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 gordydunnot:
Man that GFS run is brutal, can the state of Texas take everyone from FL. if we bring rain.
Oh, if you're bringing rain then don't stop in Texas. Come right on in to New Mexico.
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Quoting KoritheMan:


The only thing I can think of is that the convective pattern is not well-organized yet. But look at the outflow pattern. The system is definitely breathing.

The outflow is very impressive tonight, that's for sure. Looks like Isaac has moved far enough away from the ULL to his northeast to where it stops shearing him, but is still close enough to it that it is helping to ventilate him.
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Quoting KoritheMan:


The only thing I can think of is that the convective pattern is not well-organized yet. But look at the outflow pattern. The system is definitely breathing.


And it is in the so called weaker area for development-the Eastern Caribbean. The GFS must have finally figured out the ECMWF solution.
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2648. scott39
Quoting weatherboykris:


The cone is the exact same size on both sides of the track. One of the biggest misconceptions I see is that people think that the size of the cone is somehow subjective. It isn't, at all. The forecasters do not determine the size of the cone, previous track errors over the past five years do.
Go look at the current cone at the NHC site....maybe I need some new glasses??
Member Since: June 13, 2009 Posts: 0 Comments: 6912
2647. gator23
Anyone who looks at a single run to make a determination about landfall more than 5 days from now is at best a poor scientist. You should really look at trends. For all we know tomorrow the run is back East.
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Quoting scott39:
What side of a hurricane do you add the forward movement to the speed of the winds at landfall?



The right front quadrant, in this case the north and eastern side.
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Quoting hydrus:
I was wondering if anyone on the blog is having trouble with the blos being squeeed to the point one can barely read it.?


Getting the same mice-type .... think admin had to make blob space room for the nervous groom on his way to cancun.
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2644. 7544
looks like se fla will feel a great deal as its still on the dirty side of this one
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2643. JLPR2
Quoting ProgressivePulse:


Even with the dip south I am not sold that it will consolidate there. Lowest pressures were still up on the NHC track. Issac is working out some major bugs tonight though.


The GFS does pull the circulation a little to the south only to move it north 3 hrs later, that little dip to the south must have been the storm aligning with the MLC and afterwards it basically moves north and back on track.

Will be interesting to see if it materializes.
Member Since: September 4, 2007 Posts: 7 Comments: 8747
2642. KEEPEROFTHEGATE (Mod)
Quoting bdm2225:
Bonjou from Port-au-Prince. How bad is it for us??


storm will be a cane to your se on approach should cross to the right or just east of POP strong winds with heavy flooding rains can and will occur

prepare as best as you can
warn as many as able too

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Quoting scott39:
Smmmokin!



That does not mean that it developed and matured at that speed. I've no historical information ready, but my guess would be that it sailed around the western periphery of a collapsing ridge to garner that much forward speed.
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Quoting ProgressivePulse:


Yes the gulf coastans are overlooking the first landfall.


which mighteven be mainland south florida
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Mission 5 is in the air.
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Air conditioning broken at my house. Getting a taste of what going through a hurricane is like.
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Levi,will the upper low over Cuba be a player?
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Quoting scott39:
I know this can change, but take note how the cone is shrinking on the E side and growing on the W side.


The cone is the exact same size on both sides of the track. One of the biggest misconceptions I see is that people think that the size of the cone is somehow subjective. It isn't, at all. The forecasters do not determine the size of the cone, previous track errors over the past five years do.
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Quoting 1900hurricane:

Nice use of the soundings. I always try to use then whenever available. It's a fantastic way to look through the deep layer of the atmosphere that we can't get otherwise.


It tells you the big picture when the models don't.
Member Since: March 7, 2007 Posts: 602 Comments: 21231
2634. scott39
What side of a hurricane do you add the forward movement to the speed of the winds at landfall?
Member Since: June 13, 2009 Posts: 0 Comments: 6912
Quoting sullivanweather:


The overall circulation is very broad and lacks a true inner core. Most of the deep blowups of convection have been the result of convergence along the differing airmasses being entrained into the storm (dry air to the north meeting up with moist air streaming into the storm from the south). Each time one of these large bursts of convection takes place smaller vorticies at the surface develop and spiral into, or away from, the overall broad and elongated center of the storm. This is what's causing all these 'jumps' of the center as each individual vorticy becomes dominant over another. It is better to pay little attention to these little vorticies and instead track the broad center unti a true inner core develops. Until that occurs Isaac will likely struggle to intensify.

