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 StormJunkie:
What's up WCSC, good to see you. Yeah, I know it's a dangerous game to play trying to keep level heads prevailing around here...lol


im good man just doing chyron for the fox 24 news at 10 but watching blog at same time lol
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Quoting Bluestorm5:
People need to chill and not hype the storm up.
Yes!!! Hype is causing undue fear. Even though intensity forecasting has not gotten any better since the days of Andrew, I have to believe the folks at the hurricane center have a handle on it and wouldn't suggest a weak storm if it were possible it could be a major cane.
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Quoting KEEPEROFTHEGATE:


should change his name to sherlock


LOL best comment all day :D
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Quoting HuracanTaino:
Oh please don't tell me,, I'm still with power here in the west...of PR.
didnt they just shut it off for one storm?
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Quoting scott39:
Why are you so sure of yourself, that Isaac wont have room in the GOM to BOMB??


Again, I am not "so" sure; but fairly certain.

1. Most model guidance keeps it close to Fl.
2. It is a badly decoupled system ATM and it is also a large system which means it has a lot of work to do in order to work that decoupling out.
3. Most models have it tangling with mountains in Haiti and Eastern Cuba. This would likely disrupt any stacking it attempted previously.
4. Very large systems take their time getting back together after being disrupted (mountains) typically.

Sure, there is always a chance the unexpected happens...But those chances are low in this case imho.
Member Since: August 17, 2005 Posts: 26 Comments: 16230
1846. trey33
Quoting floridaT:
east of naples


Wow you are in the glades

(thought you were going to say Weston :)
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1845. Dakster
Quoting yoboi:


try to avoid using gas with ethanol, ethanol messes up small engines....


Yes, it definitely does. I have had to throw out a three year old weed-eater because of ethanol...

TOUGH as heck to find the non-ethanol stuff though. I know of only ONE station in Miami-Dade that has non-ethanol fuel - which by the way is $4.29/gal right now, while you can get regular for $3.75...
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Quoting floridaT:
did they turn off the power like they did a couple years ago?
Oh please don't tell me,, I'm still with power here in the west...of PR.
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Quoting JLPR2:


Dang... already?
Mine is holding for the moment, though I see no reason why it shouldn't. :\


I know, we haven't seen any real strong wind gusts, just some rain and lightning...but out of the blue the darn power is gone!
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Quoting AussieStorm:
Ummmmmm......

Pretty consistent with the Tampa area.
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Quoting matilda101:
Jim Cantore is suggesting that a new center may be forming to the SW of the NHC fixed position as he believes that a center of circulation may be developing with a tongue of thunderstorms wrapping around the westside of the area of concern. He also says that is just a speculation on his part


When Cantore shows up at Pensacola Beach, I'll start getting a bit concerned.
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Quoting trey33:


east or westside?
east of naples
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1839. scott39
@ Stormjunkie.... The past 24 hours have shown only trends towards the W with models( with the exception of the other outlier the GFDL) I will take the EURO over the GFDL any day of the week.
Member Since: June 13, 2009 Posts: 0 Comments: 6747
1838. HrDelta
Quoting leo305:


Just like Ike right.. Just because a system gets into the gulf, doesn't mean it will automatically become a monstrosity..

There are way more factors going into why a storm strengthens or remains at a certain intensity..

Katrina started going right before making landfall in Florida actually.. it erupted rather quickly, jumping from a weak TS to a Hurricane in a days time..


I would say Ike did explode....in size. And that size in turn was able to make the storm surge larger.

I really do think that the Saffir-Simpson Scale needs to be retired. It does not take into consideration size. There should be another system, which would take into account size.
Member Since: October 4, 2007 Posts: 0 Comments: 451
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1836. Levi32
The ridge to the northwest of Isaac is starting to weaken a bit. 500mb geopotential height is down from 5900m at 12z to 5880m at 00z at Santo Domingo, DR.
Member Since: November 24, 2005 Posts: 635 Comments: 26585
Quoting Tribucanes:
Rapid organization is now underway with Isaac, he may be pushing cat2 strength before he hits any mountainous regions in the coming days imo. If somehow Isaac misses a direct strike with the mountainous regions of Eastern Cuba, that would bode poorly for everyone in his path. Can't wait for what NHC has to say in their 11PM update.


Totally agree
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1834. trey33
Quoting floridaT:
i will i live kinda out in the everglades twords the end of the road so in the event of widespread outages im sure we are the last on the long list to get restored


east or westside?
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1833. Dakster
Quoting PRweatherWatcher10:


No, it's just the system. If you spit in the wrong place, you might loose power LOL


Makes me wonder what happens when you sneeze...
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SFC MAP shows the conus/BA highs are bridging with the eConus trof weakening and lifting out, increasing the 1016mb isobar over Isaac and westwards into the GoM all the way to the Yucatan at 20n.

Isaac's current disorganization and fast forward speed westwards at 20 mph, along with still mixing some dry air, will like continue its present motion more westwards. That will likely continue now for 24-36 hours - just will not organize that fast. All of which supports the drift westwards of many of the models, but even more so of the ECMWF.

It's a much larger system than Ernesto, a very broad circulation, but all the critical similar detrimental effects are the same: fast forward speed, dry air still, proximity to land (not as bad). The key will be forward speed. It cannot get vertical and mature at that clip - just doesn't happen. If it keeps that up, we're looking again at 75-80w before this thing really gets cranking.
Member Since: July 9, 2006 Posts: 184 Comments: 29596
1831. yoboi
Quoting floridaT:
well all is prepared almost. tried to fire up the generator today no luck carburetor is bad, good fortune though the guy on ebay assured me he will get a new one in the mail first thing in the morning. some good people out there.


try to avoid using gas with ethanol, ethanol messes up small engines....
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Quoting GetReal:


Grr. I hate the pass from visible to water vapor/infrared. :( Guess night is not an option lol
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Quoting KEEPEROFTHEGATE:


make sure ya run that thing
away from the house bubbles
ya seem to be alright
so i like for ya
to be around after the storm
as well as during
i will i live kinda out in the everglades twords the end of the road so in the event of widespread outages im sure we are the last on the long list to get restored
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The only thing I will say about the GFDL is,

Garbage In, Garbage Out

I put no weight into that model after it's failures this year.
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Quoting Dakster:


Dang. Anticipated outage?


