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 stormpetrol:


Look , I know the NHC are the experts, I give them heads up , they get it right the vast majority of the time and I'm just a layman"not even that" but if I had followed the track back in 2004 when Ivan devastated us, I'd been in deep trouble,even more than I've suffered! " Experience is the greatest of all schools , so that even the fools will learn", I will not turn my back on Isaac until he is well North of us(Cayman Islands)either way we will experience probably TS conditions from him, even if not a direct hit!!!


storm's been right a couple of times
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New AOI off the African Coast!

TROPICAL WEATHER OUTLOOK
NWS NATIONAL HURRICANE CENTER MIAMI FL
800 PM EDT WED AUG 22 2012

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

THE NATIONAL HURRICANE CENTER IS ISSUING ADVISORIES ON TROPICAL
STORM ISAAC...LOCATED ABOUT 65 MILES SOUTHWEST OF GUADELOUPE...AND
TROPICAL DEPRESSION TEN...LOCATED ABOUT 920 MILES WEST OF THE CAPE
VERDE ISLANDS.

1. SHOWERS AND THUNDERSTORMS NEAR THE WEST COAST OF AFRICA ARE
ASSOCIATED WITH A TROPICAL WAVE. DEVELOPMENT IS EXPECTED TO BE
SLOW TO OCCUR...AND THIS SYSTEM HAS A LOW CHANCE...10 PERCENT...OF
BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS AS IT MOVES
WESTWARD AT ABOUT 15 MPH.

ELSEWHERE...TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE
NEXT 48 HOURS.

&&
PUBLIC ADVISORIES ON TROPICAL DEPRESSION TEN ARE ISSUED UNDER WMO
HEADER WTNT35 KNHC AND UNDER AWIPS HEADER MIATCPAT5.
FORECAST/ADVISORIES ON TROPICAL DEPRESSION TEN ARE ISSUED UNDER WMO
HEADER WTNT25 KNHC AND UNDER AWIPS HEADER MIATCMAT5.

$$
FORECASTER BERG
NNNN
Member Since: August 31, 2009 Posts: 0 Comments: 5628
Quoting pottery:

Inflow, more like.
But yeah.


Are you well stocked with rum?!
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Quoting MiamiHurricanes09:
I must say it is quite surprising that we are already on our 10th cyclone of the 2012 season. On this exact date in 2010, we were still only by number 6, and on this date in 2005, the 11th tropical cyclone just developed. Not bad at all so far for what was supposed to be just an "average" season.


/delurks
Well, we were supposed to be deep in the throes of El Niño by now. But while the cat's away...

It has at least made for an interesting season.
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Quoting Drakoen:
NHC 8:00pm coordinates and a little extrapolation gives a good estimate of the center given the current observations.
drak does it look like the center is starting to consolodate, someone in the last post mentions that the midlevel center and the llc are not aligned because of the trade winds ???
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1546. pottery
Quoting bajelayman2:
Just a weather alert to you guys up North, we in Barbados are on the very far edges of Isaac, but tonight we are getting a LOT of water, very heavy showers sporadically.

So, this storm a lot of water with it. Clsoer to the centre must be loaded with moisture.

Looks like the worst is past you, except for an area just to your south.
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1545. LargoFl
Quoting StormJunkie:


Recon did not show a closed low. I watched the entire time they were in there, anyone else that did should have seen the same thing.
...does anybody think the impossible..that this does NOT pull together and somewhere down the line..falls apart?
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Be funny if this season also had 19 named storms XD.
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Quoting NYCyclone86:


What if he wobbles and avoids a chunk of the high terrain and is stronger? Yes, tomorrow we will have a better grip on things. Remember, land friction can throw curveball's into the track of a storm.


Yeah, and that land interaction thing goes both ways. Due to the further south position than originally thought, there is a higher chance that Isaac will have minimal interaction with Hispaniola.
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1539. Gearsts
Member Since: August 2, 2010 Posts: 0 Comments: 1739
1538. icmoore
Quoting trey33:



Yes. Worst case for Tampa is for a storm to sit just NW of the city - all that water coming into the bay will cause one heck of a storm surge.


