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 HoustonTxGal:
Wow, what just happened? One minute we are all discussing and share info on Isaac and then next minute and half a dozen arguments have broken out.
I think your wrong HoustonTxGal!! Wrong wrong wrong, lets fight about it! :) Kori's a lightening rod what can I say? Weather guru that he is. JK Kori. You put a lotta Alphas in the room together and they're bound to try and eat each other sometimes. Awesome to have you here HoustonTxGal you have a lot to offer.
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2100. HrDelta
Quoting MelbourneTom:


Hi Levi, with almost all model runs shifting West I am looking forward to your update tomorrow. Thanks for the good work.


I wishcast that storms won't hit Haiti.
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Quoting Felix2007:
Still only 45 mph?


Aye, and they barely can support that at this rate.
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Quoting eyewall99:
Kori wants everyone to talk in scientific terms on this blog? LMAO Maybe he should join a meteorology paid site if there is one out there where he can post his jargon to impress others. I find his blogs and posts way to wordy and it is obvious he wants me to be impressed with the "big" words he uses (overkill).


I don't charge for forecasting lessons, dear.

And yes, dear me. How dare I suggest we use science on a weather blog. How thoughtless of me!

This is comical.
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30 minutes until GFS begins its run.
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Sir Levi or Kori, I have a question for you. Should Isaac completely miss Cuba, and an essence only scrape the northeastern coast and remain over water, what type of implications would that have on the intensity and track?
Member Since: September 2, 2009 Posts: 130 Comments: 21091
Quoting Tazmanian:



And if they don't stop they will ikey end up banned. Wounder blog admin is lurking some where I beat wait too banned sme one


So if they don't want too get banned from the blogs they sould get back too the weather



Just saying


AGREE :o)
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Quoting angiest:
Can someone post the advisory? I don't think we've seen it yet.


You know? Sheesh. LOL
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Quoting louisianaboy444:


Why do you care so much about that recognition no one is better than anyone there are many of us that are degreed or about to be degreed meteorologist that do not post..


Because in order for your opinions to get taken remotely seriously around these parts, you have to fight for it. Sad but true. Some have said that this blog has an elitist mentality. I tend to agree.
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2092. Michfan
That upper level low is also going to help ventilate Issac as long as it stays a good distance away.
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storm.is.going.to.be.bigger.than.the.cone
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2089. angiest
Can someone post the advisory? I don't think we've seen it yet.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
He's deep down in the Caribbean at current location



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Quoting scott39:
Ok fine, I took it that you understood me. Calling me a liar without knowing me was uncalled for.


My apologies. Just provide some clarification next time.
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2085. HrDelta
Quoting Losttsol:


I fail to see how Accuweather killed 30 people after Katrina (especially since the storm had already passed in your context). People die during bad storms. The best forecasting in the world won't save them. Even the Native Americans told the white people they were crazy for building their city in a hole.


Because of them, the DHS mis-deployed their resources, lengthening the amount of time to get to New Orleans.
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So Isaac slowed down 1mph, decreased pressure by 1mb, and increased latitude by .1 degree.
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Still only 45 mph?
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Lol 1 more thing. Seems like Isaac center moved further south again based on where HH are heading.



Unless there looking for something else. Again, night everyone.
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Chicklit that looks like a giant crab monster Post2051
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Quoting Levi32:


Hey that's where I wishcast all my storms.


Hi Levi, with almost all model runs shifting West I am looking forward to your update tomorrow. Thanks for the good work.
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Looks kinda' breezy in San Juan...

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2078. trey33
Quoting Levi32:


Hey that's where I wishcast all my storms.


Well maybe if we wishcast together it might ....



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Comenzo el Mambo...



ULL over Cuba is about to take charge and will start forcing Isaac WNW

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THE HURRICANE WATCH FOR PUERTO RICO AND THE U.S. VIRGIN ISLANDS HAS
BEEN DISCONTINUED.
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Slight shift westward, but would still give the west coast of FL. bad weather.

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2074. Michfan
Quoting KoritheMan:


I just can't respect opinions that are not scientific, when we're dealing with science! I don't care if you wishcast a storm to land or out to sea. If you make a claim, you better have some good reasons for it. That is how you garner respect, if you care about that sort of thing. Me, Drak, Levi, TA, and Mississippi didn't get the recognition we have now overnight. It was a gradual process, through the presentation of demonstrable evidence.


Bingo. Back up your claims with science. Not hyperbole.
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2072. scott39
Quoting KoritheMan:


I was saying that I desire to have a hurricane so I can get a chase. The face was meant for me more than it was you.

