Isaac lashing the Keys; an eyewall is building

By: Dr. Jeff Masters , 3:48 PM GMT on August 26, 2012

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Tropical Storm Isaac is steadily organizing as it lashes the Florida Keys with heavy rain and tropical storm-force winds. Sustained winds of 44 mph and 41 mph have been observed at Molasses Reef and Sombrero Key, respectively, this morning. Radar out of Key West shows an increase in spiral banding, and the beginnings of an eyewall. An Air Force Hurricane Hunter aircraft completed its first pass through the center of Isaac near 11:30 am EDT, and did not find the pressure had fallen, or that the peak winds had increased. Infrared and visible satellite loops show that Isaac is a large and increasingly well-organized storm. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow is quite good and increasing to the north, but is lacking elsewhere. Moderate wind shear and dry air to the south are interfering with heavy thunderstorm development on Isaac's south side. Heavy rains from Isaac are lingering over Haiti and the Dominican Republic; flash floods in Haiti from Isaac's torrential rains killed at least four people.


Figure 1. Morning reflectivity image from the Radar out of Key West radar shows the northwest section of an eyewall beginning to form to the southeast of the city.

Latest model runs for Isaac
The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs have diverged significantly, and we can no longer be confident we know where Isaac will make landfall on the Gulf Coast. One camp of models, the UKMET and ECMWF, predict that a trough of low pressure moving across the Southeast U.S. will be strong enough to turn Isaac north to a landfall in the Florida Panhandle. The other set of models, the GFDL, GFS, and HWRF, predict the trough will bypass Isaac, and a ridge of high pressure will build in and force Isaac to a landfall over Louisiana. The official NHC forecast averages out these two extremes, calling for a landfall midway between the two solutions. Odds are, one of the two model solutions will turn out to be the correct one, and the NHC will be forced to make a substantial adjustment in their forecast track to the east or the west. Isaac has the potential to drop torrential rains capable of causing serious flooding and drought relief over the South. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over Southeast Louisiana, where it predicts Isaac will make landfall. The ECMWF model, however, these heavy rains will fall more over the Florida Panhandle, Alabama, and Georgia.


Figure 2. A hurricane forecaster's dilemma: which set of models is correct? The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs have diverged significantly. Our two top models--the GFS and ECMWF--have 72-hour forecasts that are about 350 miles apart. The ECMWF forecast is not shown here, but lies just to the west of the UKMET forecast (white line.)

Intensity forecast for Isaac
Isaac survived passage over Hispaniola and Cuba relatively intact. It's large size aided this. Isaac is over very warm waters of 31°C (88°F) with high total heat content in the Florida Straits, but is encountering moderate wind shear of 10 - 15 knots due to upper-level winds out of the southwest. This shear is predicted to relax to the light range tonight as an upper-level anticyclone becomes established over the storm. This should allow for more substantial intensification after Isaac passes the Florida Keys. However, the total heat content of the ocean decreases for Isaac Monday morning as it encounters a relatively cool ocean eddy in the Southeast Gulf of Mexico. If Isaac takes a more westerly track, passing due south of the Central Louisiana coast, the storm will encounter a modest warm eddy, which would aid intensification. The intensify forecasts from the various models are very divergent. The latest 06Z (2 am EDT) run of the GFDL model keeps Isaac as a strong tropical storm until landfall in Louisiana. Isaac will undergo rapid intensification into a Category 3 hurricane as it hits New Orleans, says the latest 06Z (2 am EDT) run of the HWRF model. The ECMWF model has Isaac as a strong Category 2 storm with a central pressure near 950 mb as it hits near the Alabama/Florida border.

Comparing Isaac with Ike of 2008
The current situation with Isaac is similar in some ways to that of Hurricane Ike of 2008. Ike spent considerable time over Cuba, weakening from a Category 4 to a Category 1 storm. The storm couldn't put its energy into building a strong inner core, but it was able to build up its outer rainbands that were over very warm waters. This resulted in a major expansion of its wind field, with tropical storm-force winds extending out 275 miles from the center at one point. Ike was able to intensify into a Category 2 storm on its path towards Texas, and had an unusually low pressure for a Cat 2 storm with 100 mph winds--944 mb. That's a central pressure more typical of a Category 3 storm, but Ike could only manage Category 2 winds, since it had such a large chunk of the atmosphere to keep spinning. With Isaac's TS winds already extending out to 205 miles, maybe we'll see another Ike-type situation as it intensifies--the storm will have an unusually low pressure in order to keep a huge wind field spinning, but never make it above Category 2, since it will take so long to spin up such a large wind field.

