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 AstroHurricane001:
Felix, 2007:



Isaac, 2012:



Hurricane Felix was one of the fastest intensifying hurricanes ever recorded in the Atlantic Ocean. It was the sixth named storm, second hurricane, and second Category 5 hurricane of the 2007 Atlantic hurricane season. Felix formed from a tropical wave on August 31, passing through the southern Windward Islands on September 1 before strengthening to attain hurricane status. A day later it rapidly strengthened into a major hurricane, and early on September 3 it was upgraded to Category 5 status Link
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Quoting MiamiHurricanes09:
Oh wow, does he have a blog? Could come very close to South Korea.

I'd honestly love to move to Taiwan...not only for the simplistic lifestyle, but to witness one of these massive cyclones.

My wife and I have been thinking about moving to the Philippines but after the summer they have had with all the flooding, it changed our minds. Our house about 45km north of Manila had about 6inch's of flood water through it and our house it about 2 feet above ground level. All our neighbours had about 3 feet of flood water in there houses.
Member Since: September 30, 2007 Posts: 9 Comments: 15979
Quoting NoNamePub:


DONE! Short Ribs - Brown rice with Shoyu and MAc salad....

Hoping this sucker avoids S. Fla - Own a house in Pompano....
I'm so hungry now- miss me some Brothers Cazimero music, Lei trading and all that beautiful Hawaiian spirit- there's nothing like it. I own a house near Sarasota so I might get a double whammy!
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Quoting MiamiHurricanes09:
Oh wow, does he have a blog? Could come very close to South Korea.

I'd honestly love to move to Taiwan...not only for the simplistic lifestyle, but to witness one of these massive cyclones.


I'm not sure if he has a blog or not. And yes they get some awesome storms out there. :)
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The fear I have, living in South Tampa is the potential for Isaac to slow down as it approaches the big bend of Florida. Debbie, being a minimal tropical storm and considerably smaller than Isaac, caused Hillsborough Bay to overflow its bank and flood Baysore Blvd near downtown. A prolonged SW fetch from a stronger storm with this size, would flood downtown and south Tampa extensively.
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Pretty big shift on the HWRF. It's on board for a Gulf storm now, it had mostly been staying east of FL or dissipating the storm before it got there:

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Quoting flowrida:
If the NHC projections follow suit when would Tampa start feeling the bad weather?
..probably outer bands maybe mon night, supposedly monday thru wensday..but..that could change, storm could slow down, could speed up
Member Since: August 6, 2011 Posts: 4 Comments: 42267
Quoting kmanislander:
Good afternoon

Pressure of 999 mbs at 16.1 and 61

What a confusing situation especially with winds nowhere near what you would expect with that kind of pressure
I believe the data is suspect due to the height of the aircraft, unless it's new information I haven't seen.
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Set of ULLs controlling Isaac....

Member Since: August 2, 2010 Posts: 21 Comments: 9882
If the NHC projections follow suit when would Tampa start feeling the bad weather?
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Quoting Hurricanes305:


Yup just read your blog Excellent short and to the point. COC is under Extremely intense convection We could see a Hurricane by 11am tomorrow. Thanks for clarification and stay safe man!
Thanks we will try!!
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I've only read one paper detailing the SREF's performance in forecasting tropical cyclone tracks near the United States, and that was only for the 2004 hurricane season, so I don't know how good their skill is now, but here's the SREF ensemble mean 87 hours out, showing Isaac over the northern Bahamas.

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Quoting MAweatherboy1:
NOAA plane reported 999mb, but they're at 10,000 ft so that's probably too low.
Yes, thats about 1004 at the surface
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Quoting NOLALawyer:
I think this storm will make the GOM. If and when it does, things could very well get quite interesting.
Honestly I think it's nearly a certainty that Isaac will make an encounter with the Gulf. Whether it is the Euro solution of missing southern Florida and intensifying massively to later affect the northern coast, or passing over southern Florida and just skirting along with western Floridian coast remains to be seen; but a recurvature east of Florida, or a poleward recurvature over Florida seems rather unlikely at this point.
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Quoting JLPR2:
Now what's Isaac doing?



Consolidating it time for its strengthening process to begin.
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Member Since: August 2, 2010 Posts: 21 Comments: 9882
Quoting ProgressivePulse:
I think the NHC is getting fed up as well.


8:00 PM AST Wed Aug 22
Location: 15.7°N 62.2°W
Moving: W at 21 mph
Min pressure: 1004 mb
Max sustained: 45 mph


000
WTNT24 KNHC 222038
TCMAT4

TROPICAL STORM ISAAC FORECAST/ADVISORY NUMBER 7
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
2100 UTC WED AUG 22 2012


TROPICAL STORM CENTER LOCATED NEAR 16.0N 61.2W AT 22/2100Z
POSITION ACCURATE WITHIN 30 NM

PRESENT MOVEMENT TOWARD THE WEST OR 280 DEGREES AT 19 KT


WSW jog?
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Quoting KeyWestwx:
i know! lots of locals look at projected wind speed path and think it will be a weak hurricane when it gets here though I'm afraid the models can't equate in factors like H20 temps and other things including random pockets of warmer water which could lead to a quick explosion is intensity- like charley did. This is a large storm which might take it longer to intensify though
yes, right now it sure looks like you folks will feel something from this storm, be careful down there and one good thing..your aware its coming, i asked my neighbor if he was going to board up his windows for the storm..he said..What storm..millions out there..just like him
Member Since: August 6, 2011 Posts: 4 Comments: 42267
Member Since: August 2, 2010 Posts: 21 Comments: 9882
Quoting Tazmanian:
i have noted that Isaac is S of his forcast points if he dos not start moveing more N he will likey miss PR


Watching the consolidation willl be interesting, Taz. If the southern half wins... Another shift to the left.
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i think i found its center. its the area lacking convection between the 2 blobs. around 15.5N and 62.4W moving off west at about 20mph
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Quoting AussieStorm:


Mate!!!!!
How is the shelter project in Haiti going, are they some what prepared for Isaac?