Very good post.
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Levi at least Joe Bastardi agrees with you via his twitter "GFS may be too far west given west coast trough offshore and ridge over colorado. Much more like psns for east coast, not gulf storm."
Member Since: August 19, 2011 Posts: 0 Comments: 131
Tropical depression forms far out in Atlantic
(AP) – 48 minutes ago
MIAMI (AP) — Forecasters say a tropical depression that has formed far out in the Atlantic Ocean is expected to strengthen into a tropical storm on Thursday.
While most eyes were on Tropical Storm Isaac in the Caribbean Sea late Wednesday, the National Hurricane Center in Miami says the most recent tropical depression is about 1045 miles (1682 kilometers) west of the Cape Verde islands.
It has top sustained winds of 35 mph (56 kph) and is moving west-northwest at 17 mph (27 kph). It is expected to continue on that course through Friday, possibly veering east of Florida but nearing Bermuda by Monday night. Link
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Quoting louisianaboy444:


The only difference is a shift further to the west in the gulf...hits around the central Panhandle instead of the big bend area
This storm may be serious! On the latest infrared the heavy precip dives to the southwest.
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Quoting sullivanweather:


The overall circulation is very broad and lacks a true inner core. Most of the deep blowups of convection have been the result of convergence along the differing airmasses being entrained into the storm (dry air to the north meeting up with moist air streaming into the storm from the south). Each time one of these large bursts of convection takes place smaller vorticies at the surface develop and spiral into, or away from, the overall broad and elongated center of the storm. This is what's causing all these 'jumps' of the center as each individual vorticy becomes dominant over another. It is better to pay little attention to these little vorticies and instead track the broad center unti a true inner core develops. Until that occurs Isaac will likely struggle to intensify.


Hey sully. Good to see you.
Member Since: March 7, 2007 Posts: 602 Comments: 21231
Man that GFS run is brutal, can the state of Texas take everyone from FL. if we bring rain.
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2627. pcola57
Quoting KEEPEROFTHEGATE:
by noon mon one may best be rushing preps to completion


I'm prepped, but it's like an itch you can't scratch until monday.. :)
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Quoting JLPR2:
The scary image, in my opinion it makes everything seem scarier than normal.



Although the structure Isaac seems to be getting hints at intensification, either happening now or soon.

Wow, that image makes Isaac look twice as strong as he actually is. Literally, that image makes him look like a giant 90 mph hurricane. Thankfully, we all know better.
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Quoting Tribucanes:
Levi, or anyone else, what's stopping Isaac from intensifying quickly before he reaches the mountainous regions of Cuba? Isaac looks to be organizing well and I would not be surprised at all to see Isaac reach hurricane status tomorrow. Please and thank you.


The only thing I can think of is that the convective pattern is not well-organized yet. But look at the outflow pattern. The system is definitely breathing.
Member Since: March 7, 2007 Posts: 602 Comments: 21231
2624. GetReal


The only flea in the ointment that I can see if the ULL near E Cuba. It is currently located in an ideal place and is ventilating Isaac. IMO it will back off towards the west, being pushed by the anti-cyclone over Isaac.
Member Since: July 4, 2005 Posts: 204 Comments: 8898
Quoting uncwhurricane85:
it makes landfall in the keys first


Yes the gulf coastans are overlooking the first landfall.

First "US" landfall that is...
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Quoting Tribucanes:
Levi, or anyone else, what's stopping Isaac from intensifying quickly before he reaches the mountainous regions of Cuba? Isaac looks to be organizing well and I would not be surprised at all to see Isaac reach hurricane status tomorrow. Please and thank you.


The overall circulation is very broad and lacks a true inner core. Most of the deep blowups of convection have been the result of convergence along the differing airmasses being entrained into the storm (dry air to the north meeting up with moist air streaming into the storm from the south). Each time one of these large bursts of convection takes place smaller vorticies at the surface develop and spiral into, or away from, the overall broad and elongated center of the storm. This is what's causing all these 'jumps' of the center as each individual vorticy becomes dominant over another. It is better to pay little attention to these little vorticies and instead track the broad center unti a true inner core develops. Until that occurs Isaac will likely struggle to intensify.
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Quoting KEEPEROFTHEGATE:
HR 144 FINAL



Big thanks for posting every model run +10000
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2620. Levi32
Quoting Kowaliga:


Speaking of which, Levi, do you think the GFS accounting for the dry air that Isaac will be pulling off the FL peninsula should that track verify? Seems to me its spinning up to fast in the WGOM.


