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 noraneinsite:
I really enjoy reading analysis from some of the more informed members here and wait to see what unfolds down the road to see who was more accurate. So I think I speak for alot of lurkers when I say give it a rest. I have never heard Levi attack anyone for being wrong and I value his opinion as well as yours.

JMO Weather/tropics is never set in stone unless God Himself says it..been on this blog awhile and never seen Him on here. Weather is an imperfect science which makes it so interesting and up for debate. Think thats why we are all here. Again, JMO
Member Since: February 14, 2004 Posts: 2 Comments: 664
i think i may go with the ECW
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Quoting Drakoen:
Some day you will learn when you have a real job...there is a right and a wrong. The engineer cannot miss the minus sign or the bridge collapses. I'm gonna take a little break now.


Oh, meteorology is that clear cut? Gee if I hadda' known that I would have pursued a degree in something more challenging...
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Quoting NICycloneChaser:
18Z GFS is a bit faster than the 12Z, and slightly west. Very marginal difference in track, but it would give Isaac a little more space off the west coast of FL to strengthen.


All in all though, it shows exactly what thewindman was referring to. As much of a mess as this system is, and with the mountains coming up, and then fairly close proximity to Fl. Odds say that this will be a rain maker and that is about it.
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Hi, I live in West Palm Beach, Fl. What should I expect from this storm??
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Quoting Levi32:
It forever amazes me the inability of this blog to consider views different from the consensus of experts, models, what have you, just because it's not the generally accepted explanation. Does that make such views automatically wrong? Do I have to answer that?

I love meteorological debate but you guys make it so difficult to have fun tracking the weather.


Levi don't worry about it, some of them done the same thing to others on here before and chased them away. There's alot of folks that read and listen to what you say, You are good at what you do. Don't let anyone get to you. You stand by what you say cause in the long run when you do that instead of changing your forecast over and over that means something.

sheri
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the longer TD 10 stays weak the more it gos W and a less of a ch of going out too sea


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Quoting aislinnpaps:
Evening all, just home from long day of testing kids. I've read back a few pages. It looks like the track of Issac is still about the same, or did I miss anything?

Isaac is a very problematic storm and is still keeping everyone guessing, so no I don't suppose you missed anything!
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Quoting MAweatherboy1:
Don't ignore the consistency of the GFS:



Don't worry, the GFS is front and center ATM.

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Pretty strong storm on 18z:

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Quoting thewindman:
At this point whomever gets Isaac will be happy to get some wind and rain. By the time this storm gets its act together it will be over Hispaniola or Cuba breaking it apart to a 40mph disorganized mess and at best a mid strength TS hitting the U.S.
Just too dry in the Atlantic this year for any dangerous storms which in my opinion is a good thing
I honestly hope no one here is taking that comment to heart..... Sounds like you have no idea, and you may be putting someone in jeopardy.
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Member Since: July 15, 2006 Posts: 174 Comments: 54623
Quoting Stormchaser121:

Most of em went west. Im seeing the track of Issac moving ever so slightly to the west. Will it do the same??
Remember that the climate has been changing, and hence the patterns, The path and climatology charts are sometimes more of a setback then an aid. There is the old adage that statistics are only numbers that have no control over actual events...And it is true.
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18Z GFS is a bit faster than the 12Z, and slightly west. Very marginal difference in track, but it would give Isaac a little more space off the west coast of FL to strengthen.
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126 hrs. just offshore of Tampa Bay and I find the consistency of the GFS concerning, since we're getting into that 5 day time frame.

Member Since: August 31, 2009 Posts: 0 Comments: 5628
Some day you will learn when you have a real job...there is a right and a wrong. The engineer cannot miss the minus sign or the bridge collapses. I'm gonna take a little break now.
Member Since: October 28, 2006 Posts: 57 Comments: 30575
Quoting hurricanehunter27:
Here we go. Starting to see popcorn convection.



Slight pop up T-storms creating a possible bridge to those two ball of convection. This needs to organize its CDO before it can crank up the NE quad.
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Quoting clwstmchasr:


Last pass recon has it at about 15.4 to 15.5. We're not going to see it down to 14.8.


Maybe not, but steering still calls for potentially more southerly jogs and wobbles.

1000mb




990mb




This is why the Euro thinks it's going farther west, not to mention a continental high over the SE U.S. during the final approach.