Would not be good for the barrier islands here either like the one I live on.
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1537. scott39
Isaac is in its toddler phase. Thia isnt a system that is going to go away any time soon!
Member Since: June 13, 2009 Posts: 0 Comments: 6750
more.water.more.potential
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1535. Drakoen
NHC 8:00pm coordinates and a little extrapolation gives a good estimate of the center given the current observations.
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1534. KEEPEROFTHEGATE (Mod)
T.C.F.W.
09L/TS/I/CX
MARK
15.49N/63.45W
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SAB was revised. Here are the new numbers. T3.0 support an initial intensity of 50 mph, although recon shows winds near 45 mph.

22/2345 UTC 15.6N 62.0W T3.0/3.0 ISAAC -- Atlantic
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1532. Chiggy
Quoting Levi32:
I still think the Euro is more likely to be bogus. The last time I can remember a storm running parallel along Hispaniola and Cuba, then continuing the shortcut track into Louisiana, was Georges in 1998, and it took this strong of a ridge to keep him from recurving into the Florida area:








Here the ridge is going to be broken (GFS 84 hour):


Sure Levi but you are explaining Euro runs with GFS prediction of ridges... euro's set up is different
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Quoting SLU:


Yes. Isaac is highly decoupled. That mid-level circulation passed between St. Lucia and Martinique around midday while the LLC just pass Guadeloupe.

Doesnt mean much if taking DMax tomorrow am into it, no?

Probably will look at lot better tomorrow am.

One heck of a storm.
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Quoting scott39:
Theres a fine line in between debating and just plain arrogance.


Who's being arrogant?
Member Since: March 7, 2007 Posts: 572 Comments: 20344
1528. pottery
Quoting AstroHurricane001:


Outflow bands?

Inflow, more like.
But yeah.
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Quoting Neapolitan:
FWIW, on this date in 1995, three storms formed: Hurricanes Humberto and Iris, and TS Jerry.


i believe iris was a golden one

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1525. LargoFl
Quoting bajelayman2:
Just a weather alert to you guys up North, we in Barbados are on the very far edges of Isaac, but tonight we are getting a LOT of water, very heavy showers sporadically.

So, this storm a lot of water with it. Clsoer to the centre must be loaded with moisture.
ok thanks
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Quoting MiamiHurricanes09:
I must say it is quite surprising that we are already on our 10th cyclone of the 2012 season. On this exact date in 2010, we were still only by number 6, and on this date in 2005, the 11th tropical cyclone just developed. Not bad at all so far for what was supposed to be just an "average" season.


And when you consider it's "that list".
Member Since: March 7, 2007 Posts: 572 Comments: 20344
Quoting Neapolitan:
On this date in 2009, we were only up to #3...
I believer that was one of our analogs for this year as well? Very surprising how active we've been thus far.
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Quoting hurricanehunter27:
I hope your joking. In now way shape or form is Issac a open wave. All that has happened is a center relocation that happens all the time in weak systems. Not to mention Isaac is really tilted. That being said it appears that Isaac has consolidated. Have to wait for HH pass (Next 20 min) to confirm exact details. Plus NHC say TS not an open wave and that is really all you need.



Not joking at all. The wind reading from the earlier recon was a complete mess. And yes that is all I need; but if I only cared about information I "needed" for tropical systems then I wouldn't be here discussing them.

Scott, by experts I meant the hobbyist experts that are on this blog. But you knew what I meant.
Member Since: August 17, 2005 Posts: 26 Comments: 16257
Quoting poikoo:
really! all you people are for the most part trolls! who cares what you have to say! i miss stormw! and drakoen, and all the rest:(


Who are you?
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FWIW, on this date in 1995, three storms formed: Hurricanes Humberto and Iris, and TS Jerry.
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Just a weather alert to you guys up North, we in Barbados are on the very far edges of Isaac, but tonight we are getting a LOT of water, very heavy showers sporadically.

So, this storm a lot of water with it. Clsoer to the centre must be loaded with moisture.
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Quoting Levi32:
I still think the Euro is more likely to be bogus.


