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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2751. scott39
EURO picked up on Isaac not developing as fast as the GFS.
Member Since: June 13, 2009 Posts: 0 Comments: 6890
2750. Grothar
Quoting AllStar17:


Gotta watch out for that rapic intensification!!!


Hey, Allstar, have you ever heard of the Windward Passage? :)
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Quoting MississippiWx:
Isaac doesn't seem to have a true CDO. Just looks like a large area convection racing around a mid-level center. It doesn't seem to be constant bursting over the center, so I wouldn't expect much of a difference in the strength. Just a hunch.


exactly....it has done this the past 3 nights..until the mid level and llc align, its not going to happen.
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Thats the attacking scud blob,,,What the hell was that?
Member Since: August 17, 2005 Posts: 26 Comments: 16874
2747. scott39
Quoting MississippiWx:
Isaac doesn't seem to have a true CDO. Just looks like a large area convection racing around a mid-level center. It doesn't seem to be constant bursting over the center, so I wouldn't expect much of a difference in the strength. Just a hunch.
Your right. He has to organize quite a bit to reach hurricane status. This is what I think the models are picking up on with a more left trend.
Member Since: June 13, 2009 Posts: 0 Comments: 6890
Quoting nofailsafe:
If Isaac is undergoing an RI cycle, Hispaniola better look out. With a little over a day left of prep time it's now or never more or less.

I hate seeing them get clobbered year after year by these things, just when it seems the Haitians have things put back together it's either a massive earthquake, dysentery, or a tropical cyclone...


There's not really anything they do to prepare in Haiti....that country is too poverty stricken.
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Quoting nofailsafe:
If Isaac is undergoing an RI cycle, Hispaniola better look out. With a little over a day left of prep time it's now or never more or less.

I hate seeing them get clobbered year after year by these things, just when it seems the Haitians have things put back together it's either a massive earthquake, dysentery, or a tropical cyclone...

RI is not taking place with Isaac.
It probably won't until the Islands are out of the way.
Only gradual strengthening is possible with organization the way it is.
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Quoting weatherboykris:



Nothing really, although Barbados has a band training over the island....I wonder if they flood easily?

Not sure really. I really can't say I'm familiar with the Antilles at all.

Yeah I wasn't able to spot anything with radar either. Barbados is too far east and the one from Martinique looks like is being blocked by the mountains on the islands, so that one can't get a good look either.
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Quoting 1900hurricane:

I wouldn't say that is the most likely solution at the moment, but it isn't out of the realm of possibility either. That's still over five days out, so there is quite a bit of room for error. That stretch of coastline should definitely be on the lookout.


Thank you! I am on alert to deploy and I don't want to leave home without battening down the hatches. Hopefully the shifts stop soon or I may have to deploy in my own back yard.
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Quoting KEEPEROFTHEGATE:
NO ITS A FLAG WHICH INDICATES THE POSSIBILITY FOR RAPID DEV.

sorry caps were locked


Is good. Wasn't aware there were three RI flag states.
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2741. Grothar
Member Since: Posts: Comments:
If Isaac is undergoing an RI cycle, Hispaniola better look out. With a little over a day left of prep time it's now or never more or less.

I hate seeing them get clobbered year after year by these things, just when it seems the Haitians have things put back together it's either a massive earthquake, dysentery, or a tropical cyclone...
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Quoting AllStar17:


Gotta watch out for that rapic intensification!!!

*rapid, yall know what i was trying to say heaven forbid someone have incorrect spelling on a blog site
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Quoting Levi32:
New recon mission is already on the runway. It's nice to have planes so on top of a storm like this.


There's a familiar name. Hi Levi. It's been a while since I've been here. Are you still in school?
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Isaac doesn't seem to have a true CDO. Just looks like a large area convection racing around a mid-level center. It doesn't seem to be constant bursting over the center, so I wouldn't expect much of a difference in the strength. Just a hunch.
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Isaac looking a heck of a lot better than it did just a few hours ago!

Really strong convection wrapping around the center. I think it's beginning to tighten up!