Too early to know for sure until it organizes a bit more though.
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483. TXCWC
Quoting MAweatherboy1:
Don't ignore the consistency of the GFS:



Nor the consistency of Euro. Ignoring either one would be a mistake as both are very possible solutions at this point - esp. given Isaacs recent core issues and lack of any real intensification - thus far.
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THE INITIAL MOTION IS NOW A SOMEWHAT UNCERTAIN 280/19. OTHER THAN
THE CENTER POSITION ISSUES...THERE IS NO SIGNIFICANT CHANGE TO THE
FORECAST REASONING DURING THE FIRST 48 HR OF THE TRACK FORECAST. THE
EAST-WEST ORIENTED SUBTROPICAL RIDGE TO THE NORTH OF ISAAC ALONG
30N LATITUDE IS FORECAST BY ALL OF THE GLOBAL MODELS TO REMAIN
INTACT ACROSS FLORIDA AND THE SOUTHEASTERN U.S. DURING THAT TIME.
THIS EARLY PART OF THE FORECAST TRACK IS SHIFTED A LITTLE TO THE
SOUTH OF THE PREVIOUS FORECAST BASED ON THE OBSERVED POSITION AND
MOTION. AFTER 48 HR...THE RIDGE IS EXPECTED TO WEAKEN AS A
MID/UPPER-LEVEL TROUGH MOVES ONTO THE EASTERN UNITED STATES. THE
GLOBAL MODELS HAVE COME INTO BETTER AGREEMENT SINCE THE LAST
ADVISORY...WITH THE NOGAPS AND CANADIAN MODELS SHIFTING TO THE WEST
AND THE ECMWF TO THE NORTH. HOWEVER...THE ECMWF CONTINUES TO SHOW
A STRONGER RIDGE...AND THUS SHOWS A MORE WESTERLY MOTION THAN THE
OTHER MODELS. THE 72-120 HR PORTION OF THE FORECAST TRACK IS
SIMILAR TO THE PREVIOUS FORECAST AND LIES NEAR THE CONSENSUS
MODELS. THERE REMAINS CONSIDERABLE UNCERTAINTY AS TO WHICH PORTIONS
OF FLORIDA...IF ANY...COULD BE AFFECTED BY ISAAC.

Or as my other half said after watching one update on TWC.... " They don't know where its going...."
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Quoting TropicalAnalystwx13:
Drak, I don't know what's gotten into you today, but you need to cool it. You may not agree with Levi, but to act childish towards his opinion is uncalled for. This is a tropical weather blog for weather debates, not an I'm-right-you're-wrong blog.
Quoting Tribucanes:
Drakoen your being a right fighter with condescending comments to Levi. I think you and Levi are brilliant, but if anyone is bringing down the quality of the blog at the moment it's you. Why be nasty to Levi because he disagrees with you? Never seen you so arrogant before, maybe if you left the quality would go up.
Have I missed the posts where Drak was condescending/childish/nasty towards Levi?
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Quoting noraneinsite:
I really enjoy reading analysis from some of the more informed members here and wait to see what unfolds down the road to see who was more accurate. So I think I speak for alot of lurkers when I say give it a rest. I have never heard Levi attack anyone for being wrong and I value his opinion as well as yours.

What I don't understand is why forecasting on here has to be such a competition. And I'm not addressing anyone in particular with that, just the blog in general. We're a bunch of amateurs here to learn. No one cares if you're forecast turns out wrong. We're learning and having fun.
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Quoting thewindman:
At this point whomever gets Isaac will be happy to get some wind and rain. By the time this storm gets its act together it will be over Hispaniola or Cuba breaking it apart to a 40mph disorganized mess and at best a mid strength TS hitting the U.S.
Just too dry in the Atlantic this year for any dangerous storms which in my opinion is a good thing


Don't quit your day job
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Looks like a center reformation near Dominica.

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Member Since: July 15, 2006 Posts: 174 Comments: 54623
Quoting MAweatherboy1:
Don't ignore the consistency of the GFS:

Joyce out to sea(Looks to be affecting Bermuda) and Kirk in the making.Isaac hates Florida...
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Member Since: July 15, 2006 Posts: 174 Comments: 54623
Member Since: July 15, 2006 Posts: 174 Comments: 54623
May be an indication of the true position of the center of Isaac ....

