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 CybrTeddy:
NOAA recon reported a pressure of 1002mb at 15.667N.


Still strengthening despite the competing centers?

Wow!
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Quoting Relix:
THat's it I give up on Isaac. Do whatever the heck you want


lol ...
is your uncle driving up the cost of goods yet
Member Since: August 22, 2006 Posts: 2 Comments: 1983
Quoting stormpetrol:
Time: 22:46:30Z
Coordinates: 15.6667N 61.7667W
Acft. Static Air Press: 752.4 mb (~ 22.22 inHg)
Acft. Geopotential Hgt: 2,515 meters (~ 8,251 feet)
Extrap. Sfc. Press: 1002.8 mb (~ 29.61 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 36° at 14 knots (From the NE at ~ 16.1 mph)
Air Temp: 15.2°C* (~ 59.4°F*)
Dew Pt: -*
Peak (10s) Flt. Lvl. Wind: 14 knots (~ 16.1 mph)
SFMR Peak (10s) Sfc. Wind: -
SFMR Rain Rate: -
(*) Denotes suspect data

Looks like this might the center.


1002.8 mb? Nice little drop there. Maybe a dominant center has finally been established? Too soon to say.
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Quoting Portlight:


That's a grim image; kind of like waking up to an alligator in the swimming pool


Looks like it's coming up on donating to Portlight time.
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Reminds me of sibling rivalry.
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Looks to me that reformation may be well south and west of NHC track.

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Quoting MississippiWx:


They are flying at 8500 feet, though.


this plane always does, that is the extrapolated surface pressure
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Lets see if the two centers do the Fuji dance!
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Quoting StormJunkie:



A mess, but still firing a fair amount of deep convection


What's up, SJ?
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Quoting CybrTeddy:
NOAA recon reported a pressure of 1002mb at 15.667N.


They are flying at 8500 feet, though.
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Quoting Tribucanes:
Cantore did not state where he would be sent next.

Steffi will be in Miami for the Andrew 20 yr show
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Quoting clwstmchasr:
I'm wondering if this could pull an Ivan on us (not the strength but direction). Everything pointed to Ivan running up the spine of Florida or the West Coast. Subtle changes happened and it shifted significantly West and hit Pensacola.


I've been comparing Isaac to Ivan since yesterday's ensemble models shifted west in Fla. So far, the track and development of Isaac is similar to Ivan, which is concerning to me. I don't ever want to see an "I" storm again.
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Quoting Stormchaser121:

ECMWF



.. as weird as it may sound...I wouldnt mind having Issac the way it is now coming right for TX , it'll bring us nice rain and a break from the heat.
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Quoting weatherman12345:

Do you think you will adjust your track any or are you still leaning towards and east coast of florida scraper


Still depends on model trends during the next 2-3 days. We're still 4-5 days out and I'm only 150 miles east of the NHC track, well within the general cone of possibilities. We've seen the models show great consensus on a 5-7 day forecast for days and then have to adjust right at the finish line. Until we really get Isaac going I don't see a reason to fuss over 150 miles on a 96-120 hour forecast.
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Quoting Sfloridacat5:
Didn't the ECMWF say Debbie was going to Texas?


Yup. Most of the models did. GFS scored big on that one showing an easterly track.
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Time: 22:46:30Z
Coordinates: 15.6667N 61.7667W
Acft. Static Air Press: 752.4 mb (~ 22.22 inHg)
Acft. Geopotential Hgt: 2,515 meters (~ 8,251 feet)
Extrap. Sfc. Press: 1002.8 mb (~ 29.61 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 36° at 14 knots (From the NE at ~ 16.1 mph)
Air Temp: 15.2°C* (~ 59.4°F*)
Dew Pt: -*
Peak (10s) Flt. Lvl. Wind: 14 knots (~ 16.1 mph)
SFMR Peak (10s) Sfc. Wind: -
SFMR Rain Rate: -
(*) Denotes suspect data

Looks like this might the center.
Member Since: April 29, 2007 Posts: 0 Comments: 7862
Quoting wn1995:


My latest forecast track for Isaac. A good bit different than my last several. Now forecasting it to remain a tropical storm until near florida where it will finally becoming a cat 1 hurricane. I am forecasting a west coast cruiser.
LOL
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Quoting Stormchaser121:

ECMWF


That's a grim image; kind of like waking up to an alligator in the swimming pool
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Quoting mcluvincane:


Can't argue with that..., exactly why I'm giving the euro more faith on this storm


Weirdest example of the Fujiwhara effect yet...:)
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Been driving all day and come home to a very unusual looking tropical storm, almost looks like it wants to split in half ... what in the world is going on with Isaac?
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680. Relix
THat's it I give up on Isaac. Do whatever the heck you want
Member Since: August 3, 2006 Posts: 0 Comments: 2723


My latest forecast track for Isaac. A good bit different than my last several. Now forecasting it to remain a tropical storm until near florida where it will finally becoming a cat 1 hurricane. I am forecasting a west coast cruiser.
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NOAA recon reported a pressure of 1002mb at 15.667N.
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Quoting GTcooliebai:
Can't believe but the Euro did show this in its run last night with 2 areas of low pressure.