Well, all models are wrong; its the degree that matters. What it comes down to is a good analysis each forecasting cycle. Hopefully NOAA is planning on a large number of dropsondes; given the potential costs of an unwarranted evacuation or, even worse, a failure to evacuate where it was necessary, i'd say throw the sounding equivalent of the kitchen sink at it.
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Quoting Neapolitan:
Well, I wouldn't say "almost always fail", though where TD10s are concerned, only four of the past eight have failed to mature:

hurricanes


Would be interesting to see that graph including added weight expressed in area...
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1516. SLU
Quoting Levi32:
Really interesting. Dropsonde due west of St. Lucia just now reported northeast winds at flight-level as the plane has been observing, which implies no closed circulation at flight level, but as soon as the sonde drops 5000 feet the winds switch to westerly as they should be. One heck of a vertically misaligned circulation.



Yes. Isaac is highly decoupled. That mid-level circulation passed between St. Lucia and Martinique around midday while the LLC just past Guadeloupe.
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Quoting Levi32:
Really interesting. Dropsonde due west of St. Lucia just now reported northeast winds at flight-level as the plane has been observing, which implies no closed circulation at flight level, but as soon as the sonde drops 1500 feet the winds switch to westerly as they should be. One heck of a vertically misaligned circulation.

those trade winds doing some work to him.
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1514. scott39
Theres a fine line in between debating and just plain arrogance.
Member Since: June 13, 2009 Posts: 0 Comments: 6750
Quoting hurricanehunter27:
This convection keeps growing and getting cooler. U can see a little white pixel now.



Yes, water vapor has a white pixel or two as well...




I think it's trying to wrap up around the CoC for once...could see some strengthening soon.

It's also cut off whatever dry air was to the north, as outflow has improved there.

May be some dry air coming off of S.A. continent, but it's hard to say.
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Quoting Hurricane1956:
According to the OFFICIAL NATIONAL HURRICANE CENTER! track as of 8:00PM,Isaac is not going to get closer to the Cayman Island!!! at all.


Look , I know the NHC are the experts, I give them heads up , they get it right the vast majority of the time and I'm just a layman"not even that" but if I had followed the track back in 2004 when Ivan devastated us, I'd been in deep trouble,even more than I've suffered! " Experience is the greatest of all schools , so that even the fools will learn", I will not turn my back on Isaac until he is well North of us(Cayman Islands)either way we will experience probably TS conditions from him, even if not a direct hit!!!
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Quoting Neapolitan:
Well, I wouldn't say "almost always fail", though where TD10s are concerned, only four of the past eight have failed to mature:

hurricanes


Yep,


Always is like a loaded gun, you don't point it around physical phenomena or else you might blow your foot off.
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1510. trey33
Quoting icmoore:

#1445
This (white line) looks like it would be awful for the Tampa area with the east side being worse right?



Yes. Worst case for Tampa is for a storm to sit just NW of the city - all that water coming into the bay will cause one heck of a storm surge.
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Drak and Levi, thank-you. Those are both valid arguments for a closed low even if it is extremely broad.
Member Since: August 17, 2005 Posts: 26 Comments: 16257
1508. wn1995


Yes, already a new track by me... because I put out my 18z track late. New forecast pretty much the same as the last one.. but now calling for Isaac to become a cat 2 in the eastern gulf.
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Quoting CHANGESALOT:
will the east central coast of florida still getting anything from issac? or it issac moving to far away now?
On the current NHC track, the south east coast would experience solid tropical storm conditions.
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1506. JLPR2
Quoting JLPR2:

I'm guessing the Pacman looking area west of the gap between Guadeloupe and Dominica could be very close to the center.



And SW winds at Dominica and SSE winds in Guadeloupe seems to support it pretty well.
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Quoting pottery:
Looks like we are about to get some real heavy rain coming in from the SW.


Outflow bands?
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Quoting MiamiHurricanes09:
I must say it is quite surprising that we are already on our 10th cyclone of the 2012 season. On this exact date in 2010, we were still only by number 6, and on this date in 2005, the 11th tropical cyclone just developed. Not bad at all so far for what was supposed to be just an "average" season.
On this date in 2009, we were only up to #3...
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Quoting AussieStorm:


Go see if you can find out the size of Typhoon Bolaven. it's HUUUUUUGGGGEEEEE.


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1502. Drakoen
The elongated circulation is somewhere between Guadeloupe and Martinique.

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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.