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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I believe Isaac is going to show its true face sometime soon.. and its not going to be pretty.. IMO
Member Since: August 15, 2008 Posts: 0 Comments: 2133
Quoting Chiggy:

CIMSS vorticity maps shows otherwise - further south!

CIMSS Maps < Recon.
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1649. Michfan
Quoting shfr173:
Can anyone explain to me why tropical cyclones tend to explode in the night hours???


Ocean releases the heat it has accumulated over the day into the atmosphere as air temperatures tend to be lower at night.
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Quoting Levi32:
You know the circulation is a wreck when a radar view of the entire Antilles island chain doesn't help you find the center lol.


Well, those radar graphics are horrible, at least the ones they post online.

They need to get the Wunderground software, or else NHC or something, cuz there's sucks.

At least at the speed it's moving, much of the circulation should be visible on PR radar over night and early morning.
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1646. KEEPEROFTHEGATE (Mod)
1578. serialteg 1:30 AM GMT on August 23, 2012 +0
Quoting KEEPEROFTHEGATE:
T.C.F.W.
09L/TS/I/CX
MARK
15.49N/63.45W


you see, i just don't know keep... seems to me like it's more to the NE
................................................. ...


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1645. Chiggy
Quoting TropicalAnalystwx13:
The box marks the center.


CIMSS vorticity maps shows otherwise - further south!
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1644. palmpt
Quoting CHANGESALOT:
whoes that troll thats been saying everyday for 4 days now thatthe track of issac will keep shifting west and eventually bypass most of florida? i think he was on to something ( not bad forcasting for a amateur ) so far hes been right on


You
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Tembin

Taiwan will be tembin for a while... Stationary typoon


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Quoting washingtonian115:
Yes this "list" is known to accommodate inactive hurricane seasons with El nino usually.Might be it's year...


If it's not El Nino, it's recurvature. :P
Member Since: March 7, 2007 Posts: 585 Comments: 20854
1641. shfr173
Can anyone explain to me why tropical cyclones tend to explode in the night hours???
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1640. JLPR2
Quoting TropicalAnalystwx13:
The box marks the center.



Yep, we are on the same boat.
Hoping we don't sink together. :P
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1639. Michfan
Radar is a complete mess. Its like someone is putting his hand on on his right shoulder and pressing down.
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1638. Levi32
Quoting Gearsts:
You think the will open in to a wave?


No. It's got enough westerlies in the low levels. It's just huge.
Member Since: November 24, 2005 Posts: 635 Comments: 26661
I hate storms that look good on satellite, but in reality are barely TC's. These are the storms that make Dvorak useless and expensive reconnaissance a must.
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Quoting GoWVU:


Too funny!!! I like it....


Well, you have to blame governors and mayors, and fire the FEMA director too, but yeah, it's funny...
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Quoting RTSplayer:


What's giving them so many problems is the mid-level vort max is actually about 1.5 to 2 degrees south of the alleged LLC.


And yes, steering still favors some drifting/jogging to the south anyway.


To put the amount of decoupling in perspective, if the 1.5-2 degrees is accurate, that is somewhere between 100-150 miles of separation between the mid and low level circulations. That is a long, long way off.
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Trying to get some perspective on this.
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I am out as well....Waiting to see where the COC issue shakes out overnight. Enjoy the Show.
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Quoting scooster67:
We got a night storm, meaning he likes the night life....the night time. If he keeps this up...........





Biggest thing to note is that Cirrus outflow is expanding in the NE quadrant.
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Is there any chance that Issac is being influenced some way by the remnants of Karen? LOL. Sorry, it had to be said.

Seriously, though, I live on the Jersey Shore, and I don't know if Issac or Joyce is headed here or not, but in the last few days I have re-stocked my hurricane preparedness items, and checked my plan. It cost me like $150.00 dollars on several flashlights and a radio with enough batteries to last for a month. I have enough non-perishable food and water for a significant time, and an inland bunker: my sister's concrete finished basement, same place I stayed during Irene. I live on a barrier island, Seaside Heights, where the show 'Jersey Shore' is filmed. If you've seen the show, you've seen my town. It's located precariously between the Atlantic Ocean and the Barnegat Bay. My house is about 1-3 foot higher than sea level. The entire bayside of my community is man-made land: dredged and designed for a causeway in the 1950's. A few blocks down is an area of streets and houses, but this area was once known as Cranberry Inlet, opened and closed by hurricanes. It is believed that during Irene, another inlet was observed in infantile stage on the beach. Had the storm been stronger it might have cut through this vulnerable area. The Borough is only 100 years old this upcoming year. In that time, we've not seen a really big hurricane make landfall directly. The last time was 1821, and that was estimated to be about a once in 200 year event, 191 years ago.
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Quoting Levi32:
You know the circulation is a wreck when a radar view of the entire Antilles island chain doesn't help you find the center lol.


