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 Levi32:


So far in my "career" it has served me well more times than it has bitten me in the rear.


Well you certainly shouldn't shift your track at the whim of every single model shift. However, Isaac is struggling to organize, which makes me think the Euro solution is a little more believable. Perhaps not to Louisiana, but the Florida panhandle or points thereabouts? Sure.
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Quoting KoritheMan:


No, because then you have to take into account land interaction, which could easily result in a center reformation, or at least erratic motion.
Damn, that is true. Do you believe that a center reformation would really dramatically impact the track though? I couldn't imagine that half a point of longitude/latitude would really change the situation drastically.

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Quoting JLPR2:


First we need a defined circulation.


lets call HAARP
Member Since: August 22, 2006 Posts: 2 Comments: 1975
Quoting PensacolaDoug:






inedible? To quote Inigio Montoya: "I don't believe that word means what you think it means!"

Inevitable... smart phone action
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Quoting JLPR2:


First we need a defined circulation.


No offense to the guy who said it, but people are way too quick to call rapid intensification. I don't quite think they understand the mechanisms that have to be in place for that.
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1144. pottery
Quoting BossCane:
Isaac really looks to be consolidating now... All we need is a blow up of convection and it should intensify pretty quickly.

I see plenty of convection.???
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Quoting Floodman:


The smart money says Folly Beach...something about God being very upset with some guy over that way


^^^^ See what happens when you let an adjuster in the room....
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The pressure reading is not legitimate as the recon is at a 10,000 ft altitude. Realistically, the pressure is 1003-1004mb.
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1141. LargoFl
Quoting ILikeIke:
meh, i dont think so. Every time I check the cone keeps going more west.
there is a chance the storm crosses over florida into the atlantic at some point..everyone on the gulf coast and up the eastern seaboard needs to watch this storm til it dies out
Member Since: August 6, 2011 Posts: 4 Comments: 37064
Quoting Hurricanes305:


I HAVE BEEN SAYING THAT SINCE 8pm the sudden drop in pressure is a response to a sudden monster blowup of convection RAPID INTENSIFICATION could be COMMENCING!!!


no need to shout
Member Since: August 22, 2006 Posts: 2 Comments: 1975
Quoting Chicklit:


Looks like the east flank is jumping into the Caribbean finally. Now they can unite in bathwater...


50 mph at 11pm with 1001 mb pressure. Possibly a Cat 1 tomorrow mourning.
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Quoting serialteg:


well its getting closer, this morning it was further away


This much is true.
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1136. Levi32
Quoting KoritheMan:


You have the tendency to stick to your guns, don't you? I admire that. :P


So far in my "career" it has served me well more times than it has bitten me in the rear.
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1135. hydrus
I type this with cautious enthusiasm, but the latest satellite loops look a little better for Haiti, if this trend continues.
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Quoting Floodman:


Outstanding...I still have a good number of them around somewhere. I was able to simply walk away (after 15 years of off and on scripts for them I was quite proud of that) and I was never cross nor I do anyone harm...LOL

Fingers crossed that the therapy works...I wish you the nest of luck moving forward. One thing: once you feel 100%, act like you're 100%. Don;t be dainty and bereal the habit that years with that kind of pain gives you to avoid physical things...

Mate, once I start physically feeling 100% I'll start to feel better mentally. In fact, I told my wife, once it's gone, We are going to the Philippines and I'm going to trek up Mt Pinatubo and Mt Arayat, both are very close to our place. That will be my reward for getting through this period in my life.
Member Since: September 30, 2007 Posts: 9 Comments: 15932
1133. JLPR2
Quoting Hurricanes305:


I HAVE BEEN SAYING THAT SINCE 8pm the sudden drop in pressure is a response to a sudden monster blowup of convection RAPID INTENSIFICATION could be COMMENCING!!!


First we need a defined circulation.
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for the 4th night in a row, everybody is all "hes bout to blow, convection firing, yada yada, etc" its moving to fast all its going to do is blow up a ball of convection, displace it to the south because its moving to damn fast! its just going to maintain 40mph until it slows down which will probably be near hispanola, by then it will have problems with the mountains. its not going to strengthen signifigantly until in either enters the gulf near key west, the channel, or the east side of florida, which one who knows, looking more like the eastern gulf though! who knows a few wobbles and formations can put it east of florida or into the channel into the central gulf. Irene was week and moved further north than isaac, so this whole weaker the storm further south thing is usually the case but not all the time, even shallow storms feel weaknesses and troughs.
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Quoting allancalderini:
Last year were the women names now is turn for the males ;)


Still a long way to go... It hasn't developed yet... structurally it hasn't organized.... And Isaac is going to cross the highest peak and a relative big portion of land - Hispaniola..... Then the E tip of Cuba... From then on, the existing conditions and variables will be the ones to determine where it will go and how intense it will turn to be... So...

do not suffer through the game of chance that plays....
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Quoting Levi32:


I still think it could shift back. Goodness knows we've seen several storms in this position "nailed down" 96 hours in advance only to have the models shift by hundreds of miles during the final 72 hours. If the models over the next 48 hours shift farther westward then I'll probably have to shift, but it's the cat and mouse game every track forecaster plays with the computer.


A very recent reminder of that thought process: go back to Hurricane Ike. Enough said.
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Quoting drg0dOwnCountry:
This is ridiculous Post 726 is removed because why? Community Standards? The post was to this post about the storm track with quotes from Dr Masters. Who is in charge here of the moderation? Where can i complain about this?




