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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3851. LargoFl
Quoting SFLWeatherman:
Q. cone shift East at 11AM
A.yes
B.no
.A
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Quoting SFLWeatherman:
Q. cone shift East at 11AM
A.yes
B.no


No
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Quoting stormpetrol:
Looking at the OBs from "Kermit" even though they don't post fixes, I suspect the center of Isaac to be consolidating actually below 15N , probably near 14.5N/65W, just my take though.
they have it at 15.4 north moving due west. this one looks like it will track south of the big islands into the triangle and boy the waters south of cuba are VERY HOT AND DEEP like a bathtub. the 0z Euro illustrates my exact track
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3848. Grothar
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Quoting SFLWeatherman:
Q. cone shift East at 11AM
A.yes
B.no

No
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3846. Grothar
Quoting JasonRE:
So we're basically looking at a hit on the FL/AL coast as opposed to any of the models that show it entering the GOM and heading a bit further West towards MS/LA?

I understand that the GSM compared to the Euro is a bit more reliable or makes more sense, but does this thing still have potential to turn once it crosses the islands or even enters the GOM? Last night before bed, the shift was further out in the GOM from the West coast of FL, and this morning, within less than 12 hours it's shifted back East. I guess the 'wobble factor' is going to be like this for the next few days.

What, if anything would cause this storm to shift Westward, at all? Thanks guys/gals!


That would depend on a number of factors. The strength of Isaac, the position of the weakness in the ridge and the speed of Isaac. All of these would depend on when it turns NW or North.
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3845. JasonRE
Quoting GetReal:


UKMET has shift significantly west on last run.


Where can you see this information? On this website or is it another you are looking at?

In Lafayette here.......Nice logo, WHO DAT!?
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Quoting AussieStorm:

Could be a threat to Bermuda down the road

Well, to say it more precisely, intermediate advisories are issued when watches and/or warnings are issued.
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Q. cone shift East at 11AM
A.yes
B.no
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Quoting Grothar:
most of the models have shifted slight East from the previous I just posted.



Clustered right on the SW FL coast. NHC may nudge this a tad east at 11am.

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Quoting atmosweather:
Morning folks,

Isaac's slower motion estimate for the 8AM advisory is simply because the center has reformed and jumped around the broad center of circulation multiple times overnight. The overall steering flow is not significantly slower than it was 24 hours ago, but as Water Vapor imagery shows, the longwave trough over the N-ern Gulf coast is beginning its advancement eastward towards breaking down the subtropical ridge.



Also, I'll point out that the 06z SHIPS rapid intensification index now shows a 55% chance of at least 25 kt strengthening in the next 24 hours, and a 44% chance of 30 kts or more. Both these values are the highest they have been throughout any of the SHIPS runs since Isaac formed.

A 25 kt increase in winds by tomorrow morning would make Isaac a 60 kt tropical storm with more ocean left to cover before any of the earliest interaction with Hispanola could begin.

Prob of RI for 25 kt RI threshold= 55% is 4.3 times the sample mean(12.8%)
Prob of RI for 30 kt RI threshold= 44% is 5.2 times the sample mean( 8.4%)
Prob of RI for 35 kt RI threshold= 32% is 6.3 times the sample mean( 5.0%)
Prob of RI for 40 kt RI threshold= 17% is 5.1 times the sample mean( 3.4%)


and yet ships has performed horrid.
Member Since: August 22, 2006 Posts: 2 Comments: 1983
3839. GetReal
Quoting 7544:
nhc cone shift at 11 am left or right ?


IMO it will come left some out of respect to the EURO and UKMET.
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3837. CJ5
The models are begining to get a better handle on Isaac. There are still quite a few variables but the future track has been narrowed down quite a bit. He kept his convection pretty strong overnight and with the slow in forward speed has an opportunity to consolidate and strengthen today. There is some 30kt shear ahead of him but that should drop by the time he gets there. Overall, things look good for him.
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Quoting Bobbyweather:

Ten isn't a threat to any land areas. Therefore its advisories are issued every 6 hours (5 and 11 EDT).
If a system is a threat to even one land area, then intermediate advisories are issued. It is issued every three (two if it is a big threat) hours.

