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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Very interesting to see the effect that the NE ULL used to have on Isaac... you can notice how it wraps moisture left behind in the E tail of it... But no longer affects its COC... Now the ULL over Cuba is ahead and will determine or affect Isaac's path...

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Did anyone besides me notice that it is almost always the male species of the blog who create the "blow ups"?

I think that answer is to have a few more of the female species on board.
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Quoting MysteryMeat:


Putting a smiley on a post that predicts a rapidly intensifying hurricane in the GOMEX where it will cause significant damage and loss of life.

I'm woozy from the class being shown.


You have to wonder as to the age of certain posters. Anyone who has suffered through a Katrina or Andrew-like storm will tell you there is nothing to :) about.

That said, Isaac is making me a little nervous. He's a big boy. If he doesn't lose a few "inches" the COC landfall won't be the only worry.
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Quoting 7544:
looks like a hurricane just south of so fla depending on how strong he will be at that time so fl still not out of the woods by a long shot . and they prob will relocate the center several more times imo .


No, I think this is it. Whatever center is there now will be the definite one, barring Hispaniola. Isaac is organizing now.
Member Since: March 7, 2007 Posts: 521 Comments: 19127
2196. HrDelta
Quoting SubtropicalHi:


Accuweather messed up with Gilbert. The day b4 Gilbert made landfall they predicted it would make landfall near Freeport, TX, which is near Galveston.

Galveston was evacuated.

Gilbert made landfall in Mexico. The NHC forecast was for the Brownsville area.


I really thank you for informing me of that. :) It is sad I have but one upding to give.

I was born in 1990. So my first Hurricane Season (I live in Colorado Springs, but much of my dad's family is in Florida) was 2004. Also, I didn't much have an interest in meteorology before then either.
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Quoting Felix2007:


Wait what? Was my impostor in there again?


not really... not as you lol... just come in.. you too trib!
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Quoting Chicklit:
oh oops, thanks.
then the latitude was same at 5 p.m. as it is now!
in other news:
Tampa Mayor Bob Buckhorn said his city is prepared.

"We have contingency plan after contingency plan," he said. "We are ready in the event that it happens. I don't think it's going to be a factor in this particular convention. But we are prepared in the event that it is."

really?! I mean really?


Well, he is the mayor, one could not expect him to tell everyone to stay home" at least not this early". "It not being a factor", well that is a little much.
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Quoting MiamiHurricanes09:
Thank you.

I'm going to refer towards the dynamical data to paint a more fluid picture of my thinking; basically, the slightest of poleward wobbles after emerging from Hispañola would result in the cyclone missing Cuban terrain and essentially scraping along the coast. I think it's a more likely solution than it's currently being given credit for. Going to come down to the wire Friday night into Saturday morning. I see some all-nighters in store hahaa.



Same thing the chief met at WPTV said. I went "that's the same day though!"
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Quoting Hurricanes101:


I really dont think the trend will continue, there is no evidence as Levi was saying that shows this system will go as far west as the Euro says it will, I think the track will be fairly close to what they have now

even if it goes further west, Isaac is and will be huge and will still dump tons of rainfall on Florida; which is really bad for areas already having flooding issues


I wonder if the Euro is right mainly because Isaac is a large system that is still moving very quickly. I also see how many of the other storms this season have struggled in the Caribbean and wonder if Isaac will struggle also.
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2190. Gorty
Isaac looks much better organized now than several hours ago.
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galvestonhurricane all good, thought you were being serious. :) I'm not ashamed to admit that I'm a dad without a lot of social interaction these days; these are my friends, my social outlet, and the such. Sorry to jump down your throat. I'd never seen you post anything like that before and I was taken aback. Sorry again for not asking for clarification.
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Quoting scott39:
The smiley fast is for IMHO. Your are twisting it for your own pleasure.


A smiley face has no place anywhere on a post that forecasts likely death and destruction for someone on the Gulf Coast.
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Quoting sunlinepr:


White specs.
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2185. Thrawst
Quoting sunlinepr:


got the tropical storm symbol sideways on the last frame... i think that signals something :P
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2184. Levi32
Quoting RitaEvac:
This storm is getting further and further west by each hour of over 15 mph. The storm is large and weak and will be hard for a storm like this to turn, it's basically top heavy and like a bowling ball once the momentum is one way it usually goes that way


Except that the vertical integral of air mass over a larger storm (and thus a larger depression in the air pressure field) is actually smaller compared to the same area around a smaller storm :P

Larger storms can get caught easier by troughs, actually.
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0z GFS soon. you guys ready?
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2182. GetReal
Member Since: July 4, 2005 Posts: 204 Comments: 8802
2180. 7544
looks like a hurricane just south of so fla depending on how strong he will be at that time so fl still not out of the woods by a long shot . and they prob will relocate the center several more times imo .
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Quoting RTSplayer:
CMC says it's going east.

That's as good a reason as any to believe it won't be going east.


Can't Model Crap?
Member Since: March 7, 2007 Posts: 521 Comments: 19127
Quoting RitaEvac:
This storm is getting further and further west by each hour of over 15+mph. The storm is large and weak and will be hard for a storm like this to turn, it's basically top heavy and like a bowling bowl once the momentum is one way it usually goes that way

Agree!
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Quoting Dragod66:
felix....? tropics chat? or anyone for that matter? lot of crazyness on here right now!


Wait what? Was my impostor in there again?
Member Since: July 12, 2008 Posts: 7 Comments: 383
CMC says it's going east.

That's as good a reason as any to believe it won't be going east.
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Quoting Clearwater1:
Looks like he may get to keep his job, if the westward trend continues. Or maybe they won't chance it.