Again, I took it exactly how you said it.
Ok fine, I took it that you understood me. Calling me a liar without knowing me was uncalled for.
Member Since: June 13, 2009 Posts: 0 Comments: 6730
Quoting Chicklit:


okay, here we go...
It's like it jumped forward 12 hours. adjustment needed on timing already...
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000
WTNT44 KNHC 230257
TCDAT4

TROPICAL STORM ISAAC DISCUSSION NUMBER 8
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
1100 PM AST WED AUG 22 2012

AN AIR FORCE HURRICANE HUNTER AIRCRAFT INVESTIGATED ISAAC SEVERAL
HOURS AGO AND FOUND THAT THE CENTER WAS STILL NOT WELL-DEFINED AND
THE INTENSITY ESTIMATE OF 40 KT WAS PROBABLY A LITTLE ON THE HIGH
SIDE. HOWEVER...DROPSONDE DATA FROM A NOAA AIRCRAFT CURRENTLY
INVESTIGATING THE STORM SUGGEST THAT THE CENTER HAS PROBABLY BECOME
A LITTLE BETTER DEFINED AND THAT THE MINIMUM PRESSURE HAS DROPPED A
BIT. THERE HAS ALSO BEEN AN INCREASE IN COLD-TOPPED CONVECTION
NEAR AND OVER THE ESTIMATED CENTER SO THE CURRENT INTENSITY IS HELD
AT 40 KT. ISAAC HAS DEVELOPED AN IMPRESSIVE UPPER-LEVEL OUTFLOW
PATTERN AND DYNAMICAL GUIDANCE SHOWS THAT THE VERTICAL SHEAR SHOULD
REMAIN WEAK DURING THE NEXT SEVERAL DAYS. THEREFORE THE PRIMARY
IMPEDIMENT TO STRENGTHENING SHOULD BE THE INTERACTION WITH THE
MOUNTAINOUS TERRAIN OF HISPANIOLA AND EASTERN CUBA. THE MOST
RECENT RUNS OF THE HWRF AND GFDL SHOW SIMILAR AMOUNTS OF LAND
INTERACTION BUT ARE NOT IN GOOD AGREEMENT ON THE INTENSITY FORECAST
AS THE FORMER MODEL SHOWS MUCH LESS STRENGTHENING THAN THE LATTER.
THE OFFICIAL WIND SPEED FORECAST IS IN REASONABLE AGREEMENT WITH
THE LATEST STATISTICAL-DYNAMICAL GUIDANCE.

AFTER A SLIGHT SOUTHWARD RELOCATION OF THE CENTER...THE INITIAL
MOTION CONTINUES BASICALLY WESTWARD OR 270/17. ISAAC SHOULD
CONTINUE TO MOVE WESTWARD TO WEST-NORTHWESTWARD FOR THE NEXT COUPLE
OF DAYS ALONG THE SOUTHERN PERIPHERY OF THE SUBTROPICAL RIDGE. THE
GLOBAL MODELS CONTINUE TO DEPICT A BREAK IN THE RIDGE IN THE
VICINITY OF FLORIDA IN SEVERAL DAYS...WHICH SHOULD INDUCE A MORE
NORTHWARD COMPONENT OF MOTION. THE ECMWF MODEL SHOWS LESS OF A
BREAK IN THE RIDGE THAN THE GFS SOLUTION. CONSEQUENTLY...THE ECMWF
TRACK IS SOMEWHAT TO THE SOUTHWEST OF MOST OF THE OTHER TRACK
MODELS WHICH WOULD LESSEN THE THREAT TO FLORIDA. THE OFFICIAL
TRACK FORECAST FOR THIS ADVISORY IS QUITE CLOSE TO THE PREVIOUS ONE
AND ALSO VERY CLOSE TO THE DYNAMICAL MODEL CONSENSUS.

FORECAST POSITIONS AND MAX WINDS

INIT 23/0300Z 15.8N 63.0W 40 KT 45 MPH
12H 23/1200Z 16.2N 65.4W 45 KT 50 MPH
24H 24/0000Z 16.9N 68.1W 55 KT 65 MPH
36H 24/1200Z 17.5N 70.5W 65 KT 75 MPH
48H 25/0000Z 18.6N 72.7W 55 KT 65 MPH
72H 26/0000Z 21.5N 77.0W 60 KT 70 MPH...ON NORTH COAST OF CUBA
96H 27/0000Z 24.0N 80.5W 65 KT 75 MPH
120H 28/0000Z 27.0N 83.5W 70 KT 80 MPH

$$
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He's really getting that look now!
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2067. geepy86
it's here
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BULLETIN
TROPICAL STORM ISAAC ADVISORY NUMBER 8
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
1100 PM AST WED AUG 22 2012

...ISAAC MOVING OVER THE EASTERN CARIBBEAN SEA...


