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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Wind Fields for Isaac
Last Updated On 8/23/2012 3:00:00 PM GMT

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Quoting HurricaneGroupie:
Joyce is here!

Did you finally press F5 on the NHC page? LOL.
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water vapor

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Quoting opal92nwf:
Hurricane Ivan models were farther East (six days out) than they are with Isaac now.




Issac today

img src="Image and video hosting by TinyPic">


XTRP model has this thing headed to Texas, but it's clearly an outlier and will eventually swing back in line with the other models, as time goes by.
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That latest track is much more in line with what's been going on with Isaac.

I'm interested to see what the GFS info will show and it's effect on the next NHC forecast. I expect it will be more in line with the ECMWF and now NHC tracks
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4446. bappit
.
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By the way.......Good morning RitaEvac and all bloggers...... :)
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Quoting opal92nwf:
WOW, watching the Weather Channel right now, you would think we were in a Andrew or Katrina situation. It's just ridiculous.


Have to admit, living in Caribbean - the level of "activity" and "narrative intensity" on TWC are somewhat different to what I am used to - all said, however, the attention and addition of new graphics has to be a bonus compared to the old norm!

The "attractive talking heads" still grate though IMHO.

I would almost cynically say that the TWC have an upcoming IPO on the market
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Quoting Chiggy:

150 year HIstory suggests that FL landfall is rare for storms in Issac's position


Climatology also suggests it is nigh impossible for a major hurricane to go from west to east in the Caribbean. Yet it happened a few years ago.
Member Since: August 3, 2008 Posts: 16 Comments: 5881
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4441. yoboi
Quoting uptxcoast:
On the bright side it looks like the upper Texas Coast will be in for some great weather! Will be praying for those affected.


it has not made the turn yet still heading west...
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Quoting CaneHunter031472:
The fact that it is slowing down should suggest that it is about to take a turn. Whether it is as forecasted or to the SW it will make a significant difference as to the future tract Imo.

You need to check your info, Isaac has actually forward speed.

TROPICAL STORM ISAAC ADVISORY NUMBER 10
1100 AM AST THU AUG 23 2012
PRESENT MOVEMENT...W OR 275 DEGREES AT 15 MPH...24 KM/H

TROPICAL STORM ISAAC INTERMEDIATE ADVISORY NUMBER 9A
PRESENT MOVEMENT...W OR 270 DEGREES AT 13 MPH...21 KM/H

TROPICAL STORM ISAAC ADVISORY NUMBER 9
PRESENT MOVEMENT...W OR 270 DEGREES AT 12 MPH...19 KM/H

TROPICAL STORM ISAAC INTERMEDIATE ADVISORY NUMBER 8A
200 AM AST THU AUG 23 2012
PRESENT MOVEMENT...W OR 260 DEGREES AT 20 MPH...32 KM/H

Also note the slight turn. From 260 degrees at 2am to the current(11am) 275 degrees.
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Quoting hurricanejunky:
Isaac really starting to look like he's FINALLY getting his act together on satellite:




heard that before
Member Since: August 22, 2006 Posts: 2 Comments: 1983
Isaac really starting to look like he's FINALLY getting his act together on satellite:


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Convection blowing up over Hispanola due to topographic lift, could be bad for Haiti.
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Quoting clwstmchasr:


It will not be on top of Tampa. It will be 120 miles offshore at 84.5W. Thus Tampa would not experience the worst of the storm or even hurricane force winds. Having said that, that track can shift although the bias is steady to the west.
Yes unfortunately StormTracker2K may not get his storm in CFL after all, and will probably start calling out peoples forecasts and saying he is right and they are wrong.  hehe.
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Quoting bappit:

We are masochists. :)


I'm either a misogynist or a mechanic ... I keep forgetting ...
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Accuguess which Chucktown coined earlier was hilarious. They've come a long way since Katrina and Rita in that they tend to now not give a forecast that is utterly different and contradicting to what the NHC says. Can be argued their forecasting got some folks killed in Katrina and Rita.
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Quoting AussieStorm:

it's no mystery, If you read back you'll find out why.


Well, with over 4,000 posts, I don't have the time for that. It was a bit of a mystery why so many failed to strengthen last season, and this season is a continuation of that. 'Lack of vertical instability' was suggested for last season's duds.
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4432. emguy
Quoting opal92nwf:
Hurricane Ivan models were farther East (six days out) than they are with Isaac now.




