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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The 997mb pressure was found at 10,000 feet as the Air force is out of there.
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So... what's 997 mb meant at 10,000 feet? Dang...
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Quoting AussieStorm:

Well, I'll be on a course of those magic pills for 6 months then slowly come off them and hopefully once I do my nerves wont be as sensitive and I'll be fine. That's the hope anyways.


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...
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Quoting Levi32:


And yet no wind shift yet southwest of the pressure center lol.

i have no idea how to use and understand that lol im just following what you guys are reporting :)
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1096. LargoFl
Quoting DaytonaBeachWatcher:


I dont like that one... pick another one.... LOL
..gee in years past i used to trust the GDFL model, especially when storms got close to florida
Member Since: August 6, 2011 Posts: 4 Comments: 41581
Quoting JLPR2:


Rainbow or not, that's some very strong convection.



He is getting ready to blow
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Quoting ILikeIke:
Good looks Isaacs not commin to NC.

Be aware carolinas still in play.. more unlikely yes but not out of the question...
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Quoting Bluestorm5:
Oh boy... I got question. Is that the eyewall Isaac is building up?



I have been saying there is a monster explosion of convection RIGHT OVER the new COC clearly it the dominating circulation and any other spin will be absorb also its align itself a bit more with the MLC and vertical stack should become better defined. RI is starting to commence just watch.
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Isaac really looks to be consolidating now... All we need is a blow up of convection and it should intensify pretty quickly.
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Quoting CybrTeddy:
Anyone else notice the new circle on the map? This is what the ECMWF/GFS are developing.
The left one is Isaac in Lousiana the right one is TD #10
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1090. HrDelta
Quoting winter123:
Isaac is the scariest 45 mph storm I've ever seen. Such a large wind field, I feel hispaniola couldn't kill it.


I was wondering the same thing. Once Issac gets organized, there is going to be problems.
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1089. icmoore
Quoting Clearwater1:
I can't remember what storm, but I evacuated Indian Rocks Beach, maybe around '95 or so. The island, except for a few police officers, was empty. However, some clever thieves boated over to the mostly vacant island and robbed a bunch of homes. Crazy world.


Wow really that is crappy....we are in Madeira Beach just down the road.
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1088. JLPR2
Quoting serialteg:


its also on rainbow (deceiving). im on puerto rico's southermost central side and i have only seen light rain and winds so.


Rainbow or not, that's some very strong convection.

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Quoting sunlinepr:


looks scary sunline
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He is definately building convection now....
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1085. Levi32
Quoting TropicalAnalystwx13:

Which means the pressure readings will be more accurate.

999.1 millibars confirms that Isaac has begun intensifying.


And yet no wind shift yet southwest of the pressure center lol.

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Good looks Isaacs not commin to NC.
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Quoting Relix:


Hahahaha. Waneko... what the heck :p

Only expecting rains and very light winds from Isaac. Feel kinda smart that I didn't go out and bought 2 boxes of sausage cans XD! Just stocked on water and gasoline, really wasn't expecting much from it. Guess a part of me wanted a surprise like last year with irene.


well i really want some action. last year the COC of a storm got into PR and i was expecting hell. my dad was going thru hell in trujillo, and here in ponce not a single leaf was moving.

i woke up mad as a kid with no christmas gifts lol
so i did what a good cane addict would do:
drive to arecibo! jaaaa
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Isaac:

22/2345 UTC 15.6N 62.0W T2.5/3.0 ISAAC -- Atlantic
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Isaac's getting a nice shape to him, should steadily intensify from here on out
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We would be in pair with 2011 if Kirk forms this month as of August 2011 7 name storms form from Emily to Katia and this time would probably be from Ernesto to Kirk.E to K all over again.
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Quoting Levi32:


I won't know until tomorrow morning when I update it. I see no reason to alter it significantly right now while we're still 96-120 hours away from the part of the track that is in controversy.


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.
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NOAA plane just reported 997mb, still at 10,000ft.
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1077. Levi32
The MJO upward motion pulse is moving on to the east of the Caribbean, leaving Isaac to deal with an area of downward motion ahead of it. This can have both negative and positive implications for the storm's development during the height of the hurricane season, depending on the situation.

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Quoting hurricane23:
GFDL 18z..



I dont like that one... pick another one.... LOL
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Quoting MAweatherboy1:
18z GFDL:



I think the GFDL model is spot on in terms of track it has it going south of DR and threading the needle as it crosses the Haitian peninsula and emerging north of the eastern tip of Cuba and heads NW into SE Florida. However the intensity is a bit too conservative tho.
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Quoting JLPR2:
Next buoy in Isaac's way.
16.333N 63.5W


1005 millibars
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Quoting icmoore:


The forever gentleman :) Well, we are good right now minus sleep oh and a few fingerenails have bitten :) the dust. How are you and your bride?


