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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2251. RitaEvac
3:30 AM GMT on August 23, 2012
The day 4 and 5 cone is very wide for good reason, but take note that the bulk of the cone is over the Gulf now, which is a tale tell sign that this is becoming more likely of a Gulf system downstream.

Member Since: July 14, 2008 Posts: 1 Comments: 9630
2250. FOREX
Quoting stormpetrol:
Just before I turn in for the night , IMO Isaac is a difficult forecast, basically where it is going is all up in the air at the moment, one thing same certain though, Florida is in the crosshairs , but the BIG ? is where it will make that turn , I suspect further west than the official forecast, just my take, Goodnight.


I also believe that a new center is trying to form and that the new center will stay south of the Dominican Republic and Haite. I see the storm turning more wnw eventually going over central Cuba. Once it re-emerges in the Gulf with maybe 50 mph winds and will reach a weak CAT2 before making landfall between Mobile Bay and Cedar Key. Just my take.
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2249. quante
Quoting CaneHunter031472:
I don't know about you guys and gals, but to me this look like a significant shift to the west and perhaps the usual shifting to the west trend.



Minor shift at most. And with everything clustered on Florida I would not bet against all those folks,their PhDs and supercomputers
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Winds are strongest on the north side.
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Interesting tight spin down around 14N. It is bound to be mid-level, but the only reason I brought it up is the fact that it's the only spin I can find. Lol.

Member Since: July 15, 2006 Posts: 17 Comments: 10247
It looks more and more like we can split the difference between GFS and Euro and go for Mobile to Destin to Big Bend landfall now.

NOLA-Biloxi is still in play, as is a rake of W Fl coast, but anything further W of Grand Isle or E of the Keys would be the outliers.

I'm calling my shot....let's see if it pans out.



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2245. GetReal
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Quoting GeauxGirl:


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.


When these kids can't play video games for 3 weeks and have to sleep in the heat they might change their minds.
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Quoting HoustonTxGal:


It does look like a small shift W in the pretty colors...lol

In MH09's scenario, it would lead to a track more to the East, not West.
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Quoting angiest:


The very active sea breeze indicates we are not totally protected.


However, steering patterns show that a storm moving that far west would take a track more like Ernesto's (except maybe a little farther north).
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2241. KEEPEROFTHEGATE (Mod)
Quoting Grothar:
Why wait?




it is almost time
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2240. scott39
For now....wind shear is flatening out the N side of Isaac. It is starting to ease up as he moves W. After he is far enough away, expect more organization.
Member Since: June 13, 2009 Posts: 0 Comments: 6775
2239. HrDelta
Quoting OneDrop:
Too soon?? LMAO


I was more thinking of Penn State, and the harshest attack I could concoct against them. I found one.

Issac is looking much better than even 2-3 hours ago. I wonder how bing the storm will end up being as it organizes. I know it will be large, but will it stay at "Dwarf Planet" size?
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Assuming the NHC center coordinates are correct, Isaac should pass near this buoy in the next few hours.
Member Since: March 7, 2007 Posts: 575 Comments: 20571
Hope Isaac doest whipe out the Anclote Island sand bar.........
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Quoting redwagon:

Just when I was nodding off... bring it, Isaac. Our atmo will run you down like a dog in the GOM and make you cry.


RED!
SSHUSH!
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2234. will40
Quoting floridaboy14:
its not that strong though and the actual center is farther south. this one has a good chance of slipping under cuba. lot will depend how much the ridge erodes



read it again he didnt say stronger he said larger system
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2233. quante
Issac moving briskly at 20 knots. Unless it slows,down. On this track issac will hit land before being very organized. Tough forecast scenario. A bit more west a ttad. Florida going to get some action. Where and how much stilll a big ?.?? Perhaps the perfect scenario of lots of rain for central,part of country. But Feel for the haitians. All that rain and living under tarps.
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I don't know about you guys and gals, but to me this look like a significant shift to the west and perhaps the usual shifting to the west trend.

Member Since: August 1, 2007 Posts: 0 Comments: 179
Quoting nrtiwInvragn:


This is going to Texas... Too many FL wishcaters around.


What kind of imposter are you?
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2230. Grothar
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Time to put down the Isaac for the night. Long nights a coming. One more before I go which includes the latest NHC track. Nice that SFWMD took away the annoying https tag. Just copy and paste now.

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Quoting opal92nwf:

I was talking about a direct hit. Although a partial impact like you're talking about would disrupt the convention as well.
Well it doesn't need to be a direct hit to be bad here in fact a storm skirting the coastline is the worst case scenario because of the storm surge and the water that would be pushed up onshore.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
Quoting caneswatch:


Same thing the chief met at WPTV said. I went "that's the same day though!"
LOL, you're in West Palm? I'm down south stuck with Max Mayfield hahaa.

But for real though, John Morales is the absolute best television Met in south Florida, without an once of a doubt.
Member Since: September 2, 2009 Posts: 130 Comments: 21193
Just before I turn in for the night , IMO Isaac is a difficult forecast, basically where it is going is all up in the air at the moment, one thing same certain though, Florida is in the crosshairs , but the BIG ? is where it will make that turn , I suspect further west than the official forecast, just my take, Goodnight.
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Quoting nrtiwInvragn:


This is going to Texas... Too many FL wishcaters around.


