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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3601. sar2401
Quoting StormTracker2K:
Expect Issac to really gain some wind speeds today. My guess is we will have a 60mph Issac at 5pm then 75mph Issac at 11pm.



Umm...no. Isaac is barely holding together as TS right now. There's nothing I can see that would lead to such RI over a short period of time. If it was over the Florida Straits, maybe, but not now.
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keep in mind the lines have nothing to do with time and everything to do with wind speed.

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3599. GetReal
700-850mb


500-850mb


No where to go but west for the next 24-36 hours... IMO
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Hi all...

Prep time! For those on the FL Panhandle make a plan & stay alert.
In the meantime - pray for shear!
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Quoting RTSplayer:
I hate when NHC does BS like this...

They moved the center fix 0.1N to 15.4N, even though their own data that was posted a page ago said it had moved at least 0.05 farther south.

Recon center fix was supposedly 15.25-ish if I read that right, and I bet that's being generous, given two different radar stations say the center is much south of that and also a degree farther west.




I'm laughing at their center fix, because if you look at the radar, and max out the frame rate, it's obvious their center fix is nothing but a dry pocket between two rain bands, which are obviously circulating around something at least a degree to the WSW of the official fix...
I have posted this twice but seems like everyone is choosing to ignore it. Recon posted two seperate Center Fix locations.

‎#1 :Center Fix Coordinates: 15°24'N 64°37'W (15.4N 64.6167W) #2: Center Fix Coordinates: 14°58'N 64°22'W (14.9667N 64.3667W)

Are some ignoring this due to the fact it isn't in line with their thinking ?

As far as I can tell they are still having difficulty finding the main coc.
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weatherh98 model will be uploaded in a second.

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3595. VR46L
The Caribbean in rainbow


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Quoting wxmobilejim:
Has anyone noticed that NHC hasn't updated their graphics since 2am? I wonder why they haven't.

It's updated for Isaac.

Member Since: September 30, 2007 Posts: 9 Comments: 15979
Quoting GeorgiaStormz:
LLC has relocated NW of the previous fix...

Center Fix Coordinates: 15°24'N 64°37'W (15.4N 64.6167W)


So the center of isaac is in the little blob NE of the big blob?



No, that's just wrong and ridiculous, even though that's what NHC says.

Set this to Long Range, and set animation speed to maximum, and try not to laugh at the NHC fix
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3592. HCW

Quoting TropicalAnalystwx13:

35 mph in 15 hours...you expect rapid intensification?



Since when does wind speed change equal RI ? I always thought that it was a 21mb drop in less than 24 hours that equaled RI
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Quoting gprxomstr:

If it won't dig as far south, then it will probably aid in allowing Isaac to get further into the northern Gulf Coast. The Euro model and now UK are seeing this.

Oh ok..thanks...makes sense!
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3590. Gearsts
Quoting RTSplayer:
I hate when NHC does BS like this...

They moved the center fix 0.1N to 15.4N, even though their own data that was posted a page ago said it had moved at least 0.05 farther south.

Recon center fix was supposedly 15.25-ish if I read that right, and I bet that's being generous, given two different radar stations say the center is much south of that and also a degree farther west.




I'm laughing at their center fix, because if you look at the radar, and max out the frame rate, it's obvious their center fix is nothing but a dry pocket between two rain bands, which are obviously circulating around something at least a degree to the WSW of the official fix...
The lowest pressure are in the northeast side of the system and thats where the center should be.
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3589. sar2401
The CME model has now turned a little neutral on an eastern Gulf path. Wholesale gasoline in the overnights is holding right around $3.14, which is near the close yseterday. The oil company mets, like the rest of us, are waiting to see which models actually start to confirm before advising their clients to bet gas up. Keeping an eye on the price of wholesale gas may give us better insights than some of the other models.
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3588. LargoFl
Quoting StormHype:


Trim those trees now, but do it right to keep the center of gravity low. It's possible to trim a tree and make it *worse* by taking off too many low branches without doing any topping.
thanks trouble is most are real close to me and they arent mine, we have huge tall tree's here by me.a tree two houses down could get me lol.. oh well, we have went thru storms before, we should be ok..just dont like the soaking wet ground this time..its different than times past
Member Since: August 6, 2011 Posts: 4 Comments: 42274
Quoting 69Viking:
Anybody else think the center is where the white cloud tops are below, below 15 N? Looks that way when you put it in motion too.



That's where I'd put it..but I'm no expert. It makes sense visually...but it could be MLC.
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I hate when NHC does BS like this...