Issac today

img src="Image and video hosting by TinyPic">


Huge difference between Ivan and Isaac is that Isaac's models have been generally locked in on the same thinking for days.

Ivan on the other hand had a terrible history of predicting an early turn...and on each run it just backed the turn up an additional 6 hours until Ivan had no choice but to turn. Model runs earlier in the lifecycle actually were predicting Ivan would turn out to sea.
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21Aug.06pmGMT: 15.3n53.2w (279.9* West@11.8knots) 35knots, 1005millibars, TropicalStorm
22Aug.12amGMT: 15.4n54.8w (273.9* West@15.5knots) 35knots, 1006millibars, TS
22Aug.06amGMT: 15.5n56.5w (273.7* West@16.4knots) 40knots, 1003millibars, TS
22Aug.12pmGMT: 15.9n58.5w (281.9*WNW@19.7knots) 40knots, 1006millibars, TS
22Aug.06pmGMT: 15.9n60.4w (270.3* West@18.3knots) 40knots, 1004millibars, TS
23Aug.12amGMT: 15.8n62.2w (267.0* West@17.4knots) 40knots, 1004millibars, TS

DSD-LaDesirade :: GBJ-Marie-Galante :: :: HTB-Terre-de-Bas :: DOM-Dominica

The easternmost dot is where TropicalDepressionNine became TropicalStormIsaac
The dot to the right of GBJ is TS.Isaac's 22Aug.06pmGMT position
The westernmost dot is TS.Isaac's 23Aug.12amGMT position

On 22August at ~8:54pmGMT, TS.Isaac made passage over Marie-Galante (GBJ)
On 22August at ~10:03pmGMT, TS.Isaac made passage over Terre-de-Bas (HTB)

Copy&paste stt, dsd, dom, 15.3n53.2w-15.4n54.8w, 15.4n54.8w-15.5n56.5w, 15.5n56.5w-15.9n58.5w, 15.9n58.5w-15.9n60.4w, 15.9n60.4w-15.8n62.2w, 15.9n60.4w-gbj, 15.9n60.4w-htb into the GreatCircleMapper for a larger map and other information
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
The previous mapping for comparison
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
22Aug.06pmGMT: 15.9n60.4w (270.3* West@18.3knots) 40knots, 1004millibars, TS
23Aug.12amGMT: 15.8n62.2w (267.0* West@17.4knots) 40knots, 1004millibars, TS
23Aug.06amGMT's numbers are below, before 23Aug.12pmGMT's
Derived from NHC_ATCF data for TropicalStormIsaac for 23August12pmGMT
MinimumPressure decreased from 1004millibars to 1003millibars
MaxSusWinds held to 40knots(46mph)74km/h to35knots(40.3mph)65km/h
Vector changed from 256.8*WSW@14.8mph(23.8km/h)] to 265.6*West@14.5mph(23.3km/h)

PUZ-PuertoCabezas :: PVA-Providencia :: LIO-Limon :: PTP-Guadeloupe :: DOM-Dominica ::

The easternmost dot is Isaac's position at the beginning of its 2nd day as a TropicalStorm
The easternmost dot on the longest line is TS.Isaac's most recent position

The longest line is a straightline projection through TS.Isaac's 2 most recent positions to its closest approach (within 18miles or 29kilometres) to a coastline
23Aug.12amGMT: TS.Issac had been headed for passage over LagunaLayasiksa (bottom,PUZdumbbell)
23Aug.06amGMT: TS.Isaac had been headed for passage over Portete (top,LIOblob)
23Aug.12pmGMT: TS.Isaac had been headed for passage ~3.8miles(6.2kilometres)North of Providencia in ~3days2hours from now (when this was posted)

Copy&paste peu, ptp, dom, 15.9n60.4w-15.8n62.2w, 15.9n60.4w-13.51n83.54w, puz, 15.8n62.2w-15.5n63.5w, 15.8n62.2w-10.019n83.115w, lio, 15.5n63.5w-15.4n64.8w, 15.5n63.5w-13.452n81.381w, 13.397n81.372w-13.4522n81.381w, pva into the GreatCircleMapper for a larger map and other information
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Joyce is here!
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4429. kwgirl
Quoting Stormcow6:
I am hosting a wedding in my backyard Sunday afternoon in Loxahatchee, Fl. Should I reschedule the tent delivery. I am not sure what to do. Canceling this wedding is not an option but I am nervous that it might rain. Any suggestions....
You should be good. The storm will still be south of you. It is projected to be over Cuba on Sunday. Just make sure that tent gets striked by Monday AM.
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Just for the heck of it, based on previous thoughts, current obs, I'm gonna say Isaac will likely totally miss Hispaniola. And, Jamaica will start putting up RED flags.
Member Since: July 9, 2006 Posts: 184 Comments: 29610
4426. yoboi
Quoting mikatnight:


if ya overlay the cones from the past 48 hrs it's more and more west...
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According to the weather channel we officially have TS Joyce.
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Quoting StormPro:
What we need is a good ole DJ DOOMCOM forecast.... Btw, most of us here in NOLA are not wringing our hands and sweating profusly ...yet


No but we've all got our trusty Official Nash Roberts ™ Black Markers out.
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Quoting samiam1234:
does florida even want this crap which will hardly put out 20 mph winds..this wouldnt even get them one day of work... Let tx or louisiana have this.. florida only accepts cat 2 or greater :)


REALLY? No thank you from Texas
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Here in Alabama I am keeping a wary eye on this one. I am enjoying reading all the info and opinions on here this morning.Teh East West cone movement back and forth has been espcially amusing.
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Joyce appears to be a fish....
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4419. Chiggy
Quoting opal92nwf:
Hurricane Ivan models were farther East (six days out) than they are with Isaac now.




Issac today

img src="Image and video hosting by TinyPic">

150 year HIstory suggests that FL landfall is rare for storms in Issac's position
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4418. VR46L
Maybe it wont be Florida DOOM .. Just sayin

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4417. Dakster
Quoting StormPro:
What we need is a good ole DJ DOOMCOM forecast.... Btw, most of us here in NOLA are not wringing our hands and sweating profusly ...yet


I see a key word in your post... Yet....

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WOW, watching the Weather Channel right now, you would think we were in a Andrew or Katrina situation. It's just ridiculous.
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4414. hydrus
Some of those model runs worry me terribly.
Member Since: September 27, 2007 Posts: 1 Comments: 21191
On the bright side it looks like the upper Texas Coast will be in for some great weather! Will be praying for those affected.
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NHC have changed their track to the left. the storm is now hitting close to alabama/ Mississippi border. Again this is assuming Isaac skirts dominican republican and go well north of Jamiaca going through central cuba. Based on its current movement of due west, I dont see how this happens. The only thing everyone seems to count on is a sudden surge in intensity pulling the storm northward
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Quoting jeffs713:

Thank you for your insightful, alarmist trolling. It isn't terribly obvious that you are trolling, considering your recent join date, official-looking post, and alarmist post content.

I hope you enjoy your short stay here on WU.


It only looked official for a few words. Then you get to "CATOSTRAPHIC" and know it's posted by someone who can't spell. ;)
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And there's some new convection popping up on the north side of the current cloudtops, closer to the track. No idea what that means. Will wait for wiser heads to notice and comment.

Quoting bappit:

I would expect the LLC to be more easily influenced by the convection, so I'd expect it to be near the southwest hidden by the overcast. I'd think the MLC would correspond more to the overall structure we can see on the satellite which suggests a center more to the northeast--due to the orientation of the bands on the east side of the storm.
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Quoting 7544:
no cone change
They moved to the WEST.
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4407. 7544
alot of new conv now building to the ne side looking better
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Quoting clwstmchasr:


It will not be on top of Tampa. It will be 120 miles offshore at 84.5W. Thus Tampa would not experience the worst of the storm or even hurricane force winds. Having said that, that track can shift although the bias is steady to the west.


Storm surge "funneling" I think will be a concern...
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What we need is a good ole DJ DOOMCOM forecast.... Btw, most of us here in NOLA are not wringing our hands and sweating profusly ...yet
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Well now the FL Panhandle (My House) is official dead center of the cone, joy, joy! Sure hope this thing doesn't become a monster in the GOM when it gets there.
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This is going to be very ugly!!!
Hope they are ready!!
Quoting sunlinepr:
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4402. Chiggy
Quoting CaneHunter031472:
The fact that it is slowing down should suggest that it is about to take a turn. Whether it is as forecasted or to the SW it will make a significant difference as to the future tract Imo.


It actually got 3 mph faster than 5am/8am advisory... 15mph is about an average for a TS/Hurricane...
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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.