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If this misses Haiti significantly to the south. That's going to be bad news for US/Fl. If you look at the satellites not much is going to penetrate the gulf with that frozen front. The faster this storm goes west, and a little to the south the harder it's going to turn. Not good news. IMO
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2734. Levi32
New recon mission is already on the runway. It's nice to have planes so on top of a storm like this.
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2733. KEEPEROFTHEGATE (Mod)
three stages of RI
off
flag
on
Member Since: July 15, 2006 Posts: 175 Comments: 54858
Quoting ProgressivePulse:
04:32:00Z 17.700N 64.817W

Taking off now.


Link?
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Quoting 1900hurricane:

Dr. Masters actually has the links for them at the bottom of the first paragraph in this entry, but I wasn't able to make much of them the last time I looked (about 15 minutes ago). Maybe you can find something that I'm missing?



Nothing really, although Barbados has a band training over the island....I wonder if they flood easily?
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Quoting uncwhurricane85:


if RI means rapic intensification.....then def not! all its doing in desplacing the thunderstorms to the south like it has done the past 3 night...they will turn in to band and dissipate, and a refire on convection will happen again near 15.5-15.9


Gotta watch out for that rapic intensification!!!
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Quoting KEEPEROFTHEGATE:
T.C.F.W.
09L/TS/I/CX
MARK
15.51N/65.13W
R.I.FLAG FLAG

So Isaac has moved half a degree N and 4 deg W in 6 hours from the 6ish fix.
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Quoting MiamiHurricanes09:
Sunday I got 1 hour of sleep, Monday I got 2, last night I got a miraculous 3 and half, and tonight ain't looking too good hahaa.

Now I'm really signing off lol. Night.

I would have died by now if this was me. :P
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2727. KEEPEROFTHEGATE (Mod)
Quoting nofailsafe:


RI FLAG: ON, eh?
NO ITS A FLAG WHICH INDICATES THE POSSIBILITY FOR RAPID DEV.

sorry caps were locked
Member Since: July 15, 2006 Posts: 175 Comments: 54858
Quoting nofailsafe:


RI FLAG: ON, eh?


if RI means rapic intensification.....then def not! all its doing in desplacing the thunderstorms to the south like it has done the past 3 night...they will turn in to band and dissipate, and a refire on convection will happen again near 15.5-15.9
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2725. bappit
Quoting MBSCEOCHam:


Which means that 33% of the errors are outside of the cone--that's not terribly comforting.

Yep. More reason not to focus on the skinny line.
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Going to call it a night. Catch everyone in the morning.
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Quoting cirrocumulus:


This storm already defied every model except the ECMWF. It also is already developing a history of being unpredictable, large and having strong rains. Imagine if it bypasses the island of Cuba and gets into the gulf! It could have the makings of a monster!
GFS ensemble mean has been on same track since 00z 8/20
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2722. scott39
Quoting MiamiHurricanes09:
Sunday I got 1 hour of sleep, Monday I got 2, last night I got a miraculous 3 and half, and tonight ain't looking too good hahaa.

Now I'm really signing off lol. Night.
Your past sleep deprived...yea you need to go to bed now! Lol
Member Since: June 13, 2009 Posts: 0 Comments: 6890
If you run this for 20 frames and then speed it up, well it looks like one blob on the Ease side got really pissed and high tailed it at breakneck speeds to a blob on the west side, That was a little strange...

Member Since: August 17, 2005 Posts: 26 Comments: 16874
Quoting weatherboykris:



That'd be nice does anyone have a link to a Lesser Antilles radar? assuming the core of the storm is still in range?

Dr. Masters actually has the links for them at the bottom of the first paragraph in this entry, but I wasn't able to make much of them the last time I looked (about 15 minutes ago). Maybe you can find something that I'm missing?
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Quoting scott39:
Your young...you can handle it! Lol
Sunday I got 1 hour of sleep, Monday I got 2, last night I got a miraculous 3 and half, and tonight ain't looking too good hahaa.

Now I'm really signing off lol. Night.
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Quoting KEEPEROFTHEGATE:
T.C.F.W.
09L/TS/I/CX
MARK
15.51N/65.13W
R.I.FLAG FLAG


RI FLAG: ON, eh?
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So Issac has the RI Flag on?
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2716. Grothar
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Quoting scott39:
So the outer edge of the cone, doesnt have a less chance of getting hit on either side? That is wild!