They moved the center fix 0.1N to 15.4N, even though their own data that was posted a page ago said it had moved at least 0.05 farther south.

Recon center fix was supposedly 15.25-ish if I read that right, and I bet that's being generous, given two different radar stations say the center is much south of that and also a degree farther west.




I'm laughing at their center fix, because if you look at the radar, and max out the frame rate, it's obvious their center fix is nothing but a dry pocket between two rain bands, which are obviously circulating around something at least a degree to the WSW of the official fix...
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Quoting StormTracker2K:
Expect Issac to really gain some wind speeds today. My guess is we will have a 60mph Issac at 5pm then 75mph Issac at 11pm.


35 mph in 15 hours...you expect rapid intensification?
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23/1145 UTC 15.0N 41.7W T2.5/2.5 10L -- Atlantic
Member Since: May 21, 2006 Posts: 5091 Comments: 115456
Quoting presslord:
Yet again...people are coming outta the woodwork to help Portlight respond to this....This is truly a remarkable community we're part of here...


That's awesome!
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3582. LargoFl
Quoting GeorgiaStormz:


GFDL?
Certainly not...It couldnt even get up there if it was doping..
im sorry but the GDFL hasnt budged from its track, unlike the other models, I am for sure NOT discounting it yet, its track is very dangerous to us
Member Since: August 6, 2011 Posts: 4 Comments: 42274
Wow! Waking up this morning to see Isaac still very far south and not turning. The tracking doesn't even look right, with a huge NW turn needed in the next 6 hours.
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Quoting sunlinepr:
Traped between two ULLs...

Both are weakening ULLs and should not cause much of a problem now.
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Quoting gprxomstr:


How would you rate the REED model?

It's pretty good.
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Quoting LargoFl:
thanks..in that position we are in danger if it comes that way..well anyway..i think my place can withstand a cat-1 storm, unless the tree's come down..the ground is soaking wet here and more rain coming before the storm gets here..for me the worry is about the tree's coming down..only takes one to make for a very bad day huh


Trim those trees now, but do it right to keep the center of gravity low. It's possible to trim a tree and make it *worse* by taking off too many low branches without doing any topping.
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Expect Issac to really gain some wind speeds today. My guess is we will have a 60mph Issac at 5pm then 75mph Issac at 11pm.

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3576. Gearsts
Quoting cat6band:


Hahaaa...I'm not so sure of that....it's south of every NHC point....
Not even the nhc knows where the center is exactly because it barely has one. We don't know if is moving west or wnw until we get a center fix.
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3574. LargoFl
best advice the NHC can give you please listen to them....................IF YOU HAVE NOT DONE SO
ALREADY...INTERESTS ARE ENCOURAGED TO BEGIN PREPARING THROUGH
SATURDAY. VISIT WWW.READY.GOV FOR DETAILED PREPAREDNESS PLANS AND
INFORMATION.
Member Since: August 6, 2011 Posts: 4 Comments: 42274
Quoting StormTracker2K:
If this were the "Model Olymipics" then the GFS would get the Gold Medal, GFDL the Silver Medal, and the Euro the Bronze Medal.


GFDL?
Certainly not...It couldnt even get up there if it was doping..
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DMJ I saw someone comment on your question. I agree with their answer bring her in Sat. Although whatever she has to do in Ft. Myers might be delayed by a day or two anyways depending on future track. Always check with the National Hurricane Center website for official information.
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If this were the "Model Olymipics" then the GFS would get the Gold Medal, GFDL the Silver Medal, and the Euro the Bronze Medal.
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Has anyone noticed that NHC hasn't updated their graphics since 2am? I wonder why they haven't.
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Quoting StormHype:


Yup... and fwd motion has slowed too. Looks like he wants to intensify.
15.2 N was the previous center fix. This is the last one from HH
Latest VDM:

B. Center Fix Coordinates: 14°58'N 64°22'W (14.9667N 64.3667W)
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LLC has relocated NW of the previous fix...

Center Fix Coordinates: 15°24'N 64°37'W (15.4N 64.6167W)


So the center of isaac is in the little blob NE of the big blob?

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Morning all....the trough that was suppose to lift out across the Gulf coast is lingering...wondering if/how that might affect the forecast? If at all....thoughts?
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3566. GetReal
<
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Yet again...people are coming outta the woodwork to help Portlight respond to this....This is truly a remarkable community we're part of here...
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3563. Drakoen
Quoting 69Viking:


Sure doesn't look like it when you watch the satellite but then I don't have a plane in it getting me the exact center. I just look at the fact it was at 16.0 N yesterday and is still only at 15.4 N, so far he has not made a single forecast point, keeps going South and West of each one.


