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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The sudden wind shift from northerly to southwesterly as the plane turned west of Martinique is a great illustration of the WSW to ENE elongation of the center.

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GFS latest run, pressure fields...backing a little right now...homing in?

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Word up guys I am pretty much a lurker, but live in Panama City, please keep us posted as to probabilities, I need an extra day or two to prep, have seven adopted kids five are handicapped. I am really depending on you all and thank you so much.
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I am just waiting for the 18z or 00z GFS to come out and it have shifted way east to the coast or off the coast of FL.
Not because i think it will or want it to or any of that...just because it would throw such a "what the heck" into everyone's plans, forecasts they are swearing by, and everything else.... it would be interesting seeing all the blog replies :)
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145. HCW
Quoting MAweatherboy1:
New forecast cone didn't change much. Either they're not putting much stock in the center reformation or they're waiting for a little more data.


Track might start changing after some better data from the hunters gets into the models on Friday. Heck if it keeps shifting west it may end up in Mexico like others have done this season.
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Quoting TropicalAnalystwx13:

Perhaps a little, but the shift would not be significant.
I think it may move farther south than you think it may very well pass south of Hispaniola hope that happens so Haiti would not receive a huge blow.
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Quoting Levi32:
The only place dry air has been wrapping in during Isaac's entire life has been the western side, and that hasn't affected him greatly. He has a very nice moisture bubble that has remained perfectly intact, with again the only intrusion from the west, and minor at that.


He still looks kinda flat in the NE quad, tho.
Member Since: August 3, 2008 Posts: 16 Comments: 5874
The deepest convection is now out in front of the system, while the earlier burst has weakened some but also expanded a bit. And he still looks kind of stretched out.

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Quoting medic2luv:
Am I correct in saying that the Euro is now coming into more agreement with GFS? Yesterday the Euro had Isaac going to the Yucatan, GFS has consistently been in the same area. Just curious why the big change in the Euro when its been said that its the top model.


The Euro was probably forecasting a weaker storm and a high pressure building stronger well into the Gulf due to a retreating trough of low pressure. Things have changed.
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The only place dry air has been wrapping in during Isaac's entire life has been the western side, and that hasn't affected him greatly. He has a very nice moisture bubble that has remained perfectly intact, with again the only intrusion from the west, and minor at that.

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Quoting wunderkidcayman:
ok looks like forecast cone has moved N and E for the first half then also cone has moved S and W for the second half


I expect when the LLCOC is found and is better defined I expect the whole cone to be Shifted S and W


I highly think they will find the LLCOC further S of where the NHC 5pm update puts it


Of course you expect....never mind.
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Quoting Patrap:


Yeah, my Radios are going bonkers as well as TWC Flash Tone on the scroll.



Yeah im seeing it on wdsu right now. It seems very serious all of a sudden
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960
WGUS55 KVEF 222055
FFWVEF
CAC071-222300-
/O.NEW.KVEF.FF.W.0163.120822T2055Z-120822T2300Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000 Z.OO/

BULLETIN - EAS ACTIVATION REQUESTED
FLASH FLOOD WARNING
NATIONAL WEATHER SERVICE LAS VEGAS NV
155 PM PDT WED AUG 22 2012

THE NATIONAL WEATHER SERVICE IN LAS VEGAS HAS ISSUED A

* FLASH FLOOD WARNING FOR...
SOUTH CENTRAL SAN BERNARDINO COUNTY IN SOUTHERN CALIFORNIA...

* UNTIL 400 PM PDT.

* AT 152 PM PDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED
VERY HEAVY RAIN FROM A THUNDERSTORM OVER EXTREME SOUTH CENTRAL SAN
BERNARDINO COUNTY...OR 24 MILES EAST OF TWENTYNINE PALMS. THE STORM
PRODUCING VERY HEAVY RAIN WAS MOVING NORTHEAST AT 15 MPH...AND
WILL AFFECT HIGHWAY 62 BETWEEN MILE MARKERS 50 AND 67.