Storm surge forecast for Isaac
Isaac is a very large storm, with tropical storm-force winds that extend out 205 miles from the center. For comparison, Hurricane Katrina at landfall had tropical storm-force winds that extended out 230 miles from its center. Isaac's large size will enable it to set a large area of the ocean into motion, which will generate a large storm surge once the storm approaches land on the Gulf Coast. The latest 3:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 0 on a scale of 1 to 6, but the destructive potential of Isaacs's storm surge was 2.1 on a scale of 1 to 6. A 2008 paper by Irish et al., The influence of storm size on hurricane surge, found that large storms like Isaac are capable to delivering a 30% larger storm surge to the coast than a smaller storm with the same maximum wind speeds. The angle with which the storm hit the coast is important, too--a storm moving due north or slightly east of north will deliver a storm surge about 10% greater than a storm moving NNW or NW. Consult our Storm Surge pages for detailed information on what the risk is for the coast. I expect that Isaac's storm surge will be about 30% higher than the typical surge one would expect based on the maximum wind speeds.

Invest 97L off the coast of Africa
A tropical wave (Invest 97L) is located about 650 miles west of the Cape Verde Islands off the coast of Africa. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 50% chance of developing by Tuesday morning. None of the reliable models foresee that 97L will be a threat to the Lesser Antilles Islands. However, both the GFS and ECMWF models predict that a tropical wave that has not yet emerged from the coast of Africa may develop late this week, and potentially take a more westward track towards the Lesser Antilles, arriving around September 2.

Angela Fritz will have a new post here by 6 pm EDT. For the next few days, I plan to do the morning blog post, and Angela will be doing the late afternoon post. I'm in Atlanta to help out The Weather Channel with their on-air hurricane coverage, and will be on either in the afternoon or evening on Sunday, Monday, and Tuesday.

Jeff Masters

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Quoting flcanes:

full DOOM ahead
flcanes.What did I tell you about that?
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And boom goes the dynamite.

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GFS is showing another caribbean bound storm upcoming.
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Quoting seflagamma:
Oh my goodness, I just looked.. this last squall has dropped over 1.5" in past 20 mins.

I am now at 3" of rain!

and we are just getting started.. YIKES!!


Now through the overnight hours will be worst for us. Especially as Isaac builds in intensity and beefs up these rain bands. Heavy rain just started in Jupiter.
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Quoting Levi32:
This storm is a huge win for the ECMWF, redemption for its performance with Debby, as it led the models into the Gulf of Mexico from the very beginning.

It is so ridiculous to have two storms like this in one year.


absolutely agree, the GFS was lagging on this one and yet it is getting huge praise today, when in reality it was wrong all the way up until yesterday
Member Since: March 10, 2010 Posts: 1 Comments: 7871
Quoting MiamiHurricanes09:
Not as impressive as I anticipated. 996.1mb.

152630 2340N 08057W 8431 01462 9961 194 161 054004 007 024 002 00


The radar now has enough frames offshore to show that the path is northwest, and will hit or go just barely west of Key West (4pm). This is confirmed by a much faster pressure drop at Key West buoy. Last reading was 11:42am and the pressure was 1002Mb. Updates every 6 minutes:

http://www.ndbc.noaa.gov/station_page.php?station =kywf1


So we can see from these pressure readings whether the storm is able to take advantage of its movement to open water.
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Quoting SykKid:


why? isaac is struggling greatly right now everytime he's forcasted to get stronger he never does. im just saying he looking really ragged right now jeeze why is everybody so harsh for me being excited that he isnt becoming strong? :( i just want whats best for everybody in the gulf.
So if Isaac looks ragged now in your opinion he will never get strong heading up to the northern gulf..........Right?
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currently experiencing the strongest feeder band since this event started in West Broward. Heavy sideways rains, not sure of the gusts, but probably in the 30's range?? So far over an 1.5 inch of rain but this band will probably get us closer to 2.

