalmBeachWeather:
Tomball joined years ago under another name... He had a password issue and changed his handle
I knew he was familar, but trolls have been pretending to be someone. That's why I snapped.
Member Since: August 1, 2011 Posts: 28 Comments: 7880
Quoting bappit:
What effect does this have?


It's keeping the storm off-centered. The dry air punching into the core of the system along its western flanks is creating a pseudo-frontal boundary - much in the same way a dry line develops over the High Plains and sparks a line of thunderstorms. This creates most persistent convection associated with the storm along the southwest quadrant, hence elongating the center and pretty much keeping it from anything other than gradual intensification. As the pressure slowly drops and the storm slowly consolidates this dry air should become more diffuse and the storm will likely change structure quite rapidly.
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4369. bappit
Quoting AussieStorm:

I always get those two mixed up. Why is English so hard.

We are masochists. :)
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AND, the better convergence is under the MLC southwest of the official coordinates.
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FWIW...

5am Thursday 8/23/12

Summary:

At 5am EDT Thursday, Tropical Storm Isaac was located about 255 miles south-southeast of San Juan Puerto Rico, or approximately 1,275 miles from Miami, Florida.After a wobble to the southwest and a reformation of the center of circulation last night, Isaac is moving west at 12 mph. Although additional “wobbles” are possible, a general west-northwest motion is expected for the next day or 2 as it is steered around the southern edge of a large high pressure system in the central Atlantic Ocean.

Computer model track guidance is coming into better agreement (the outlying westward models have shifted east and the outlying east models shifting west) and many of these, along with the official NHC forecast, take Tropical Storm Isaac south of Puerto Rico and then across southern Dominican Republic and Haiti tomorrow.

Beyond that time, high pressure should weaken, which will allow Isaac to curve northwest across eastern Cuba and into the Florida Straits and eastern Gulf of Mexico. Some models still signal a more westward movement across central or western Cuba. The official forecast track has Isaac moving across the Florida Keys early Monday and then moving towards the Florida Panhandle, to a position roughly 85 miles west of Tampa early Tuesday morning.

Maximum sustained winds have decreased and are now near 40 mph, but conditions are favorable to promote strengthening. There is still a chance Isaac could become a hurricane before reaching Hispaniola, but the highest chances are that it may remain a tropical storm.

Isaac is a large storm, and tropical storm force winds now extend up to 140 miles from the center.

Any land interaction with Haiti and Cuba will likely weaken the storm, but warm water in the Florida Straits will allow for some re-strengthening.

Several Hurricane Hunter reconnaissance missions will be deployed today that will help further increase confidence in the long-range track and intensity. It is important to note that computer model guidance, especially beyond 72 hours, can still be unreliable.

Elsewhere in the tropics:

At 5am Thursday. Tropical Depression Ten was located roughly halfway between the Cape Verde Islands and the Leeward Islands.

Maximum sustained winds remain near 35mph. Some strengthening is possible and it could become Tropical Storm Joyce as early as this afternoon.

T.D. 10 is moving west-northwest at 16mph and this motion is expected to continue through Saturday before turning more to the north on a track towards Bermuda.

A new tropical wave has emerged from western Africa. Development should be slow to occur and the National Hurricane Center is currently forecasting a low (10%) chance of becoming a tropical cyclone within the next 2 days.

Florida outlook:

It is too early to forecast the extent of any affects Tropical Storm Isaac will have on Florida. Everyone in and near the error cone should continue to closely monitor this system and begin to review their preparedness plans.

There are no tropical watches or warnings in effect for Florida, but much of Florida is within the 5 day error cone. Remember that the cone does not necessarily forecast impacts.

Portions of South Florida from Palm Beach to Ft. Myers now have a 30-40% chance of receiving tropical storm force winds within the next 5 days. Tampa currently has a 22% chance of receiving tropical storm force winds and Tallahassee has a 10% chance. All of Florida currently has less than a 5% chance of seeing hurricane force winds within 5 days.

Flood Warnings are in effect for 6 Florida rivers.

Another update will be issued Thursday afternoon.

Amy Godsey

State Meteorologist

Florida Division of Emergency Management
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TROPICAL STORM CENTER LOCATED NEAR 15.6N 65.4W AT 23/1500Z
POSITION ACCURATE WITHIN 30 NM

PRESENT MOVEMENT TOWARD THE WEST OR 275 DEGREES AT 13 KT

ESTIMATED MINIMUM CENTRAL PRESSURE 1003 MB
MAX SUSTAINED WINDS 35 KT WITH GUSTS TO 45 KT.
Member Since: August 6, 2007 Posts: 0 Comments: 693
4365. luigi18
Blue skies in San Juan a lot of dry air to the NE
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Quoting AussieStorm:
I guess when we have a US threat. It brings out all the trolls. Not what we need right now. What we need is the correct information, not misleading or incorrect information.
Which is what I like. I am getting really annoyed.
Member Since: August 1, 2011 Posts: 28 Comments: 7880
The tail of Tropical Storm Isaac has driven aground a yacht called Utopia at Prince Rupert Bay, Dominica, where there is a heavy swell coming in from the south west. Please remind me how to put a photo on here. I did it once but that was a few years ago.
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Quoting RTSplayer:
Who else has 13 weather related tabs open in multiple browsers?!

LOL.


I'm a lightweight, only 4 LOL but I'm a girl of multiple interests...or scattered some may say :P

Early this morn TD10 was lookin a bit poorly, so wasn't expecting poss naming when I woke back up.

Isaac's really trying to do a full blown Ernesto. At least on the annoyance scale ;D

Have some nice waves at the local'ish' surf spot today, courtesy of the remnants of Gordon, ta dude!
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Yet another storm that mysteriously fails to ramp up.
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4360. bappit
Quoting jeffs713:

Holy jeebus. Bolaven is HUGE.

Indeed.
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Quoting Bluestorm5:
You just joined few days ago... likely to troll on us. I ain't gonna chill just because you said so. What a dumb statement.
Tomball joined years ago under another name... He had a password issue and changed his handle
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No change with the 11am advisory i bet
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Quoting JasonRE:


That's what I see. Almost, and I mean, ALMOST a SW jog for a split second. It's hard to judge but if anything, I still see W movement. Nothing to show a NW shift at all.
Just another wobble in a long run of wobbles,still an average of west over the next few hours,then a change in direction.
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4356. bappit
Quoting weathermanwannabe:
The heaviest ball of convection is doing a loop-de-loop at the moment in the S-SE quadrant and the coc is probably outrunning the convection somewhere to the NE. The Hunters are going to have a tough time figuring out exactly where the dominant coc is located.

I would expect the LLC to be more easily influenced by the convection, so I'd expect it to be near the southwest hidden by the overcast. I'd think the MLC would correspond more to the overall structure we can see on the satellite which suggests a center more to the northeast--due to the orientation of the bands on the east side of the storm.
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WE HAVE TS JOYCE
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4354. Chiggy
150 year history suggests Fl landfall is rare..!!
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Well mission 7 has a bit of a long flight in to the storm. Should be there in a couple hours.
Member Since: August 17, 2005 Posts: 26 Comments: 15626
Quoting USCGLT:
I wonder how long it will be before the "Herbert" box discussion begins :)


Now you've done it! LOL
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Meanwhile in the North West Pacific basin...

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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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