Could be a threat to Bermuda down the road
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Quoting 7544:
nhc cone shift at 11 am left or right ?


Right
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3833. 900MB
Quoting 7544:
nhc cone shift at 11 am left or right ?


Gotta be left.
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3832. GetReal
Quoting Grothar:
The CMC shifted west and the EURO shifted East.



UKMET has shift significantly west on last run.
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3831. 900MB
Quoting gprxomstr:
Yes 900MB. How's the weather in Brooklyn?


I'm in Manhattan. A tad overcast and humid, but only in the 70's, for which I am thankful. Unlike Feb-July which was very warm, August has given us a little break.
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3830. TheMom
Just a reminder that this storm and / or it's sister and brother tagging along behind is going to hit somewhere and lives are going to be effected. Portlight.org started here many moons ago and is one relief organization we all KNOW gives 100% of donations TO THOSE IN NEED! We may not be able to give a lot but I know that they make sure every little bit we can give helps someone. http://portlight.org/
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3829. 7544
nhc cone shift at 11 am left or right ?
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3828. Chiggy
for now I am done with guessing the images on sat loops for the CoC and just watching the radar loop out of PR.. clearly showing rotation! I urge folks to have a look and post their opinions, thank you!
Link
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Morning folks,

Isaac's slower motion estimate for the 8AM advisory is simply because the center has reformed and jumped around the broad center of circulation multiple times overnight. The overall steering flow is not significantly slower than it was 24 hours ago, but as Water Vapor imagery shows, the longwave trough over the N-ern Gulf coast is beginning its advancement eastward towards breaking down the subtropical ridge.



Also, I'll point out that the 06z SHIPS rapid intensification index now shows a 55% chance of at least 25 kt strengthening in the next 24 hours, and a 44% chance of 30 kts or more. Both these values are the highest they have been throughout any of the SHIPS runs since Isaac formed.

A 25 kt increase in winds by tomorrow morning would make Isaac a 60 kt tropical storm with more ocean left to cover before any of the earliest interaction with Hispanola could begin.

Prob of RI for 25 kt RI threshold= 55% is 4.3 times the sample mean(12.8%)
Prob of RI for 30 kt RI threshold= 44% is 5.2 times the sample mean( 8.4%)
Prob of RI for 35 kt RI threshold= 32% is 6.3 times the sample mean( 5.0%)
Prob of RI for 40 kt RI threshold= 17% is 5.1 times the sample mean( 3.4%)
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Quoting Drakoen:
The 12z TVCN is a little to the west of the NHC forecast maybe only a slight shift westward of the NHC track at 11 am.


That would put the bullseye on my house! Hopefully getting the bullseye this early means it will miss the Panhandle of FL all together!
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Looking at the OBs from "Kermit" even though they don't post fixes, I suspect the center of Isaac to be consolidating actually below 15N , probably near 14.5N/65W, just my take though.
Member Since: April 29, 2007 Posts: 0 Comments: 7810
Quoting AussieStorm:

Yes exactly, the NHC had not named it Joyce.
Why didn't they name it at the 8am update??

Ten isn't a threat to any land areas. Therefore its advisories are issued every 6 hours (5 and 11 EDT).
If a system is a threat to even one land area, then intermediate advisories are issued. It is issued every three (two if it is a big threat) hours.
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3823. DDR
The feeder band over Trinidad and Tobago has more precipitation than Isaac,floods in north western areas again,other areas including Tobago.
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Quoting CypressJim08:
When can we expect the next HH mission?


NOAA is in there now, next Air Force plane should be in the air. Looks like a long flight so they may be coming from Biloxi? Take off was at 12Z for a 18Z mission.

I. ATLANTIC REQUIREMENTS
1. TROPICAL STORM ISAAC
FLIGHT ONE --TEAL 75
A. 23/1800Z,
B. AFXXX 0709A ISAAC
C. 23/1200Z
D. 17.0N 66.3W
E. 23/1730Z TO 23/2000Z
F. SFC TO 15,000 FT
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3821. Grothar
The very latest is still very close. 12z

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3820. JasonRE
So we're basically looking at a hit on the FL/AL coast as opposed to any of the models that show it entering the GOM and heading a bit further West towards MS/LA?