Which is why I emphasize the GFS and UKMET tracks, and also warn not to discount the ECMWF.
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Quoting weatherh98:
Weather h 98 model is running

That's one of the unreliable models. Down there with the NOGAPS.
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3561. LargoFl
Quoting TampaFLUSA:
Largo:

thanks..in that position we are in danger if it comes that way..well anyway..i think my place can withstand a cat-1 storm, unless the tree's come down..the ground is soaking wet here and more rain coming before the storm gets here..for me the worry is about the tree's coming down..only takes one to make for a very bad day huh
Member Since: August 6, 2011 Posts: 4 Comments: 42274
Quoting dmjmd:
Hi - total newbie here, hoping for some help from you experts.

My daughter HAS to be in Southwest Florida (Ft. Myers) on Monday morning. Currently she is scheduled to fly in Sunday afternoon, leaving New York at 2:30p, arriving at RSW at 5:30p Sunday. Is it pretty likely that Isaac will already be causing flight disruptions by then?

We could change her flight to Saturday evening, leaving NYC at 6pm and getting into RSW at 9pm, though it will be pretty costly to change; should that be pretty safe or is there a chance that flight disruptions could start that soon?

I know there are no guarantees, just looking for advice and educated guesses from people with more knowledge of these things, so any help is greatly appreciated...


Hello, I am in SW Fort Myers so I can give you some insight. Issac's outer bands will likely be going over us strong by the time she is going to be heading in. RSW is further inland, 15-20 miles from the coast. We will see impacts from Isaac if it stays in the forecast cone, Hurricane warnings would be issued for our coast Saturday night if the exact track and intensity of the NHC is realized. Flights cancellations into RSW would start Sunday. But, you greatly increase her chance of going if you schedule an earlier flight on Sunday. Being here on Monday could be dangerous though.
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Quoting Drakoen:


Maybe be trying to start that turn


Sure doesn't look like it when you watch the satellite but then I don't have a plane in it getting me the exact center. I just look at the fact it was at 16.0 N yesterday and is still only at 15.4 N, so far he has not made a single forecast point, keeps going South and West of each one.
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Quoting WxLogic:
LLC has relocated NW of the previous fix...

Center Fix Coordinates: 15°24'N 64°37'W (15.4N 64.6167W)


Yup... and fwd motion has slowed too. Looks like he wants to intensify.
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Quoting IKE:
Been doing ok. Didn't want to hog the blog during Isaac. Large system.


Obviously reports of your demise were greatly exaggerated, LOL.
Good to see you man!
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3556. Chiggy
Issac missing the next NHC forecast point by about >1deg to the south. May not sound much but it's actually huge this early in the game...
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Quoting LargoFl:
OMG..is this radar true?..it has to be a mistake..right?

Yep thats wrong somehow just checked radar all clear except for a couple of bits of rain
Member Since: June 6, 2010 Posts: 0 Comments: 4466
3554. sar2401
Quoting mcluvincane:


May not? It's a certain gulf system


I wouldn't say certain, but the odds of a Gulf track seem to be increasing. Isaac has to start making a turn no later than today for all this to come true, however. A more southerly and westerly track is still on the table. Isaac's lack of a clearly defined center and weak organization are throwing the models for a loop.
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Quoting IKE:
isaac looking a little better this morning vs. last night. Good morning everyone!


How the hell are you buddy! We miss you on here. I was wondering if Sir Issac would get you back in gear on here.

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Quoting gprxomstr:

12mph.

Link

Watch satellite. Our buddy has slowed and will begin to move WNW or NW. We may not see a Gulf system.


Hahaaa...I'm not so sure of that....it's south of every NHC point....
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Quoting dmjmd:
Hi - total newbie here, hoping for some help from you experts.

My daughter HAS to be in Southwest Florida (Ft. Myers) on Monday morning. Currently she is scheduled to fly in Sunday afternoon, leaving New York at 2:30p, arriving at RSW at 5:30p Sunday. Is it pretty likely that Isaac will already be causing flight disruptions by then?

We could change her flight to Saturday evening, leaving NYC at 6pm and getting into RSW at 9pm, though it will be pretty costly to change; should that be pretty safe or is there a chance that flight disruptions could start that soon?

I know there are no guarantees, just looking for advice and educated guesses from people with more knowledge of these things, so any help is greatly appreciated...


I would have her fly in Sat. Things will be fine until late Sun pm at the earliest. Mon pm is too late.
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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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