* LOCATIONS IMPACTED INCLUDE...
HIGHWAY 62 EAST OF TWENTYNINE PALMS.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

FLASH FLOODING IS EXPECTED ON AREA ROADWAYS...AROUND UNDERPASSES...
AND IN NORMALLY DRY WASHES AND LOW LYING AREAS. FLASH FLOODING IS
NATURES NUMBER ONE KILLER. NEVER TRY TO CROSS A FLOODED ROADWAY OR
WALK THROUGH FLOODWATERS. TURN AROUND...DONT DROWN!

REPORT FLOODING BY POSTING YOUR REPORT TO THE LAS VEGAS NATIONAL
WEATHER SERVICE FACEBOOK PAGE...OR BY USING THE TWITTER HASHTAG...
NWSVEGAS.



LAT...LON 3419 11575 3419 11548 3412 11548 3412 11553
3407 11555 3409 11560 3404 11560 3403 11561
3403 11575



MORGAN

CLICK HERE TO GO TO PREVIOUS BULLETINS.
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p3 has already been taken off from close by. kermit mission on google earth.
Member Since: August 6, 2007 Posts: 0 Comments: 693
"For people that think Florida will get nothing... WRONG!!! 100% WRONG. Issac could go to Texas and florida wuld still b impacted.... by high waves,strong rip currents and beach erosion... and the rip currents are just as dangerous as Issac itself..."
doppler i think your right about rip currents next week. we will have all the tourist from out of town at the beach having fun. even if this goes to texas lots of big waves will be rolling through im sure and that is a recipe for disaster with tourist out there. didnt even think of that.
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Quoting lottotexas:
Euro has moved from west side of Yucatan to west Cuba. Moving to the right
Tomatoe....tomato.....GFS was father E and has done more moving to the W, than the EURO has done to the E. They will meet sooner than later.
Member Since: June 13, 2009 Posts: 0 Comments: 6740
Time: 20:28:00Z
Coordinates: 13.35N 60.1333W
Acft. Static Air Press: 692.5 mb (~ 20.45 inHg)
Acft. Geopotential Hgt: 3,171 meters (~ 10,404 feet)
Extrap. Sfc. Press: 999.8 mb (~ 29.52 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 199° at 4 knots (From the SSW at ~ 4.6 mph)
Air Temp: 9.8°C (~ 49.6°F)
Dew Pt: 9.7°C (~ 49.5°F)
Peak (10s) Flt. Lvl. Wind: 6 knots (~ 6.9 mph)
SFMR Peak (10s) Sfc. Wind: 35 knots (~ 40.2 mph)
SFMR Rain Rate: 13 mm/hr (~ 0.51 in/hr)
(*) Denotes suspect data

New HH mission just found this
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Quoting MAweatherboy1:

A center reformation to the south would seem to mean the cone would shift south/west, right?
2-3 day time frame but the rest should remain in tact
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Quoting bigwes6844:
hey Pat tornado warning for grand isle right now


Yeah, my Radios are going bonkers as well as TWC Flash Tone on the scroll.



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Quoting VAbeachhurricanes:



Why is it?


A combination of the light northeasterly shear the last couple of days and the abnormally strong monsoonal southwesterly fetch from the south that helped spawn Isaac. That fetch is still with him to some extent, and has been supporting convection to the east and south of the center, but such a flow pattern does not favor low-level convergence in the NE quad. The monsoon fetch will be weakening as he enters the Caribbean, which should allow a more uniform convective pattern, but so far Isaac has been quite a unique case with its own unique struggles. We don't see this again for a while.
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Quoting Doppler22:
For people that think Florida will get nothing... WRONG!!! 100% WRONG. Issac could go to Texas and florida wuld still b impacted.... by high waves,strong rip currents and beach erosion... and the rip currents are just as dangerous as Issac itself...
Still cant see how Florida would escape TS Watches/ Hurricane Watches it would have to hit the yucatan to not give us anything look how big Isaac is
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121. Relix
Quoting serialteg:


You know better. The North will be less vulnerable than the South.


Oh yeah absolutely. But the north is to dry at the moment, plus the increasing distane, that I believe we won't be getting the tropical storm winds forecast. Except maybe the mountains. Getting any surf though? :P
Member Since: August 3, 2006 Posts: 0 Comments: 2648
Quoting MAweatherboy1:

A center reformation to the south would seem to mean the cone would shift south/west, right?