I think it fits more with the presentation of Isaac satelitte.

CIMSS 850mb


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472. TXCWC
GFS consistent today with a large FL West Coast rider.
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Quoting stormpetrol:
It appears to me that even REcon is having a hard locating a true center, just sayin....


I am not completely convinced that this is even technically a "tropical storm" at this point. Not saying it won't ever get its act together, but right now it very closely resembles a very weak TD, or possibly an open wave. Good microwave would be nice.
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Quoting Drakoen:


I think you have supplanted some of the so-called experts with your simple post about the dry air. Have a good one.
I really enjoy reading analysis from some of the more informed members here and wait to see what unfolds down the road to see who was more accurate. So I think I speak for alot of lurkers when I say give it a rest. I have never heard Levi attack anyone for being wrong and I value his opinion as well as yours.
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Don't ignore the consistency of the GFS:

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Drakoen your being a right fighter with condescending comments to Levi. I think you and Levi are brilliant, but if anyone is bringing down the quality of the blog at the moment it's you. Why be nasty to Levi because he disagrees with you? Never seen you so arrogant before, maybe if you left the quality would go up.
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Local Miami radio station is throwing the dart. It may be going up the east coast, but the florida keys are in for it and if they are it is going to curve and head to tampa. Bloody hell
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oh yeah ... I heard the hurricane plane above me also. NOBODY else flying right now.

Though we did just see the ferry from Guadeloupe arriving.
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Member Since: July 15, 2006 Posts: 174 Comments: 54623
Post. 446 I wonder why they didn't push for an earlier start date like say in June for the RNC, as opposed to now as we enter into the heart of the hurricane season, knowing very well that activity increases now?
Member Since: August 31, 2009 Posts: 0 Comments: 5628
Here we go. Starting to see popcorn convection.

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Drak, I don't know what's gotten into you today, but you need to cool it. You may not agree with Levi, but to act childish towards his opinion is uncalled for. This is a tropical weather blog for weather debates, not an I'm-right-you're-wrong blog.
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Quoting Drakoen:


You're right nobody is missing their baby daddy. This is a weather blog. I'm surprised this is even up for debate the quality of this blog has really gone down.
Lol.

I hope by quality of the blog you weren't referring to Levi and the debate you two are having now. It is obvious the upper level environment is limiting outflow and thunderstorms to that side. Some dry air is also present in the low to mid levels. All factors at all levels of the atmosphere have an influence on the storm, so to exclude one or the other is wrong. In other words, you're both right.
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Member Since: July 15, 2006 Posts: 174 Comments: 54623
18z GFS basically has a southern Florida landfall, then a poleward turn to scrape the western Floridian coast. Best...or worst...of both worlds.
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Quoting KEEPEROFTHEGATE:


All of S. Florida is getting to worse side of the storm the East side.
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well here I am in Dominica. NOTHING in the way of wind yet.

I am guessing ... maybe that the center is forming right above my head.

http://www.wunderground.com/weatherstation/WXDail yHistory.asp?ID=IXXPORTS2 my station.
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Evening all, just home from long day of testing kids. I've read back a few pages. It looks like the track of Issac is still about the same, or did I miss anything?
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HH RECON I think found one at 15.7N 61.4W

NOAA's P-3 is near antigua flying SW likely going to check out that area near 14.8N 63W
Member Since: June 13, 2009 Posts: 2 Comments: 12180
Quoting TXCWC:
"...THERE REMAINS CONSIDERABLE UNCERTAINTY AS TO WHICH PORTIONS OF FLORIDA...IF ANY...COULD BE AFFECTED BY ISAAC..."

NHC is not discounting Euro - nor should they as long as it too is being pretty consistant right now.

Wow... seeing them actually say it... that's strong language.
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GFS not too different than the last run as far as impact with FL goes.
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At this point whomever gets Isaac will be happy to get some wind and rain. By the time this storm gets its act together it will be over Hispaniola or Cuba breaking it apart to a 40mph disorganized mess and at best a mid strength TS hitting the U.S.
Just too dry in the Atlantic this year for any dangerous storms which in my opinion is a good thing
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Quoting stormpetrol:
It appears to me that even REcon is having a hard locating a true center, just sayin....


That's been the case all day. There are a couple of different circulation centres, which will probably end up meeting somewhere in the middle. Doesn't mean the circulation isn't closed.
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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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