Please stay a disorganized mess Isaac, biblical Isaac was really old, maybe he was senile too, hope you are too and forget how to become a hurricane. :P
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So according to the dr one of the more reliable models still could have it be an Alabama Pensacola landfall?
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Quoting Levi32:
This storm is a huge win for the ECMWF, redemption for its performance with Debby, as it led the models into the Gulf of Mexico from the very beginning.

yep, gas stock has gone down though, but it is still the most precious commodity
Member Since: August 20, 2012 Posts: 13 Comments: 1254
Quoting seflagamma:
Hi everyone, Isaac is here in Broward County also.
Not hurricane wind but nasty windy bands of heavy rain.

we already got 1.5" in my neighborhood after just last night and early this morning and raining hard now.
Another band is coming thru and they are bad.

our side yard is flooded and now streets are flooding.

This storm may bring us flooding like Irene did in 1999. We had boats going down street and water was thigh high in center of street..

Hope you all stay safe.. I am posting in my blog as I can and a few others from Broward or S FLA are there also if you want to check in .



Hi seflagamma, thanks for the info. I'm on the opposite side, Collier County, and although the radar looks really bad, we've had very little of anything here, west of US 41. Still holding my breath. Stay safe.
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Quoting Chiggy:

Raleigh Site is probably overloaded from folks like us getting 6-hourly time-stamp runs... lol


Lol. Guilty as charged. :)
Member Since: August 24, 2010 Posts: 0 Comments: 252
is anyone having problems with http://raleighwx.americanwx.com/ page?
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161. jpsb
Quoting gustaveye:
Since their is no much uncertainty, the cone should include the gfs and the euro...this would stretch from Cameron Parish Louisiana to Destin Florida.....don't understand why NHC just doesn't make the conde wider since the EURO and GFS are over 300 miles apart...
I am not expert on the models and really only lurk when a storm is heading towards the USA. But my recollection is that the New Improved GFS is the more reliable model. It certainly was with Debby earlier this year. And one other storm whos name I forget (right after Debby).
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Quoting DataNerd:



NO

full DOOM ahead
Member Since: August 20, 2012 Posts: 13 Comments: 1254
This storm is a huge win for the ECMWF, redemption for its performance with Debby, as it led the models into the Gulf of Mexico from the very beginning.

It is so ridiculous to have two storms like this in one year.
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Quoting PalmBeachWeather:
Kid..Please don't do that....You hane no idea.... Please don't give people that are not in the know some relief for not being aware.... I guess what I am saying is .Stop you BS posts


why? isaac is struggling greatly right now everytime he's forcasted to get stronger he never does. im just saying he looking really ragged right now jeeze why is everybody so harsh for me being excited that he isnt becoming strong? :( i just want whats best for everybody in the gulf.
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It has problems for now but that would soon change!
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.
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School closures for South Florida was ordered so they'd have a day to assess the damage.

Perspective for the South Floridians that aren't impressed- this is only a 65 mph tropical storm over 100-200 miles away. We aren't talking a cat 2+ hurricane.
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Quoting DataNerd:
NCEP is updating their National maps at the moment, and the Trough looks far less strong now in terms of its representation, also, looks far less formidable on sat imagery as well.


I think GFS may be right......again.

Gotta learn to stop doubting that model this season.


to be fair, the GFS did not have this scenario until yesterday, lets not praise it unless it truly deserves it

other models were further west to begin with and for a longer period of time
Member Since: March 10, 2010 Posts: 1 Comments: 7871
Quoting flcanes:

kudos to him, too


Appreciate that, you all! Anything WE'VE learned that helps someone else, I'm glad to pass on! I'm reposting at intervals through-out the day and it's ON my WunderBlog, if you need to print it to check stuff off or add your own good ideas.
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Quoting seflagamma:
Oh my goodness, I just looked.. this last squall has dropped over 1.5" in past 20 mins.

I am now at 3" of rain!

and we are just getting started.. YIKES!!
forecast of 8-12 inches of rain over next 48 hrs
Member Since: August 20, 2012 Posts: 13 Comments: 1254
Quoting WxGeekVA:
Seven years ago today, to the hour:



Le Deja Vu, anyone?





NO
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Quoting treehuggingsister:
From our blog (originally written to help w/ Irene): (A very large list of stuff you should have done last week)

Think I'll go out and work on my roof now.
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If Isaac takes a more westerly track, passing due south of the Central Louisiana coast, the storm will encounter a modest warm eddy, which would aid intensification.

Lovely...
Member Since: August 24, 2010 Posts: 0 Comments: 252
Is anyone else in SE Fla getting this type of rain???

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Quoting seflagamma:
Oh my goodness, I just looked.. this last squall has dropped over 1.5" in past 20 mins.

