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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3951. drs2008
Quoting JasonRE:


Where could I see the EURO model? Sorry, trying to keep up with everything.......thanks!
Im ,now , offically concerned about the North central gulf.
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Quoting 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


Turn the lat/lon on, still appears to moving straight West at 270 ish.
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3948. Relix
Quoting Chiggy:

Moving straight WEST


I don't think so. Its definitely climbing, heck I'd say its over the Forecast points... IF the center is the NNE area. Maybe 290 degrees was too extreme, but its definitely climbing.
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3947. GetReal
Quoting 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


I just do not see that occurring at this time; but then again I do not have owl eyes either.
Member Since: July 4, 2005 Posts: 204 Comments: 8802
Quoting Bluestorm5:
Not yet. Trolls will be telling you that you guys are clear, but that's not the case. Florida is not out of woods.
How can anybody said that South FLorida is out of the woods??,please look at the official National Hurricane Center projected track,the center of the storm is just south of the tip of Florida or over the upper Florida keys!,this is a HUGE!! storm!!!,and as the Met's said don't concentrate on a single point,this is not a single point this is a massive system!!.South Florida will feel the effect of this storm not matter what it does a little bit to the East or to the West.
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Quoting hurricanehanna:
so...Isaac is weaker, center keeps reforming, it has slowed down...and the models are all over the place. I see where the two High pressure systems are, but can a system this weak "feel" the pull to the North ?


Explains every problem discussed
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I used to live in Florida (Tallahassee) but now in DC - have family in north Florida and Tallahassee still so keeping a close eye on Isaac.

Tallahassee is due statistically for a big hit - hope this is not the one - and as of now, Isaac is not strengthening. Until the CDO blows up OVER the center, it won't strengthen. Right now, it is SW of the center (where the mid-level center is). If we see that change, we will know it is strengthening.
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What's wrong with this image?





Give up?



It has nested white inside of double nested gray, far to the south-west of the official center fix, including it's own automated center fix.

Nested "weak" color inside of double nested stronger color usually signifies a CoC...


Even if you assume the radar graphic is foreshortened and move it half a degree north to compensate, it's still far WSW of the official fix.
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3942. Grothar
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.


True. Even if the center of circulation moves to the West coast, the right side of the storm is usually the "dirty side" Southeast Florida could see the worst weather with this. Also, if it slows down, which is also expected, all of South Florida, the Keys and perhaps even the Bahamas could see TS force for many hours.
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OUTLOOK VALID 28/0600Z 27.4N 83.7W
MAX WIND 70 KT...GUSTS 85 KT.

From latest point forecast...
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so...Isaac is weaker, center keeps reforming, it has slowed down...and the models are all over the place. I see where the two High pressure systems are, but can a system this weak "feel" the pull to the North ?
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The only good thing about Issac is that it's weaker than it was projected to be a day ago at this time. If it can only stay weak and disorganized with respect to stacking...
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3937. Max1023
Issac still isn't vertically stacked, but the SFMR supports at least 40 knot surface winds with a few data points indicating 45 (but with 11mm/hr rain). The winds have come up since early this morning and it appears that the MLC may be elongating towards the LLC indicating slow organization..
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Quoting connie1976:


Maybe the islands will tear the thing apart???
Hope for a Tropical Storm, Prepare for Category 2.
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Quoting weatherh98:


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

This thing will keep moving west until its a cane


yep but even if it did turn into Hurricane it would still be on a gereral W-N of due W-WNW look at 999 steering still show W even if we get up even further with stronger Hurricane 989 steering still says W bound with a small N bias-WNW
Member Since: June 13, 2009 Posts: 2 Comments: 9592
3934. bassis
To my eyes it looks like he is consolidating by getting rid of the excess baggage on the NW & SE sides. Still very large. TD10 has a nice symmetrical look to it right now
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3933. JasonRE
Quoting LeithScot:
From where Isaac is at 65W he would have to go straight NW NOW to hit the eastern edge of Hispaniola and straight WNW to hit the western edge.

Given his immense size, moisture field and comparative weakness, his straight west movement over the past few days to anything north of that will be very gradual I feel, despite the slowing of forward speed.

The longer he spends south of the predicted track could have big implications down the line.

Fractions of a degree now could widen to two degrees latitude 600 miles further west (south of Hispaniola's western tip) and double that after 900.

The Euro is (more or less) on the money, IMO.



Where could I see the EURO model? Sorry, trying to keep up with everything.......thanks!
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Quoting 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
Looks like it is getting back on track with the NHC forecast track.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
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Quoting connie1976:


Oh, I hope so!! I love my power and my neighborhood floods...

anyone that says South Florida is 80% in the clear is nuts.

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Bulk of storm nearly due south of Puerto Rico, deep in the Caribbean

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Easily a Category 4 right now...