Statistically, yes, that is correct. Which is my point...the cone is based completely on statistics.
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Quoting Stormbugn:
Hey guys ...and the incredibly smart ladies....what are the chances of it hitting between Mobile and New Orleans? I keep seeing it moving west. Shouldn't it be headed more WNW and turning to keep in line with the forecasts?

I wouldn't say that is the most likely solution at the moment, but it isn't out of the realm of possibility either. That's still over five days out, so there is quite a bit of room for error. That stretch of coastline should definitely be on the lookout.
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2713. scott39
Quoting MiamiHurricanes09:
...on to another night of less than 4 hours of sleep.

Isaac should be there in the morning. ;) Good night y'all.
Your young...you can handle it! Lol
Member Since: June 13, 2009 Posts: 0 Comments: 6890
Quoting scott39:
A cone will bend and shrink over time. There is more of the cone in the GOM than the E side of Fl, in the Atlantic.



A cone bends and shrinks based on the forecasted track of the storm, not the relative difficulty of the forecast. The cone is drawn using a simple computer program that uses the past five years' forecast errors. There is nothing subjective or specific about the cone itself, just the skinny black line. The forecasters have no input on the size or shape of the cone.
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Quoting bappit:

Nah, he's right. I looked it up not so long ago. The info is on the NHC site somewhere.

"the size of each circle is set so that it encloses 67% of the previous five years official forecast errors. The cone is then formed by smoothly connecting the area swept out by the set of circles."


Which means that 33% of the errors are outside of the cone--that's not terribly comforting.
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2710. scott39
Quoting weatherboykris:


A faster storm will have a smaller cone not because the track is more certain (although usually, a quickly moving storm is easier to predict, Wilma for example), but simply because the size of the error circles surrounding each forecast point need to stay the same regardless of storm's speed.
So the outer edge of the cone, doesnt have a less chance of getting hit on either side? That is wild!
Member Since: June 13, 2009 Posts: 0 Comments: 6890
2709. KEEPEROFTHEGATE (Mod)
T.C.F.W.
09L/TS/I/CX
MARK
15.51N/65.13W
R.I.FLAG FLAG
Member Since: July 15, 2006 Posts: 175 Comments: 54858
...on to another night of less than 4 hours of sleep.

Isaac should be there in the morning. ;) Good night y'all.
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Quoting cirrocumulus:
Yes! At this point one needs to go with the center of the overall cloud mass represented on satellite.


Don't just follow the convective, cold cloud tops. Use the channel2 IR and pay attention to the low-level circulation. Looking at the other channels which won't pick up the low-level clouds well gives the impression the storm is zipping along to the west even faster than it actually is.
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Quoting southfla:
I need to mention the ants. Maybe it is the approaching fall season (yes Miami has one) or maybe the less than stellar housecleaning on my part, but several of the window sills have been overrun with ants this past week. Just saying...


For the past 2 days we too have ants EVERYWHERE (I'm in South Palm Beach County, FL) Same happened before Wilma and Faye. However, this is a big system so we'll probably get a lot of rain.

Good to see old friends!
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Quoting DataNerd:
Howdy All!



Been busy with work lately so I have not been on hardly at all.


Interesting to see that after Ernesto, we have seen so many systems so quickly, and now two brand new cyclones.


Isaac appears to be intensifying at this hour and as it poses a threat to the GOM I will be watching it very carefully.


This storm already defied every model except the ECMWF. It also is already developing a history of being unpredictable, large and having strong rains. Imagine if it bypasses the island of Cuba and gets into the gulf! It could have the makings of a monster!
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Quoting 1900hurricane:

I would love a good microwave pass about now. It would give us a nice look to see how things are organizing at the moment.



That'd be nice does anyone have a link to a Lesser Antilles radar? assuming the core of the storm is still in range?
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Quoting southfla:
I need to mention the ants. Maybe it is the approaching fall season (yes Miami has one) or maybe the less than stellar housecleaning on my part, but several of the window sills have been overrun with ants this past week. Just saying...

glad it was you and not me but I thought about it! Lol
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Quoting yqt1001:
Tembin still strengthening at a good clip, Taiwan might be in for quite a bit.



Looking very impressive now.

Good looking storm for sure.



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2701. bappit
Quoting yqt1001:
Tembin still strengthening at a good clip, Taiwan might be in for quite a bit.



Looking very impressive now.

A beast. Hey, little piggy ...
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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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