Not sure how it's FL wishcasters because the official cone still includes all of FL. I guess the NHC are Florida wishcasters? Texas isn't anywhere in the equation at this point. LOL!
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2224. GetReal
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Just seems to be like the Ernesto situation where a COC never materializes, keeps going west because of this, and Master's did bring that up as a possibility. Just moving too fast, and if this keeps up....all bets west. Now if it starts getting it's act together than yes the NW track seems reasonable, but when will that process start....
Member Since: July 14, 2008 Posts: 1 Comments: 9630
2222. angiest
Quoting galvestonhurricane:


Texas Death Ridge will prevent that from happening.


The very active sea breeze indicates we are not totally protected.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
2221. scott39
The 11pm track from the NHC has Isaac with less land interaction over all. If the trend with the track continues to go more W before the turn, then Isaac could miss alot of the mountains.
Member Since: June 13, 2009 Posts: 0 Comments: 6775
Quoting nrtiwInvragn:


This is going to Texas... Too many FL wishcaters around.

Just when I was nodding off... bring it, Isaac. Our atmo will run you down like a dog in the GOM and make you cry.
Member Since: August 4, 2007 Posts: 0 Comments: 3237
Quoting nrtiwInvragn:


This is going to Texas... Too many FL wishcaters around.


Texas Death Ridge will prevent that from happening.
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Quoting GTcooliebai:
That's a complete lie, Tampa has been affected by numerous hurricanes even recently, Frances and Jeanne in '04, Gabrielle in '01, Alma in '66, Donna in '60, Easy in '50 where a wind gust of 93 mph. was reported in New Port Richey and I'm sure there was winds sustained well past that, since it was a 126 mph. storm just offshore of Pinellas County.

I was talking about a direct hit. Although a partial impact like you're talking about would disrupt the convention as well.
Member Since: May 12, 2012 Posts: 11 Comments: 2608
2217. Grothar
Why wait?


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Quoting Tribucanes:
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.


Absolutely no offense taken. I apologize again. There are not that many people on here that I trust and respect, and you (and many others) are on here tonight.
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ULL Over eastern Cuba is interesting...may steer Isaac WNW to NW soon as it remains large and shallow at the moment
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2214. OneDrop
Quoting HrDelta:


You remember correct. Also, Accuweather was formed by Penn State Alums, and many meteorologists at Accuweather come from Kid Fondle U.
Too soon?? LMAO
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Off topic but

"Authorities late Wednesday were evacuating Baton Rouge residents near Interstate 10 where a tanker carrying flammable isobutane crashed early Wednesday as they prepared to burn off the substance and remove the truck." 2theadvocate.com
Member Since: August 23, 2010 Posts: 0 Comments: 1005
hmmmm... Im thinking the 0z gfs will be right in line with the center of our new cone of doom.

If so, just one question, are they getting the GFS ahead of time, so it can be plugged into the advisory?

Member Since: September 30, 2008 Posts: 0 Comments: 108
2211. KEEPEROFTHEGATE (Mod)
09L/TS/I/CX

Final T# Adj T# Raw T#
3.3 3.7 4.4
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Funny how the southward looking dive of convection makes it look like Isaac is shifting south. Convection can play tricks on your mind...
Member Since: May 12, 2012 Posts: 11 Comments: 2608


Isaac looks like the Goodyear blimp
night all.
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Hey Tallahassee, start getting ready.
Member Since: August 26, 2011 Posts: 0 Comments: 72
2207. GetReal


Isaac continues to develop a core, and if this trend continues I would not be surprised to see a 60 mph TS by daylight.
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Quoting opal92nwf:
"I don't think it's going to be a factor in this particular convention."

No disrespect, but Famous last words? ^^^^

It's been 91 years too late since a significant hurricane hasn't been a factor for Tampa.
That's a complete lie, Tampa has been affected by numerous hurricanes even recently, Frances and Jeanne in '04, Gabrielle in '01, Alma in '66, Donna in '60, Easy in '50 where a wind gust of 93 mph. was reported in New Port Richey and I'm sure there was winds sustained well past that, since it was a 126 mph. storm just offshore of Pinellas County.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
Quoting Levi32:


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.
its not that strong though and the actual center is farther south. this one has a good chance of slipping under cuba. lot will depend how much the ridge erodes
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting Levi32:


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 :P


Ima have to have a tidbit to understand that one. lol ;)
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2203. KEEPEROFTHEGATE (Mod)
T.C.F.W.
09L/TS/I/CX
MARK
15.51N/64.33W
R.I.FLAG FLAG
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Isaac doesn't need to go to the big bend area of FL. There are still roads impassable from the flooding 2 months ago. With all of the rain that has fell over the past few weeks, the water levels are not receding very quickly. More rain will only aggrevate the situation and bring flood levels back to or higher than they were.
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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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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.