Not at all Aussie, not at all. If Isaac hits even the south peninsula as anything over a TS, it will be very very bad. Over a quarter miilion people still in tents after the earthquake and all this is going to do is reduce their numbers I'm afraid...

How have you been, mate? Long time (all my fault I'm afraid)...hows the back?
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979. GoWVU
Have not been on all day.... What does it look like for Charleston SC? Should we be ok or should I be worried? Thanks in advance
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Quoting MAweatherboy1:

I think it's undergoing/has been undergoing RI:



Yeah it could be, probably should be clear to intensify up until landfall.
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Quoting Grothar:
Intensifying




this thing is beasting
i'll have to keep you posted from my post, mind you
looks like a direct hit!
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Quoting RickWPB:
Here's the 18z TVCN:

I think the TVNC is the one consensus model that the nhc use and relies on quite a bit. I could be wrong.
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Good afternoon

Pressure of 999 mbs at 16.1 and 61

What a confusing situation especially with winds nowhere near what you would expect with that kind of pressure
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Quoting Bluestorm5:
The new wave is pretty good sized...

If ten becomes Joyce and that area at 10% becomes Kirk before August it will be 7 name storms before September that is amazing.
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NOAA plane reported 999mb, but they're at 10,000 ft so that's probably too low.
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Quoting JLPR2:
Now what's Isaac doing?



I'm waiting on the next steering map, but it should continue to drift South of due west for several more hours, unless something subtle has changed in steering.

It may yet make it back as far south as 15N, depending on the ridge and just where the CoC is when it finally closes off completely.
Member Since: January 25, 2012 Posts: 33 Comments: 1520
Quoting lobdelse81:
The large size of this storm reminds me of Gilbert in its formative stages. I believe someone else on this blog mentioned that.


Isaac has Ivan written all over him.
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Quoting JLPR2:
Now what's Isaac doing?



getting ready to dance reggaeton!
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Good grief!!!! Who let the Floodman in?!?!?!?!
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Quoting Tazmanian:

i have noted that Isaac is S of his forcast points if he dos not start moveing more N he will likey miss PR



all so has of note that it may all so miss Hispaniola this would be good news for them and bad news for others
Member Since: May 21, 2006 Posts: 5091 Comments: 115456
Quoting AtHomeInTX:


There's a WU blogger in Korea,Bobby, watching that one as we are Isaac.
Oh wow, does he have a blog? Could come very close to South Korea.

I'd honestly love to move to Taiwan...not only for the simplistic lifestyle, but to witness one of these massive cyclones.
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Issac looks just like Irene path when Irene was 3-4 days out. Wouldnt be suprised if Issac shifted its track east over next day or two. I believe Issac is going to feel the weakness and begain a NW turn over central or Northern Hispanolia. Issac structure appears to be organizing well. A strengthening system would drive it more poleward. Levis forecast track is still a favorable solution in my opinion.
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Quoting Jwd41190:
When do the new models runs come out? The newest ones I see are from 2pm still...


These videos explain a fair amount about the basics of the models including the times they come out and how to read them on a basic level. The next runs will start at 8pm or 00z, but we won't see the early cycles until between 9-10 pm; then we will see the globals between 12-2am.
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Quoting KeyWestwx:
ha, I moved here from Hawaii. I lived there when Ineki hit. Have a plate lunch with macaroni salad and 2 scoop rice for me!


DONE! Short Ribs - Brown rice with Shoyu and MAc salad....

Hoping this sucker avoids S. Fla - Own a house in Pompano....
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If one follows where the convection is headed (pointing), Isaac is actually right on track.
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Quoting LargoFl:
yes, with your water temps at 90, you dont want this near you
i know! lots of locals look at projected wind speed path and think it will be a weak hurricane when it gets here though I'm afraid the models can't equate in factors like H20 temps and other things including random pockets of warmer water which could lead to a quick explosion is intensity- like charley did. This is a large storm which might take it longer to intensify though
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Quoting JLPR2:


Seems male names are the problem this year.

bucks the trend of bad women names storms.
Member Since: September 30, 2007 Posts: 9 Comments: 15979
Intensifying


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Quoting Tazmanian:
i have noted that Isaac is S of his forcast points if he dos not start moveing more N he will likey miss PM


miss the night?
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T.C.F.W.
10L/TD/XX/CX
MARK
12.25N/37.69W
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HH RECON and NOAA is finding stronger wind on the N half
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Quoting yqt1001:
Tembin's eye is reappearing..just in time for landfall tomorrow. :/


I think it's undergoing/has been undergoing RI:

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Quoting NOLALawyer:
I think this storm will make the GOM. If and when it does, things could very well get quite interesting.


How goes it, my litigative friend?
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Quoting captainmark:
Has anyone posted on oil rig evacuations yet?
I am sure it is being considered.


Yes there have been a few on already who said they were prepping and watching.
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18Z GFS is ..well interesting..especially what goes on
in the atlantic while Issac is west coat fl scraping
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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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