SUMMARY OF 1100 PM AST...0300 UTC...INFORMATION
-----------------------------------------------
LOCATION...15.8N 63.0W
ABOUT 270 MI...440 KM SE OF SAN JUAN PUERTO RICO
MAXIMUM SUSTAINED WINDS...45 MPH...75 KM/H
PRESENT MOVEMENT...W OR 270 DEGREES AT 20 MPH...32 KM/H
MINIMUM CENTRAL PRESSURE...1003 MB...29.62 INCHES

Blog crashes after too many posts of advisory.
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BULLETIN
TROPICAL STORM ISAAC ADVISORY NUMBER 8
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
1100 PM AST WED AUG 22 2012

...ISAAC MOVING OVER THE EASTERN CARIBBEAN SEA...


SUMMARY OF 1100 PM AST...0300 UTC...INFORMATION
-----------------------------------------------
LOCATION...15.8N 63.0W
ABOUT 270 MI...440 KM SE OF SAN JUAN PUERTO RICO
MAXIMUM SUSTAINED WINDS...45 MPH...75 KM/H
PRESENT MOVEMENT...W OR 270 DEGREES AT 20 MPH...32 KM/H
MINIMUM CENTRAL PRESSURE...1003 MB...29.62 INCHES
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...ISAAC MOVING OVER THE EASTERN CARIBBEAN SEA...
------------------------------------------------- -------------------------------

11:00 PM AST Wed Aug 22
Location: 15.8°N 63.0°W
Moving: W at 20 mph
Min pressure: 1003 mb
Max sustained: 45 mph

.1N
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...ISAAC MOVING OVER THE EASTERN CARIBBEAN SEA...

11:00 PM AST Wed Aug 22
Location: 15.8N 63.0W
Moving: W at 20 mph
Min pressure: 1003 mb
Max sustained: 45 mph
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Quoting cat6band:



Stop beating your chest...this is a weather blog....Lighten up!!


This is how I always am, lol. Frightening, I know. :P
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Quoting KoritheMan:


I just can't respect opinions that are not scientific, when we're dealing with science! I don't care if you wishcast a storm to land or out to sea. If you make a claim, you better have some good reasons for it. That is how you garner respect, if you care about that sort of thing. Me, Drak, Levi, TA, and Mississippi didn't get the recognition we have now overnight. It was a gradual process, through the presentation of demonstrable evidence.


Why do you care so much about that recognition no one is better than anyone there are many of us that are degreed or about to be degreed meteorologist that do not post..
Member Since: August 29, 2006 Posts: 22 Comments: 1342
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Quoting HoustonTxGal:
Wow, what just happened? One minute we are all discussing and share info on Isaac and then next minute and half a dozen arguments have broken out.



And if they don't stop they will ikey end up banned. Wounder blog admin is lurking some where I beat wait too banned sme one


So if they don't want too get banned from the blogs they sould get back too the weather



Just saying
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Quoting KoritheMan:


I just can't respect opinions that are not scientific, when we're dealing with science! I don't care if you wishcast a storm to land or out to sea. If you make a claim, you better have some good reasons for it. That is how you garner respect, if you care about that sort of thing. Me, Drak, Levi, TA, and Mississippi didn't get the recognition we have now overnight. It was a gradual process, through the presentation of demonstrable evidence.



Stop beating your chest...this is a weather blog....Lighten up!!
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Tropical Storm ISAAC NESDIS Satellite | NDBC Obs | Storm Archive
...ISAAC MOVING OVER THE EASTERN CARIBBEAN SEA...
------------------------------------------------- -------------------------------

11:00 PM AST Wed Aug 22
Location: 15.8°N 63.0°W
Moving: W at 20 mph
Min pressure: 1003 mb
Max sustained: 45 mph


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Quoting HurricaneDean07:
NHC taking their sweet time with Isaac's Advisory.
]

Probably not much difference, we shall see.
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2055. Levi32
Quoting trey33:


I know, I know!!! Alaska right? :)

What do I win???


Hey that's where I wishcast all my storms.
Member Since: November 24, 2005 Posts: 635 Comments: 26547
Monstrette stopped reporting. Last report was winds from the ESE 37

Guadalupe is reporting winds from the West
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Come on NHC, I can only hit F5 so many times:)
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Quoting scott39:
Thats BS and you know it! You even posted a crazy face when i said I wasnt normal jokingly......


I was saying that I desire to have a hurricane so I can get a chase. The face was meant for me more than it was you.

Again, I took it exactly how you said it.
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okay, here we go...
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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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