Minused into oblivion? LOL!!!
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Quoting shfr173:
Am I understanding correctly, we have intensification now?

uh, well unification anyway and that usually means intensification in cyclogenesis-speak.
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Quoting Levi32:


I still think it could shift back. Goodness knows we've seen several storms in this position "nailed down" 96 hours in advance only to have the models shift by hundreds of miles during the final 72 hours. If the models over the next 48 hours shift farther westward then I'll probably have to shift, but it's the cat and mouse game every track forecaster plays with the computer.


You have the tendency to stick to your guns, don't you? I admire that. :P
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Quoting MiamiHurricanes09:
Everyone agree we'll have a near-certain handle on Isaac by Friday night (should be passing between Hispañola and Cuba at that time)?


No, because then you have to take into account land interaction, which could easily result in a center reformation, or at least erratic motion.
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Quoting sunlinepr:


This morning it looked more scary.... will be for us similar to Irene...


well its getting closer, this morning it was further away

Member Since: August 22, 2006 Posts: 2 Comments: 1975
1122. shfr173
Am I understanding correctly, we have intensification now?
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Quoting TropicalAnalystwx13:

Which means the pressure readings will be more accurate.

999.1 millibars confirms that Isaac has begun intensifying.


Looks like the east flank is jumping into the Caribbean finally. Now they can unite in bathwater...
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Quoting Hurricanes101:


but it is going lower, which could mean that the overall pressure is


I HAVE BEEN SAYING THAT SINCE 8pm the sudden drop in pressure is a response to a sudden monster blowup of convection RAPID INTENSIFICATION could be COMMENCING!!!
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1119. LargoFl
Quoting icmoore:


Wow really that is crappy....we are in Madeira Beach just down the road.
..its great that alot of us from the same general area are on here..watching and reading..so many are still saying..what storm
Member Since: August 6, 2011 Posts: 4 Comments: 37064
I guess the Nern center won. That convection is racing there
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1117. Levi32
Quoting NYCyclone86:


Yeah I hear you on that one. I was the dark horse on this site and I was agreeing with your track in the beginning 100%. But now with trends naturally I am second guessing but I feel things will trend back again. Do you believe so? Or are we starting to settle on a south west Florida/ East gulf track? I ask these questions to increase the intellectual conversations on the blog so it stays focused and informative.


I still think it could shift back. Goodness knows we've seen several storms in this position "nailed down" 96 hours in advance only to have the models shift by hundreds of miles during the final 72 hours. If the models over the next 48 hours shift farther westward then I'll probably have to shift, but it's the cat and mouse game every track forecaster plays with the computer.
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Quoting mcluvincane:


Its inedible, the NHC will change their cone south and west. This will likely be a central gulf coast to Florida panhandle strike






inedible? To quote Inigio Montoya: "I don't believe that word means what you think it means!"
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1115. icmoore
Quoting Floodman:


We are very good, all things being equal...unfortunately she is in PA right now and I'm back in Texas, but we are hoping to remedy that situation very soon...

As for me being a gentleman, that depends on who you talk to...LOL

So when are you doing a book of the amazing photos you've taken? I have a fair collection of them myself...


Aw I hope you are both back in the same location soon. Ack I am just someone having fun with a camera, get your mind out of the gutter Press, but thank you for asking!
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Curious what you all think.... How likely do you think South Florida is going to be hit? I went to open house at my son's school and the parents were talking about how it isn't going to hit... A lot of them were saying that it is going to be a really weak storm too...
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I acknowledge most of us are very focused on Isaac at the moment, but has anyone taken a look at western Pacific's Typhoon Bolaven and how huge it is? The convection is also very intense with very cold cloud tops.
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Quoting hydrus:
Floodman....funny how you roll in at the same time i,m rollin.....Hope you are well ..:)


I'm good brother and I'm glad I got to see you...how have things been?
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1109. MahFL
The western banding is sure intense.
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Everyone agree we'll have a near-certain handle on Isaac by Friday night (should be passing between Hispañola and Cuba at that time)?
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Quoting lurkersince2008:
Is there a trough or something that would keep Isaac away from central Gom?


There is a trough, but it might not stick around for long. The weaker Isaac stays, the less pull it gets from the trough also.

No one is out of the woods from Louisiana to the Carolinas.
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Quoting Bluestorm5:
Oh boy... I got question. Is that the eyewall Isaac is building up?


Eyewall, what eyewall???
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Quoting MiamiHurricanes09:
Wow...that is blatantly awful. I'd assume you'd just have to really, really prepare yourself for such events by constructing a house that could withstand such weather disasters. But that's just all a part of living there, like hurricanes are to south Florida, earthquakes to California, and so forth.

All the houses in our Village are made from Cement blocks and have tilled or concrete floors, so they clean up very easily and the flooding didn't just appear it gradually rose up so things where able to be moved so very few things actually got damaged, in our house anyways. We opened our house to our neighbours that had there house flooded out until the water went down, my sister-in-law told us there was about 20 people, about 4 families, staying in our house. We are one of the more fortunate properties in the village and we are more than happy to help other's.
Member Since: September 30, 2007 Posts: 9 Comments: 15932
Quoting CybrTeddy:
The 997mb pressure was found at 10,000 feet as the Air force is out of there.


but it is going lower, which could mean that the overall pressure is
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Quoting serialteg:


looks scary sunline


This morning it looked more scary.... will be for us similar to Irene...
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Is there a trough or something that would keep Isaac away from central Gom?
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The 997mb pressure was found at 10,000 feet as the Air force is out of there.
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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.