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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3901. Pettee
Been following this one closely. Live in the Lower Keys, and making my preparations today anyway. Shutters are out and ready, and the boat is being pulled today and tied down. Rather be prepared for nothing, then have nothing prepared for something!
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Quoting LargoFl:
yes we are in the cone, could be dangerous come monday and tuesday, at least..this storm is giving us plenty of time to watch and prepare huh..no one should be caught off guard this time...unless your saying right now..oh what storm?
However, time is running out....
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3899. 7544
Quoting SFLWeatherman:
Q. cone shift East at 11AM
A.yes
B.no


a yes and right
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3898. Relix
Isaac is climbing it seems. The whole thing seems to be moving at 290 degrees.
http://www.ssd.noaa.gov/PS/TROP/floaters/09L/flas h-vis-short.html
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3897. KEEPEROFTHEGATE (Mod)
Quoting CJ5:


With so many of the ensemble models have Isaac moving across S FL, 80% is a pretty big number.
cj thats a troll not the real person
Member Since: July 15, 2006 Posts: 175 Comments: 54827
Quoting southfla:
Katrina, Aug 24, 2005



Katrina was 45mph at that point as well and increased to 85mph while rolling into Broward County. Just goes to show that when these systems are ready to go then they go in a hurry.

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Quoting nrtiwInvragn:


Look at that dry air NE of Isaac, that should hinder any strengthening for the next 48 hours, expect the west shift to continue yall!


I didn't say anything about whether I believe rapid intensification could and will occur. Dry air in the mid levels will be mixed out and left behind by the end of the day so that won't be a huge hinderance. It's the low level organization. The SHIPS index cannot analyze inner core dynamics and predict their changes well enough to account for this situation (although the 2011 version has improved in that department considerably from earlier products). If he can get a consolidated, tight pressure center and begin growing its circulation vertically sometime today, I don't think a 55% chance of a 25 kt wind increase is crazy at all. But we have to see evidence of a better inner core.
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3894. LargoFl
Quoting StormTracker2K:


Because it's terrible. GFS has been top dog all year. Ever since the upgrade the GFS has been our best performing model by far!
hey stormtracker..do you have an animated GDFL track, if you do could you post it please
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Quoting weatherh98:


In australia, has no chance of being hit

Oh but we all got a horse in the race. And Aussie has done an exceptional job illustrating that!
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3892. CJ5
Quoting nrtiwInvragn:


80% positive that south Florida is in the clear.


With so many of the ensemble models have Isaac moving across S FL, 80% is a pretty big number.
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Quoting alvarig1263:


South Florida is by no means in the clear yet. S FL is right in the middle of the cone 4-5 days from now. S FL will be in clear by next Wednesday or Thursday once Isaac is out of the way and has eventually made landfall.


Maybe the islands will tear the thing apart???
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Quoting 900MB:


Where are you? Heading out now, but have a great day!

Gulfport, MS. Good day to you, too!
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Quoting GTcooliebai:


NHC is on the left side of the guidance now.

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Quoting connie1976:
Hey all!! Do you think that South Florida is almost in the clear with the latest model runs? I got some water anyway this morning and I had to go to a bunch of stores before I could find any.... We drink the water rather then soda anyway, so there is no loss if the storm doesn't affect us...


Yes we are!!!
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NHC cone will shift west. I can't see this curving so sharp anymore. I am really afraid this will go to Gulf.
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Isaac takes over the entire Eastern Caribbean:

Member Since: August 31, 2009 Posts: 0 Comments: 5628
3885. LargoFl
...................
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As I have said many times Fort Lauderdale/Miami dodged another one. I love this shield we always seem to have around us. The cone just keeps shifting west!!! I keep saying it but I wont be suprised if eventually we are out of the cone. Looks to be a nice weekend afterall.. Unfortunately my wife went out and bought a $100 worth of water, batteries and canned food. Looks like its time to go exchange the water for some beer and get ready for some football!!!!
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3883. 900MB
Quoting gprxomstr:

Glad to hear. Have friends in Chicago and they have been experiencing below average temperatures for the past month.


Where are you? Heading out now, but have a great day!
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Just what we need more rain....

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Quoting JasonRE:
Why is the UKMET model so far West? What information are they relying on, or information are they basing their models on? Thanks


Because it's terrible. GFS has been top dog all year. Ever since the upgrade the GFS has been our best performing model by far!
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G'morning, geeks! I've not finished the 1st coffee yet, and one eye is still quite foggy, but look at sats, I'm thinking Isaac is still disjointed with a dominate mid circ further west at 15n,67w and a weaker, exposed low circ at the official coordinates.

The official forecast says fwd speed has slowed, but Isaac still does not yet have his act together, not vertical with the mid circ doing all the heavy lifting.

I'm still expecting loose organization until 75/80w and a weaker storm until. Meanwhile, high pressure is still overhead and with a weak system, Isaac will continue to track more westwards than northwards.

Now, please excuse me, must have 2nd coffee to see and think more clearly! :)
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Quoting nrtiwInvragn:


80% positive that south Florida is in the clear.


Oh, I hope so!! I love my power and my neighborhood floods...
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3878. Drakoen
NHC really doesn't even need to change the track, other than extrapolation from the last position.
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Quoting gprxomstr:

Ha! Hardly!