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3928. hulakai
I's coc seems to be consolidating at the moment near or below 15N and he is still moving West. If so, and this continues, models and offl. will have to shift West. Jamaica should put up its antennae and get ready to put down its shutters.
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3927. Chiggy
Quoting 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

Moving straight WEST
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3926. 7544
someone ask for the gfdl

Link
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Quoting GTcooliebai:
Isaac takes over the entire Eastern Caribbean:


There's an...Eastern Caribbean? It thought there was just the Central Atlantic, Isaac and the Western Caribbean.
Member Since: July 7, 2010 Posts: 0 Comments: 5521
Got supplies for the event Isaac does hit fl. A lot of supplies I have has been sitting there since 2006 so, I'll hope to put them to good use. Not too terribly worried at this time though but like everyone else.. I am watching it.
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954SoxFan goodbye... I am glad you are married though.
You wife seems to be pretty smart...
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Quoting StormTracker2K:



That guy is a troll. Not the real handle and no way in hell is FL in the clear especially when Issac is atleast 600 MILES WIDE.



I moved to South Florida 6 yrs ago from Orlando.... We have had a couple of thunderstorms that scared me to death with the wind here!!! I hope that the thing goes away!!!
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3921. hubcity
Check this link out on model history for Katrina.
Lots of changes 3 days out.

http://www.wunderground.com/hurricane/at200511.as p?feature=verification
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Quoting 954Soxfan:

Troll alert.
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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...
Not yet. Trolls will be telling you that you guys are clear, but that's not the case. Florida is not out of woods.
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3918. Relix
DP, sorry haha
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3917. 7544
Quoting mikatnight:


Not to mention that the NHC issued it's 1st Hurricane Warning on Katrina (in Florida) only 18 hours before landfall...


and might do the same with issac imo
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3916. LargoFl
Quoting Waltanater:
However, time is running out....
yes just a few more days to prepare
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Quoting Grothar:


Well, Storm, You and Largo and a few others know, this is the exact tract we have had from the beginning. You bloggers know I NEVER seriously write that. I'll tease with lines like "I saw it first" and "I nailed It", but you all know I am joking. However, My original track forecast has not changed.

I said it would strengthen in the Easter Caribbean, move south of the DR and move over southern Haiti, then move into the Windward Passage over Easter Cuba and move towards Florida. I have not changed on this.

Now Doctor Masters will come along and no one will ever see this LOL. Just joking Doc.

Quoted For Truth.
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3914. 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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3912. JasonRE
Quoting weatherh98:


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

This thing will keep moving west until its a cane


So HOW FAR WEST will it continue to shift until it becomes a 'cane?

Trying to feel out if I should be going out of town next week with this thing still so far away. Don't want to leave the wife home alone to have to deal with anything associated with this storm. Will be gone Mon-Thurs.....

Honestly, even though this storm is forecasted to run up into the SE GOM, does anyone see it turning West at all or will we just have to wait until tomorrow for a really acurate, final model?
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From where Isaac is at 65W he would have to go straight NW NOW to hit the eastern edge of Hispaniola and straight WNW to hit the western edge.

Given his immense size, moisture field and comparative weakness, his straight west movement over the past few days to anything north of that will be very gradual I feel, despite the slowing of forward speed.

The longer he spends south of the predicted track could have big implications down the line.

Fractions of a degree now could widen to two degrees latitude 600 miles further west (south of Hispaniola's western tip) and double that after 900.

The Euro is (more or less) on the money, IMO.

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Quoting connie1976:


Oh, I hope so!! I love my power and my neighborhood floods...



That guy is a troll. Not the real handle and no way in hell is FL in the clear especially when Issac is atleast 600 MILES WIDE.

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3909. Grothar
Quoting StormTracker2K:


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



Well, Storm, You and Largo and a few others know, this is the exact tract we have had from the beginning. You bloggers know I NEVER seriously write that. I'll tease with lines like "I saw it first" and "I nailed It", but you all know I am joking. However, My original track forecast has not changed.

I said it would strengthen in the Easter Caribbean, move south of the DR and move over southern Haiti, then move into the Windward Passage over Easter Cuba and move towards Florida. I have not changed on this.

Now Doctor Masters will come along and no one will ever see this LOL. Just joking Doc.
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Quoting StormTracker2K:


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.



Not to mention that the NHC issued it's 1st Hurricane Warning on Katrina (in Florida) only 18 hours before landfall...
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3907. LargoFl
Quoting 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!
..good for you
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Still no defined circulation in this thing (this is crazy) therefore west he still goes
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3905. Chiggy
Quoting wunderkidcayman:


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



Left and South
\

Link
Can you guys look at this radar out of PR aswell... thx
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Quoting connie1976:


Oh, I hope so!! I love my power and my neighborhood floods...

I wouldn't listen to that person, they are a troll.

Just look at the models and they tell you the story.



I would say... To soon to tell but I would prepare for the worst.
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Quoting StormTracker2K:


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.



Exactly!
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Quoting connie1976:


Maybe the islands will tear the thing apart???


Entirely possible but by no means guaranteed. 50/50 if you ask me.
Member Since: August 10, 2010 Posts: 2 Comments: 1970
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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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.