I understand that the GSM compared to the Euro is a bit more reliable or makes more sense, but does this thing still have potential to turn once it crosses the islands or even enters the GOM? Last night before bed, the shift was further out in the GOM from the West coast of FL, and this morning, within less than 12 hours it's shifted back East. I guess the 'wobble factor' is going to be like this for the next few days.

What, if anything would cause this storm to shift Westward, at all? Thanks guys/gals!
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Quoting CypressJim08:
When can we expect the next HH mission?

The NOAA G-IV jet will be sampling the steering mechanisms over the west Atlantic this afternoon.
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3818. LargoFl
Folks remember yesterday my local met gave us where Issac would go..northward inbetween the two highs...ok look at this and WHAT IS..between the two highs?....prepare now
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Time...
Time will tell the truth.

Ten will be renamed Joyce sooner or later.
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3816. Grothar
most of the models have shifted slight East from the previous I just posted.

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3815. ncstorm
Quoting LargoFl:
..yes right now we cant trust the models..back and forth they go


I am not putting too much stock in the models till

1: NOAA Flight Jet gets in there and gets better atmospheric data for the models runs

and
2: Isaac crosses Hispanola or Cuba..

Too many variables still left open for the track of Isaac..
Member Since: August 19, 2006 Posts: 13 Comments: 15082
Quoting Bobbyweather:

Well, isn't the ATCF (Automated Tropical Cyclone Forecasting) owned by the NHC?
Let's just wait. Ten will be renamed Joyce sooner or later.

Yes exactly, the NHC had not named it Joyce.
Why didn't they name it at the 8am update??
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3813. GetReal
<
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Quoting sullivanweather:
Good morning, everyone!

I see the most pronounced change in Isaac this morning is the slowing down of the forward speed of the storm, now at nearly half the speed if was chugging westward yesterday. With this Isaac should finally start getting its act together later this morning and this afternoon. It will be much easier now for the moist air encompassing the southern and eastern quadrants of the storm to wrap into the center and mix out the dry air hindering it.
You as well as the others need to go back & look at the numbers/fixes over the last several hours - as far as MY mathematics are concerned - I see NO lack in FORWARD speed! JMO
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Quoting AussieStorm:

Have a look at this loop and you'll say the same thing....




OMG...

Now it has the Typhoon Tembin coming back for a third pass...and the size and intensity on Bolaven is unbelievable...at least that one is mostly going over sparsely populated islands...
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The Weather Channel8:52 AM - Public

A look inside Tropical Storm #Isaac from the NOAA Hurricane Hunters via @NWSKeyWest



I will be back around lunchtime guys
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3808. Relix
Tightening around the NE core it seems. Hmmm....
Member Since: August 3, 2006 Posts: 0 Comments: 2696
Quoting RTSplayer:


Looks very Katrina-ish/Camille-ish in terms of final outcome.

Hits or grazes lower Plaquemines, with a slight right hook in1to Biloxi or Mobile Bay.


DOOM

ECMWF shifted east away from us (whew!) toward Mobile/Pensacola. What worries me (see chatter below) is all of the satellite looks like the center of circulation and coldest cloud tops are south of the forecast track. /s/ Completely seasonable, amateur spaghetti eater from the Hurricane Coast (aka as NOLA).

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3805. Grothar
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Quoting gordydunnot:
I don't want to start a panic. but Fl. did have a Gov. Kirk. I leave out the politics. Although he was a bit of a Claude if you as me.
Claude Kirk, wow, he was the Gov who personally went to rock concerts to bust the hippies. lol
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Quoting StormTracker2K:


That maybe a troll and not the real handle.


It is a troll just look at the join date. And no avatar.
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Quoting Grothar:


I don't remember seeing the HWRF on the east coast.

Other than the Euro and the CMC almost all models have been pretty consistent in the general direction for a few days. They have been a lot closer than some other storms.
This is what Largo posted.

Member Since: August 31, 2009 Posts: 0 Comments: 5628
Still 2 hours until 11 am. TD 10 may have the name taken back by then... wait until NHC confirmed it.
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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.