Perhaps a little, but the shift would not be significant.
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Quoting Levi32:
As some already pointed out yesterday, Isaac's development cycle is very similar to monsoon cyclones in the western Pacific, right down even to the lack of convection on the north side, which, by the way, is still not because of dry air as keeps being said.

What is it, then?
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Quoting Patrap:


hey Pat tornado warning for grand isle right now
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Quoting 954Soxfan:


Looking better and better for us in Fort Lauderdale!!! whoo hoo!!!! Dodged another one!


Really? If the track shifts only 50 miles to the east, it would be pretty bad for Fort Lauderdale.
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THE
GLOBAL MODELS HAVE COME INTO BETTER AGREEMENT SINCE THE LAST
ADVISORY...WITH THE NOGAPS AND CANADIAN MODELS SHIFTING TO THE WEST
AND THE ECMWF TO THE NORTH. HOWEVER...THE ECMWF CONTINUES TO SHOW
A STRONGER RIDGE...AND THUS SHOWS A MORE WESTERLY MOTION THAN THE
OTHER MODELS. THE 72-120 HR PORTION OF THE FORECAST TRACK IS
SIMILAR TO THE PREVIOUS FORECAST AND LIES NEAR THE CONSENSUS
MODELS. THERE REMAINS CONSIDERABLE UNCERTAINTY AS TO WHICH PORTIONS
OF FLORIDA...IF ANY...COULD BE AFFECTED BY ISAAC.



Some grudging respect for Euro finally?
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ok looks like forecast cone has moved N and E for the first half then also cone has moved S and W for the second half


I expect when the LLCOC is found and is better defined I expect the whole cone to be Shifted S and W


I highly think they will find the LLCOC further S of where the NHC 5pm update puts it
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Quoting lottotexas:

you forgot to refresh. that is an old run.


...No it isn't...
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Quoting TropicalAnalystwx13:

Why would it?

A center reformation to the south would seem to mean the cone would shift south/west, right?
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Quoting tropicalmommie:


I am in Key West and those darn sugar ants have been driving me crazy for a couple days now. They won't go away!

Rot roh, my little guys just putz around doing their biz calmly. Not liking the way they are acting at the moment
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Quoting scott39:
The exact opposite.... the GFS is coming into line with the EURO.
Euro has moved from west side of Yucatan to west Cuba. Moving to the right
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For people that think Florida will get nothing... WRONG!!! 100% WRONG. Issac could go to Texas and florida wuld still b impacted.... by high waves,strong rip currents and beach erosion... and the rip currents are just as dangerous as Issac itself...
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Quoting Levi32:
As some already pointed out yesterday, Isaac's development cycle is very similar to monsoon cyclones in the western Pacific, right down even to the lack of convection on the north side, which, by the way, is still not because of dry air as keeps being said.

I don't know as much about these things as you but that dry air can't be helping it:

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i agree with the guy posting itaintgonnahappen but he or she is being a bit abrasive i think. it just seems history repeating itself. dean ivan etc etc. hopefully that history will continue and the storm keeps trucking east and tampa can makes lots of money on those hardcore partying RNC members next week lord knows the local economy needs it.
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Quoting MAweatherboy1:
New forecast cone didn't change much. Either they're not putting much stock in the center reformation or they're waiting for a little more data.

Why would it?
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Quoting ITSNOTGONNAHAPPEN:
THE TRUTH HURTS AND I AM RIGHT
Yes, you're right... into my ignored list.
Member Since: August 1, 2011 Posts: 28 Comments: 7906
Quoting Levi32:
As some already pointed out yesterday, Isaac's development cycle is very similar to monsoon cyclones in the western Pacific, right down even to the lack of convection on the north side, which, by the way, is still not because of dry air as keeps being said.



Why is it?
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Quoting MAweatherboy1:
New forecast cone didn't change much. Either they're not putting much stock in the center reformation or they're waiting for a little more data.


Or a center reformation (in this particular case) is not as relevant to long term track.
Member Since: August 17, 2005 Posts: 26 Comments: 15656

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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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