No, it's just the system. If you spit in the wrong place, you might loose power LOL
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1824. GetReal
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GFS 500 MB Chart 126 HR - 126 Hour



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Rapid organization is now underway with Isaac, he may be pushing cat2 strength before he hits any mountainous regions in the coming days imo. If somehow Isaac misses a direct strike with the mountainous regions of Eastern Cuba, that would bode poorly for everyone in his path. Can't wait for what NHC has to say in their 11PM update.
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What's up WCSC, good to see you. Yeah, I know it's a dangerous game to play trying to keep level heads prevailing around here...lol
Member Since: August 17, 2005 Posts: 26 Comments: 16230
Quoting KEEPEROFTHEGATE:


should change his name to sherlock



Lol...nothin' gets by Jim, I tell ya'!
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1819. GoWVU
Well with all of this going on I am watch a good baseball game. The little league world series and lurking on here..
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1818. JLPR2
Quoting PRweatherWatcher10:
Well my house is without power or water already!


Dang... already?
Mine is holding for the moment, though I see no reason why it shouldn't. :\
Member Since: September 4, 2007 Posts: 7 Comments: 8641
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1816. trey33
Quoting matilda101:
Jim Cantore is suggesting that a new center may be forming to the SW of the NHC fixed position as he believes that a center of circulation may be developing with a tongue of thunderstorms wrapping around the westside of the area of concern. He also says that is just a speculation on his part


he really said tongue wrap?
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I know that this is

OLD

but I totally missed that Isaac was seen by Windsat this morning....


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Quoting TXCWC:
Consensus is now saying FL Panhandle for landfall - BUT NOTE THERE ARE NOW MORE TRACKS POINTING EVEN FURTHER WEST - INCLUDING THE HWFI. If I was a betting person I would say another slight track shift West further off FL West Coast is coming and EURO tonight holds to landfall West of Panhandle.



Jim Cantore just said on the weather channel that he expected the models move more west.
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1813. KEEPEROFTHEGATE (Mod)
Quoting floridaT:
1778. floridaT 2:10 AM GMT on August 23, 2012 0
well all is prepared almost. tried to fire up the generator today no luck carburetor is bad, good fortune though the guy on ebay assured me he will get a new one in the mail first thing in the morning. some good people out there.
?


make sure ya run that thing
away from the house bubbles
ya seem to be alright
so i like for ya
to be around after the storm
as well as during
Member Since: July 15, 2006 Posts: 170 Comments: 53446
Be prepared for news of catastrophic inland flooding if this verfies.



GFS has been sticking with the theory of a stall or second pass for some time now, and now the official forecast is similar.
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1810. Michfan
While anything can happen there is no reason to overhype this storm up. A 100 miles or so will make a huge difference over how well organized this system is once it crosses Haiti or Cuba if it in fact follows that path. You have to take this system day by day and stay prepared for anything. There is a reason your kits should be set and ready to go right now.
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1809. trey33
Quoting KEEPEROFTHEGATE:


should change his name to sherlock


still giggling
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Quoting RTSplayer:


Obviously SHIP expects near-zero land interaction, so it must be favoring a westward track.

maybe so
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1807. scott39
Quoting StormJunkie:


Shakes head...

Katrina was not a severely decoupled system, it crossed Fl, not mountainous terrain, and it had plenty of room once it got in to the Gulf.

Currently there is no reason to think that we have a potential Katrina on our hands. There is a remote chance that changes down the road, but right now it is fairly irrational thinking.
Why are you so sure of yourself, that Isaac wont have room in the GOM to BOMB??
Member Since: June 13, 2009 Posts: 0 Comments: 6747
1806. Dakster
Quoting PRweatherWatcher10:
Well my house is without power or water already!


Dang. Anticipated outage?
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Quoting HrDelta:


Thing is, I don't see it going that far into the gulf. I think it will graze the West Coast of Florida. So, while I think it may get to major, it will not be going into the 160+, -910 mbar region. It will, at most, be your average Category 3 Hurricane. Except for being the size of a Dwarf Planet.


Gotcha and lol, he is rather large
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Quoting Bluestorm5:
People need to chill and not hype the storm up.


Good luck of trying to stop some ppl or a lot of ppl on here.. its a blog and your going have dummy's out there.. Just ignore them!
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Jim Cantore is suggesting that a new center may be forming to the SW of the NHC fixed position as he believes that a center of circulation may be developing with a tongue of thunderstorms wrapping around the westside of the area of concern. He also says that is just a speculation on his part
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Quoting StormJunkie:


Shakes head...

Katrina was not a severely decoupled system, it crossed Fl, not mountainous terrain, and it had plenty of room once it got in to the Gulf.

Currently there is no reason to think that we have a potential Katrina on our hands. There is a remote chance that changes down the road, but right now it is fairly irrational thinking.


Careful junkie i tried to talk someone off the ledge early this morning and got called a little kid who doesnt respect people and wished upon to have a bad day. Channel 5 has been great today :)
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Does it look like the center may be trying to relocate in the area that is around 14N 64W?
Jim Cantore was talking about this on TWC.
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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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