In australia, has no chance of being hit
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Quoting gprxomstr:

Ha! Hardly!

????????
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3875. yqt1001
You don't get adjusted T's at 140kts (category 5) often O_o



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Quoting connie1976:
Hey all!! Do you think that South Florida is almost in the clear with the latest model runs? I got some water anyway this morning and I had to go to a bunch of stores before I could find any.... We drink the water rather then soda anyway, so there is no loss if the storm doesn't affect us...


South Florida is by no means in the clear yet. S FL is right in the middle of the cone 4-5 days from now. S FL will be in clear by next Wednesday or Thursday once Isaac is out of the way and has eventually made landfall.
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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%)
I think the trough in N gom starts breaking down before a turn into the DR - JMO
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3872. LargoFl
Quoting connie1976:
Hey all!! Do you think that South Florida is almost in the clear with the latest model runs? I got some water anyway this morning and I had to go to a bunch of stores before I could find any.... We drink the water rather then soda anyway, so there is no loss if the storm doesn't affect us...
..it is unclear right now, good your preparing just in case
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Katrina, Aug 24, 2005

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Quoting unknowncomic:
Maybe turning?

I think soon. Yep.
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Quoting wunderkidcayman:


yeah I think your right sattelite presentation and surface obs are suggesting that



Left and South


I agree. Look at the 1000 mb steering layer it significantly west.

This thing will keep moving west until its a cane
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Member Since: August 31, 2009 Posts: 0 Comments: 5628
Quoting weatherh98:
Aussie is the only true unbiased one here:)

Ha! Hardly!
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3865. GetReal
Quoting JasonRE:


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




B.
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3863. pottery
Quoting DDR:
The feeder band over Trinidad and Tobago has more precipitation than Isaac,floods in north western areas again,other areas including Tobago.

Seeing some images of flooding in and around POS already this morning.
Got some pretty strong southwest winds here last night, but not a lot of rain.
Heard it was bad up Toco and Tobago.
Member Since: October 24, 2005 Posts: 0 Comments: 24648
Quoting SFLWeatherman:
Q. cone shift East at 11AM
A.yes
B.no


Yes
Member Since: Posts: Comments:
Hey all!! Do you think that South Florida is almost in the clear with the latest model runs? I got some water anyway this morning and I had to go to a bunch of stores before I could find any.... We drink the water rather then soda anyway, so there is no loss if the storm doesn't affect us...
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Aussie is the only true unbiased one here:)
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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.


yeah I think your right sattelite presentation and surface obs are suggesting that

Quoting 7544:
nhc cone shift at 11 am left or right ?


Left and South
Member Since: June 13, 2009 Posts: 2 Comments: 12376
3858. JasonRE
Why is the UKMET model so far West? What information are they relying on, or information are they basing their models on? Thanks
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So it looks like the Keys, and the entire West coast of Florida will be getting the full forced right side of a potential hurricane riding up the coast and then making landfall on the panhandle?  Absolute worst scenario i'd say right now, worse than just making landfall and riding up the spine of the state.  Talk about increases in Homeowners insurance next year!
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3856. HadesGodWyvern (Mod)

Japan Meteorological Agency
Tropical Cyclone Advisory #29
TYPHOON BOLAVEN (T1215)
21:00 PM JST August 23 2012
====================================

SUBJECT: Category Three Typhoon In Sea South Of Japan

At 12:00 PM UTC, Typhoon Bolaven (955 hPa) located at 20.2N 134.1E has 10 minute sustained winds of 75 knots with gusts of 105 knots. The cyclone is reported as moving west northwest at 8 knots.

Dvorak Intensity: T4.5

Storm Force Winds
==================
80 NM from the center

Gale Force Winds
==================
240 NM from the center

Forecast and Intensity
======================

24 HRS: 22.4N 131.9E - 85 knots (CAT 4/Very Strong Typhoon) South Of Japan
48 HRS: 24.5N 129.2E - 90 knots (CAT 4/Very Strong Typhoon) Naha, Okinawa
72 HRS: 27.1N 126.7E - 90 knots (CAT 4/Very Strong Typhoon) Okinawa Island Waters

Japan Meteorological Agency
Tropical Cyclone Advisory #45
TYPHOON TEMBIN (T1214)
21:00 PM JST August 23 2012
====================================

SUBJECT: Category Three Typhoon Sea South Of Okinawa

At 12:00 PM UTC, Typhoon Tembin (955 hPa) located at 22.7N 122.5E has 10 minute sustained winds of 75 knots with gusts of 105 knots. The cyclone is reported as moving west slowly.

Dvorak Intensity: T4.5

Storm Force Winds
================
55 NM from the center

Gale Force Winds
=================
120 NM from the center

Forecast and Intensity
========================

24 HRS: 22.6N 120.2E - 50 knots (CAT 2/Severe Tropical Storm) Taiwan Strait
48 HRS: 21.3N 118.2E - 45 knots (CAT 1/Tropical Storm) South China Sea
72 HRS: 20.9N 118.3E - 45 knots (CAT 1/Tropical Storm) South China Sea
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Quoting gprxomstr:
starting to slow.
Maybe turning?
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3854. LargoFl
Quoting Grothar:
..gee that GDFL just wont budge, wish i knew what it knows
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Quoting 7544:
nhc cone shift at 11 am left or right ?


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