Storms running right up the west coast of Florida do tend to struggle. The 1935 Labor Day hurricane is a great example of this. The key is how much separation there is. If the storm moved up 100 miles west of the peninsula and made landfall in the panhandle instead of getting sucked right into the bight of Florida, then significant strengthening would be likely. It just can't get too close.
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Quoting Grothar:


Those things can do that very quickly.

Indeed. I sure hope not.
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Quoting yqt1001:
Oooh! pretty outflow.


On this loop you will see how the cold front is starting effect Tembin's outflow to the NW.
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48 Hours- US Surface Forcast
Link
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2616. 7544
will the plane be there in time for dmax looks like thats the time hes going to get stronger tia
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Quoting JLPR2:
In other news the barometer in my room (Carolina,PR) went down another millibar in the past hour, now down to 1011mb with Isaac still hundred of miles away.


Even with the dip south I am not sold that it will consolidate there. Lowest pressures were still up on the NHC track. Issac is working out some major bugs tonight though.
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2614. JLPR2
The scary image, in my opinion it makes everything look scarier than normal.



Although the structure Isaac seems to be getting hints at intensification, either happening now or soon.
Member Since: September 4, 2007 Posts: 7 Comments: 8747
2613. bappit
Quoting angiest:


The canonical example is the end of Wagner's Goetterdaemerung, where Bruennhilde has a ~15 minute aria to conclude the work (typically 4-4.5 hours), which also marks the conclusion of Der Ring des Nibelungen, a cycle of four operas. Wagner in particular typically requires big singers to project over the orchestra, which is usually louder than in other operas.

He also has them fly around I think.
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2612. scott39
Quoting Grothar:


The hurricane of 1938 (The Long Island Express) was moving at over 60 mph when it hit.
Smmmokin!
Member Since: June 13, 2009 Posts: 0 Comments: 6912
Don't know if anyone has posted this yet or not, but if anyone's interested, HH was tweeting during tonight's mission. Twitter handle is @HRD_AOML_NOAA And now I'm off to bed. Nite all!
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2610. Grothar
Quoting Progster:


That's actually kind of bleak, Mr. G.


Those things can do that very quickly.
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Quoting MississippiWx:
Still looks like a slight bubble of upper level ridging develops between Isaac and Joyce on the 500mb map from the GFS.

But look at that opening straight north
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2608. Grothar
Quoting Tribucanes:
How Grothar sneaks into the assisted living computer room every night to blog I'll never know. :) May we all live as full a life as you have Grothar. You putting your exploits into a book? If not, please do, Gro.


Who do you think wrote the Encyclopedia Brittanica?
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2607. KEEPEROFTHEGATE (Mod)
Quoting pcola57:


Starting to get my undevided attention Keep..(goes to pull cover off generator and checks cords...again)
by noon mon one may best be rushing preps to completion
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2606. JLPR2
Quoting JLPR2:
In other news the barometer in my room (Carolina,PR) went down another millibar in the past hour, now down to 1011mb with Isaac still hundred of miles away.


Actually now down to 1010.7mb, seems to be dropping faster.
Member Since: September 4, 2007 Posts: 7 Comments: 8747
Quoting GetReal:
well this run of GFS is now calling for a lanndfall near Panama City Beach....The trending WEST of the GFS continues slowly as it comes more in line with the EURO.
it makes landfall in the keys first
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2604. scott39
I know this can change, but take note how the cone is shrinking on the E side and growing on the W side.
Member Since: June 13, 2009 Posts: 0 Comments: 6912
Quoting Grothar:


Nothing.



That's actually kind of bleak, Mr. G.
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Quoting bdm2225:
Bonjou from Port-au-Prince. How bad is it for us??


Going to be a heavy rain maker for sure.
Member Since: August 17, 2005 Posts: 26 Comments: 16874
2601. JLPR2
Quoting ProgressivePulse:


No need to be shy, all satellite data points to that fact.


I seen a few get clobbered tonight for speaking of such blasphemy.
LOL!
Member Since: September 4, 2007 Posts: 7 Comments: 8747

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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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