its on the nor-easternmost clearing IMO
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Wish we were getting obs from Isla de Aves...
I thought once I found a way to get their data from Venzuela Met website
and it included skew charts


http://www.wunderground.com/cgi-bin/findweather/g etForecast?query=zmw:00000.1.80400
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1627. Gearsts
Quoting Levi32:
You know the circulation is a wreck when a radar view of the entire Antilles island chain doesn't help you find the center lol.
You think the will open in to a wave?
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Quoting SLU:


The only other TS this large I can remember entering the Caribbean was TS Jose in 1999 which eventually tightened up as it became a hurricane.

But Isaac is really amazing to maintain this large size almost all the way across the Atlantic in the face of dry air at times.



We better hope he finds some mountains, otherwise he's gonna be pushing a chunkload of water somewhere eventually.
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The box marks the center.

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Quoting sunlinepr:


You're down in Ponce... my sister came out from NOLA and went down to Pastillo... In Villa Flores also very tranquil


ima go sleep :(
i hope i see some action tomorrow.
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Evening All.

At least on this map models have trended eastward this evening.

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1622. Levi32
You know the circulation is a wreck when a radar view of the entire Antilles island chain doesn't help you find the center lol.
Member Since: November 24, 2005 Posts: 635 Comments: 26661
1621. scott39
Quoting HoustonTxGal:


Well, I guess if Katrina was Bush's fault, Isaac can be Obama's.... it's only fair
Hahahaha..... Both of those statements are more of a hot mess than Isaac is right now
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Quoting stormjake:
It is trying to condense. I think it will be a tight core in 12 hours.


This would be a truly amazing feet considering the fact that the storm is so far from being stacked.
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Nite all, note pottery and stay safe, dont reduce your rum too much even after you get back from airport, it needs to be in DMAX tomorrow am.... ;-)
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Quoting hurricanehunter27:
Further south?


What's giving them so many problems is the mid-level vort max is actually about 1.5 to 2 degrees south of the alleged LLC.


And yes, steering still favors some drifting/jogging to the south anyway.
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Recon found some 44-45 knot winds. The rain-rate wasn't too high, but GREarth isn't showing what flight level winds were.
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Not sure if the blog has been updated...just jumped on for the first time in years... is it looking like a FL landfall as of now?
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Quoting serialteg:


so quiet and tranquil in ponce :( (((((

even in south storms

im moving from here


You're down in Ponce... my sister came out from NOLA and went down to Pastillo... In Villa Flores also very tranquil
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1613. trey33
Quoting LargoFl:
well good night folks..i bet tomorrow is going to be a LONG day posting huh...have a safe one


'night Largo
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1612. vince1
Quoting HoustonTxGal:


Well, I guess if Katrina was Bush's fault, Isaac can be Obama's.... it's only fair

Partisans will be partisans.

And wow, first thinkprogress and now mediamatters? What's next, MSNBC? ;)
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We got a night storm, meaning he likes the night life....the night time. If he keeps this up...........



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It is trying to condense. I think it will be a tight core in 12 hours.
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Quoting LargoFl:
TREY......
82 degrees? That's a warm low.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
1608. GoWVU
Quoting HoustonTxGal:


Well, I guess if Katrina was Bush's fault, Isaac can be Obama's.... it's only fair


Too funny!!! I like it....
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1607. LargoFl
Quoting trey33:


To do list: Fix roof on Friday :)

yes way before he gets here..good nite
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1606. LargoFl
well good night folks..i bet tomorrow is going to be a LONG day posting huh...have a safe one
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Quoting Tropicsweatherpr:


So far no power outage here in Santurce with those bands moving thru.


Here.... (checking out)

Well, seems like if I am posting here and have internet and I don't have a power plant... Yes!!! no power outage here either....
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1604. trey33
Quoting LargoFl:
TREY......


To do list: Fix roof on Friday :)

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Quoting LargoFl:
TREY......


I like that "Windy" bit. Real understatement.
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1601. Gearsts
Quoting Tropicsweatherpr:


So far no power outage here in Santurce with those bands moving thru.
I was with no power for 1 hour here in Aguadilla lol
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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.