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...
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Oh boy... I got question. Is that the eyewall Isaac is building up?

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Into GOM and W coast of Florida.... GFS

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Quoting AussieStorm:

bucks the trend of bad women names storms.
Last year were the women names now is turn for the males ;)
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Quoting presslord:
Have you brilliant minds determined where this thing is gonna hit yet?


For now, left center to extreme left of the forecast cone...

maybe to be revised even farther left later, will depend on when the CoC closes up.
Member Since: January 25, 2012 Posts: 33 Comments: 1520
Quoting Clearwater1:
I can't remember what storm, but I evacuated Indian Rocks Beach, maybe around '95 or so. The island, except for a few police officers, was empty. However, some clever thieves boated over to the mostly vacant island and robbed a bunch of homes. Crazy world.



They should have been shot on sight
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GFDL 18z..

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Quoting AussieStorm:

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.
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.
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1064. Levi32
Quoting NYCyclone86:


As of tonight, do you plan on changing it at all?


I won't know until tomorrow morning when I update it. I see no reason to alter it significantly right now while we're still 96-120 hours away from the part of the track that is in controversy.
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1063. hydrus
Floodman....funny how you roll in at the same time i,m rollin.....Hope you are well ..:)
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Quoting LargoFl:
..im closer to clwtr off east bay
I can't remember what storm, but I evacuated Indian Rocks Beach, maybe around '95 or so. The island, except for a few police officers, was empty. However, some clever thieves boated over to the mostly vacant island and robbed a bunch of homes. Crazy world.
Member Since: Posts: Comments:
1061. Relix
Quoting serialteg:


me neither LOL


Hahahaha. Waneko... what the heck :p

Only expecting rains and very light winds from Isaac. Feel kinda smart that I didn't go out and bought 2 boxes of sausage cans XD! Just stocked on water and gasoline, really wasn't expecting much from it. Guess a part of me wanted a surprise like last year with irene.
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Quoting aislinnpaps:
Hmmmm....

TWC conversation between Cantore and Greg Posell (sp)

Cantore: does the cone include Alabama, MS or LA at 11

Posell: probably not quite that far yet

Cantore: But that's where we're trending

Posell: But that's where we're heading

Then about five minutes later:

both talking again and Posell says about it moving west

"then, it may not happen at the eleven, but maybe shortly after"


Its inedible, the NHC will change their cone south and west. This will likely be a central gulf coast to Florida panhandle strike
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Quoting presslord:
Have you brilliant minds determined where this thing is gonna hit yet?


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


Stono River near Buzzards Roost Marina


I ssoo wish it was still called that
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1057. ackee
I see Isaac centre has reform Further south I still think Issac will miss hispanola track just south of cuba just my thought Iam thinking the Fl Panhandle will be where it will end up
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Quoting winter123:
Isaac is the scariest 45 mph storm I've ever seen. Such a large wind field, I feel hispaniola couldn't kill it.


its also on rainbow (deceiving). im on puerto rico's southermost central side and i have only seen light rain and winds so.
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1055. LargoFl
Quoting Torgen:


I just agreed to purchase a pinball machine in Davenport, supposed to drive from Tampa and pick it up Saturday. I wonder if that is even going to be possible?
oh and by friday they should have a much better idea where its going..check out the local warnings before you head out
Member Since: August 6, 2011 Posts: 4 Comments: 41581
Quoting wunderkidcayman:

NOAA's P-3 is flying at different height (higher) than the HH RECON C-130

Which means the pressure readings will be more accurate.

999.1 millibars confirms that Isaac has begun intensifying.
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Latest infrared satellite indicates that very intense convection is firing over the new LLCOC and is connecting to the western blob once this consolidate we could see a drop in pressures by the 11pm advisory.
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Quoting Floodman:


Spectacular, with the exception of the occasional bout of spasms...

As for the pain doctor, enjoy the magic pills, but continue to strive for a non-pharmaceutical solution; expereince speaking, those little fellows can become a habit

Well, I'll be on a course of those magic pills for 6 months then slowly come off them and hopefully once I do my nerves wont be as sensitive and I'll be fine. That's the hope anyways.
Member Since: September 30, 2007 Posts: 9 Comments: 15962
Anyone else notice the new circle on the map? This is what the ECMWF/GFS are developing.
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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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