Now granted this was 72 hrs. out, but it looks like it occurred earlier.





It showed it on a couple night time runs anyway.
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Quoting Felix2007:


No it's Kirk, why would Joyce take a whole 16 days just to get over there? lol


We dont even have Joyce yet..
Member Since: August 19, 2006 Posts: 13 Comments: 15302
Quoting GTcooliebai:
Can't believe but the Euro did show this in its run last night with 2 areas of low pressure.

Now granted this was 72 hrs. out, but it looks like it occurred earlier.





Noticing that those two ball of convection is becoming less like a ball and more elongated N to S which is indicting they are going to wrap around and consolidate. Stay tuned..........
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Didn't the ECMWF say Debbie was going to Texas?
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18z GFS has run through 126 hours so far.... it still insist on raping the SW FL coast from Keys through St Pete.
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In the weather business, always expect the unexpected!
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Quoting lottotexas:

KIRK @ 324 hrs


Hope he will find that weakness like what the GFS shows and not get caught under that ridge!
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Quoting GTcooliebai:
Fried shark is the best if you ever get a chance to try it.

Who said fried shark, that is one of my favs. In OZ we call it flake.
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669. Vero1
Quoting WeatherNerdPR:

XTRP is spot-on.
And always is correct...and basically the only thing to rely on at landfall.
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Quoting MiamiHurricanes09:
Not necessarily a shift; the model only goes out to 120 hours on Allan Huffman's website, so all we've seen is the fact that it impacts southern Florida and that's it. The fact that the UKMET still impacts the Bahamas, as well as the eastern coast of Florida, and then the Gulf of Mexico, shouldn't really come as a surprise.
Thanks for the clarification Miami
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Quoting GTcooliebai:
Can't believe but the Euro did show this in its run last night with 2 areas of low pressure.

Now granted this was 72 hrs. out, but it looks like it occurred earlier.




two center lol crazy storm!
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Quoting GTcooliebai:
Fried shark is the best if you ever get a chance to try it.
to tuff and filled with mercury.... So ,uch better saltwater fish like snapper,grouper, mahi mahi. flounder, pompano, snook,wahoo, i could keep going..........
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Quoting GTcooliebai:
Can't believe but the Euro did show this in its run last night with 2 areas of low pressure.

Now granted this was 72 hrs. out, but it looks like it occurred earlier.





Can't argue with that..., exactly why I'm giving the euro more faith on this storm
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Quoting GTcooliebai:
Can't believe but the Euro did show this in its run last night with 2 areas of low pressure.

Now granted this was 72 hrs. out, but it looks like it occurred earlier.



I sure hope it's not right about Isaac in the Gulf.
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Quoting PalmBeachWeather:
Quite a shift Levi
Not necessarily a shift; the model only goes out to 120 hours on Allan Huffman's website, so all we've seen is the fact that it impacts southern Florida and that's it. The fact that the UKMET ensembles still impacts the Bahamas, as well as the eastern coast of Florida, and then the Gulf of Mexico, shouldn't really come as a surprise.
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What would it take to get the COC between those two big areas of convection ?
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ECMWF
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Quoting ncstorm:
through 360..Im assuming that is Joyce over there in the east with all that rain..



No it's Kirk, why would Joyce take a whole 16 days just to get over there? lol
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Levi, you asked me the other night about the UKMET and whether or not it was the model that showed a northward bias with Hurricane Irene. Well, I went back and read through the discussions and the UKMET/CMC were the ones with the southern bias, while the GFS/ECMWF were the ones that had a northern bias.
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Quoting Levi32:
12z UKMET ensembles, mostly gulf:



Since none of those hit the US, I'm assuming almost all of the models have crossed that out?
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Can't believe but the Euro did show this in its run last night with 2 areas of low pressure.

Now granted this was 72 hrs. out, but it looks like it occurred earlier.



Member Since: August 31, 2009 Posts: 0 Comments: 5628
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Quoting ncstorm:
through 360..Im assuming that is Joyce over there in the east with all that rain..

Models showed Kirk paying a visit to us.Lol.
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Quoting Drakoen:
Some day you will learn when you have a real job...there is a right and a wrong. The engineer cannot miss the minus sign or the bridge collapses. I'm gonna take a little break now.


Aren't you just a Richard Edward today....give it a rest or perhaps leave for awhile...
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Cantore did not state where he would be sent next.
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Quoting stormpetrol:


Recon fixes equal a WSW movement for now, argue with Recon and WKC, not me :)) J/K.

yeah lol

Quoting superpete:


....and Grand Cayman on the way...

oh boy

Quoting PalmBeachWeather:
stormpetrol.... The saying is "Don't shoot the messenger"


lol

Quoting MAweatherboy1:
The NOAA plane is finding 50mph winds...


HH RECON was finding some near the area too
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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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