Too early to call it, but it is starting to look that way, which means that it will now become a nightmare for me.
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Quoting sunlinepr:


Looks like that NE quadrant finally wants to show itself. Time for some strengthening.
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Quoting GTcooliebai:
Intensity models are all over the place with Isaac:



I feel like the SHIPS model has been too aggressive with every storm this season. Ex: Ernesto
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Quoting GTcooliebai:
I noticed the middle of the cone is line with the last GFS run.



I almost believe that the NHC could simply use GFS track for it's exact track. You are right. The cone and position are very much the same.
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Quoting galvestonhurricane:


Wow. I didn't even make the thing. It seems like everyone is trying to complain or argue about every little thing on the blog tonight. Honestly, I'm sick of it and going to bed.

I just thought it was funny.

"If the true meaning of significant data is detrimental, then it's appearance on file does not warrant a thought."
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I am wishcasting this storm to myrtle beach...we need an infusion of federal money to put all these pan handlers to work clearing streets and such!

Seriously, I think the storm can still hang off FLs east coast if the WNW and NW turns can be influenced ahead of time by the ULL over cuba.
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2168. DDR
Quoting JLPR2:


The islands are going to end up as swamps soon. :\

Also a feeder band is currently over Pottery's area.

Tobago's getting slammed.
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Quoting MiamiHurricanes09:
Thank you.

I'm going to refer towards the dynamical data to paint a more fluid picture of my thinking; basically, the slightest of poleward wobbles after emerging from Hispañola would result in the cyclone missing Cuban terrain and essentially scraping along the coast. I think it's a more likely solution than it's currently being given credit for. Going to come down to the wire Friday night into Saturday morning. I see some all-nighters in store hahaa.



It does look like a small shift W in the pretty colors...lol
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I post what I see....and sometimes just my hunches. I think we're ok doing that. I don't claim to be a met just a very interested observer. No offense to anybody.
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It's entertaining in here tonight.
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Quoting chevycanes:

it was at 62.2 at 8pm 60.2 at 5pm.
oh oops, thanks.
then the latitude was same at 5 p.m. as it is now!
in other news:
Tampa Mayor Bob Buckhorn said his city is prepared.

"We have contingency plan after contingency plan," he said. "We are ready in the event that it happens. I don't think it's going to be a factor in this particular convention. But we are prepared in the event that it is."

really?! I mean really?
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This storm is getting further and further west by each hour of over 15 mph. The storm is large and weak and will be hard for a storm like this to turn, it's basically top heavy and like a bowling ball once the momentum is one way it usually goes that way
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Quoting RTSplayer:


Commentator Jinx:

Queue RI...



Wait, that only applies to sports...
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In all reality, we're really not going to have a clear picture on the exact track or intensity until splash down in the Atl. The width of FL is little more than a 24 hour error cone.
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Intensity models are all over the place with Isaac:

Member Since: August 31, 2009 Posts: 0 Comments: 5628
Thinking that I may need to go over my storm list. So thankful that we have a generator with multiple outlets and a small ac unit..lol
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Quoting KoritheMan:


First of all, I don't see it taking that path unless it deepens significantly over the next day or so. It appears to be organizing now, but we'll see if that is just a hiccup or not. Furthermore, even if he does organize, I still think the trough over the eastern US is weakening a little based on water vapor loops. UW-CIMSS data seemed to support this idea as well.

But assuming he did take that track, that would likely mean a much stronger storm for Florida, and possibly the Carolinas as well (but more so Florida since the pattern doesn't appear amplified enough for a sharp recurve after landfall).
Thank you.

I'm going to refer towards the dynamical data to paint a more fluid picture of my thinking; basically, the slightest of poleward wobbles after emerging from Hispañola would result in the cyclone missing Cuban terrain and essentially scraping along the coast. I think it's a more likely solution than it's currently being given credit for. Going to come down to the wire Friday night into Saturday morning. I see some all-nighters in store hahaa.

Member Since: September 2, 2009 Posts: 130 Comments: 21032
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Quoting LBAR:
Hahaha. Jim Cantore said it's "basically an open wave". Yikes.


Commentator Jinx:

Queue RI...
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Quoting HrDelta:


Accuweather failed both ways in 2005. They made predictions that Katrina would go into Florida, while bashing the NHC.

FEMA used the NHC, but the Department of Homeland Security used Accuweather. As a result, at least 30 people died in the catastrophic week after Katrina.

Then they hyped up Rita hitting Houston, causing a hysteria induced evacuation. Killing somewhere between 90-117 people.

There are many companies I get ticked at for various reasons. The Super-Banks, Big Oil, and that company that sucked up Union Carbide (Bhopal). But no company on the planet makes me more angry than Accuweather. For the inaccuracy of reports, then lobbying to sabotage the NWS. For Katrina and Rita, I wish Accuweather could be sued.

I f---ing hate Accuweather. With the fury of a thousand suns.


Accuweather messed up with Gilbert. The day b4 Gilbert made landfall they predicted it would make landfall near Freeport, TX, which is near Galveston.

Galveston was evacuated.

Gilbert made landfall in Mexico. The NHC forecast was for the Brownsville area.
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as I said I'll wait for huurers to fly in in about 3 hour time
Member Since: June 13, 2009 Posts: 2 Comments: 9592
Quoting Tribucanes:
No, I'm nice to everyone except those who spread propaganda and lies. Those I eat, bones and all. Jerk to everyone much?


Hey, calm down. I was messing with you - sorry for not being clear and coming off as rude.
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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.