I am now at 3" of rain!

and we are just getting started.. YIKES!!


Looks like things are going to clear up sooner rather then later for Broward. No worries.
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Quoting nofailsafe:


We don't have a death ridge this year. A smaller ridge by far.


I don't think it's much stronger than normal, and has spent much of the summer further NW than normal, I think.
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From the Evangeline Parish Office of Emergency Preparedness:


The 10:00 conf call with the NWS in Lake Charles advised the following:

TS Isaac is just south of the FLorida Keys. There is a HC hunter aircraft on the way to get updates. Isaac will probably drift further to the west and possibly make landfall in SE Louisiana Wednesday Morning. It may become a hurricane by late afternoon today.
Now is a good time to get prepared
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NCEP is updating their National maps at the moment, and the Trough looks far less strong now in terms of its representation, also, looks far less formidable on sat imagery as well.


I think GFS may be right......again.

Gotta learn to stop doubting that model this season.
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Quoting Chiggy:


Dumb-A** Comment lol


reported
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Quoting SykKid:
isaac pressure 996? wow he really struggling im so happy that he cant get his act together. look like any chances of him ever becoming a major hurricane are dreasing by the hour! :)))))
Kid..Please don't do that....You have no idea.... Please don't give people that are not in the know some relief for not being aware.... I guess what I am saying is .Stop your BS posts
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Quoting STXHurricanes2012:

Huh?

yeah he got !
Member Since: August 20, 2012 Posts: 13 Comments: 1254
Oh my goodness, I just looked.. this last squall has dropped over 1.5" in past 20 mins.

I am now at 3" of rain!

and we are just getting started.. YIKES!!
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138. yoboi
Quoting aislinnpaps:
If Patrap hears the latest of Issac and NO, he is not going to be happy.



is he in tampa working at the rnc??? i have not seen him on here...
Member Since: August 25, 2010 Posts: 7 Comments: 2386
137. LBAR
I dunno, maybe the Bermuda ridge is ready too prime itself, but what timing! I'm not convinced yet that the ridge is going to be that strong, though.

I wish everyone along the Gulf Coast good luck!
Member Since: July 7, 2008 Posts: 0 Comments: 242
Quoting AussieStorm:

Here is a nice list for you.

Click for full size.

Goodnight all.

thx, although i don't really need it for isaac
Member Since: August 20, 2012 Posts: 13 Comments: 1254
Quoting Chiggy:


Dumb-A** Comment lol

Huh?
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Quoting AtHomeInTX:


I think my model site crashed. Can anyone tell where this makes landfall first. Looked like SWLA to me. If this plays out theyll have to move the cone again.

Raleigh Site is probably overloaded from folks like us getting 6-hourly time-stamp runs... lol
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Quoting gustaveye:
Since their is no much uncertainty, the cone should include the gfs and the euro...this would stretch from Cameron Parish Louisiana to Destin Florida.....don't understand why NHC just doesn't make the conde wider since the EURO and GFS are over 300 miles apart...


The NHC cone is not a "smart" cone based on the discrepancy in the models. Instead it is just the average NHC track error for that time period. So, sometimes it is far too wide when they have a great handle on what the storm will do. In this case it is definitely too narrow.
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Quoting Chiggy:


Dumb-A** Comment lol

ummm.....
Member Since: August 20, 2012 Posts: 13 Comments: 1254
Quoting ITCZ:



AWESOME. Thank you so much for taking the time to do this! I started a "detailed" action plan last night, but you have some wonderful ideas (necessity=mother of invention) I wouldn't have dreamed of in a million years. Kudos to you and glad you survived.

Here is a nice list for you.

Click for full size.

Goodnight all.
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Quoting flcanes:

what texas death ridge of doom that killed don and lee


We don't have a death ridge this year. A smaller ridge by far.
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I see those models for the landfall on Norther Coast of GOM are doing their back and forth thing again..

since you are all going to get something probably big..
I would prepared if I lived anywhere from New Orleans to Big Bend area of N Fla...


good luck to you all on pinning down that track that keeps flopping back and forth!

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Quoting Stormchaser121:

102 hrs


I think my model site crashed. Can anyone tell where this makes landfall first. Looked like SWLA to me. If this plays out theyll have to move the cone again.
Member Since: August 24, 2010 Posts: 0 Comments: 252
Quoting STXHurricanes2012:

?? No its not! LOL!


but hes struggling so much! how can it not impact his future
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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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