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 Hurricanes101:


It didnt shift west either lol

That's okay, it didn't widen the gap we already have, lol.
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Quoting WatchingThisOne:


Perhaps we can agree with Levi that this is the first CV storm since 1842 to take this particular track, while not diminishing Grothar's observation about tracks of storms from Isaac's current location.


Agreed, I was in no way questioning Levi. As posted before, I one of his biggest fans and have learned a great deal from him.
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So far so good in Fort Lauderdale, some heavy rain coming through in squall lines, but nothing horrible or more than a normal thunderstorm down here. 17-20mph sustained winds with some stronger gusts. No loss of power or internet.

We shall see if things pick up this afternoon.
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Quoting LSUCaneGirl:
Gov Jindal has a press conference scheduled for 3:15..things are headed to a frenzy in SE Louisiana..I'm on edge :/


ME TOO LSUCaneGirl..I am in Central LA and I am on edge as well....:(
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Quoting WDEmobmet:


00z had Central LA.. 12z had MS/AL state line... I believe that is west


Lol...00z comes before 12z.
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Key West radar

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GUYS TUNE INTO TWC DR JEFF MASTERS IS GONNA BE ON AT 130!!!!!!!!!
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Quoting WDEmobmet:


00z had Central LA.. 12z had MS/AL state line... I believe that is west


??? MS and AL are east of LA
Member Since: March 10, 2010 Posts: 1 Comments: 7874
Quoting WDEmobmet:


00z had Central LA.. 12z had MS/AL state line... I believe that is west

Lol no, that's east.
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Quoting IKE:

Thanks. no...dont need Isaac here. Rainfall may be a big issue.


Hey Ike! Long time no see :)
Member Since: September 5, 2007 Posts: 0 Comments: 3488
Quoting IKE:

Good afternoon to you and all.
good day ike
Member Since: July 15, 2006 Posts: 178 Comments: 56140
Quoting Bluestorm5:
And the news get worse and worse...
remember folks when this is all over down there..gators in the backyards..and the snakes etc
Member Since: August 6, 2011 Posts: 4 Comments: 42254
DR Masters is about to be on TWC, tune in now.
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12Z GFS Ensembles, operational on the western edge of them



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Quoting Hurricanes101:


cmc shifted east, not west.

It didn't shift by too much though


00z had Central LA.. 12z had MS/AL state line... I believe that is west

EDIT**********
LOL BEEN UP WAY TO LONg... MY APOLOGIES thats what I mean lol wow i think a nap is in order
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Quoting IKE:

Good afternoon to you and all.


Dude. I heard rumors....but I didn't believe'm.
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Jeff Masters coming up on TWC
Member Since: August 13, 2007 Posts: 0 Comments: 10492
Quoting IKE:

Good afternoon to you and all.



Oh lord IKE is back can mean only one thing.




Also: 2:00 PM EDT Sun Aug 26
Location: 23.9°N 81.5°W
Moving: WNW at 18 mph
Min pressure: 994 mb
Max sustained: 60 mph


Still WNW
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Quoting weatherh98:
Lake pontchartrain up 2 feet. Not tlfrom isaac but will allow for more surge than previously thought
And the news get worse and worse...
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TWC just said Dr. Masters is making an appearance today... sorry if this was already posted.
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Quoting PalmBeachWeather:
27.Makes one wonder where these people come from
I suggest you read DR.M's report on the temp. levels in the GOM.SST gives you the measured Ses Surface Temp. but does not give you the total available heat content of the ocean.In spite of global(haha) warming the specific heat content of the gulf is low.Looking at SST alone doesn't give you the big picture.
Member Since: July 30, 2012 Posts: 4 Comments: 614
Dr. Jeff Masters on TWC?!?
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Quoting Hurricanes101:


maybe but with TS force winds up to 210 miles from the center, there is a very good chance we see that here
I don't know, if you look at the wind Tropical Storm Force Wind Speed Probabilities, nhc chart, remains at 50% chance of tropical storm winds. So, like I sated, we will know soon enough


Edit: However, now that I look at the animated chart, I can see that the wind field possibility is more circular and larger, so as it move more to the nw, I see your point more clearly.
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Quoting ringeaux:


Your Welcome--ASH '77 ;-)



ASH 89 :)
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Well, the European models want Isaac to make landfall around Gulf Shores, while the North American models/hurricane models want Isaac to make landfall in SE Louisiana. You have to wonder what the difference is between the models since they should be receiving mostly the same data.
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Dr. Masters is on the Weather Channel check it out!
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Quoting LargoFl:
no, just took in the lawn stuff that could blow around,dont think i'll be getting much sleep tonight lol..if this changes course and comes closer, i can board the windows pretty quickly if need be, years ago for charley when we WERE supposed to get hit, i had the bolts cemented in the walls around the house and had the plwood cut to order..all i need to do is put the wood on and secure it..my neighbor loves the idea
I didn't put the shutters up. I hope I don't regret this..like you said, I don't trust this storm
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Heavy rain in downtown Orlando now.
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not boasting, but looks like Mobile remains in a bad spot regardless...reality is really setting in.
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Quoting leftlink:


from the Radar, it all still appears to be moving west northwest, but the center was 35 miles wide and then it tightened to 15 miles wide. When it did this the smaller circle formed in the southwest of the larger one, moving the center about 30 miles WSW in less than an hour.

Now, the latest Buoy observation shows that the new smaller center is continuing to move Northwest. Pressure is dropping again, at Key West.

So which is coc? SW or NW center?
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UKMET 12Z



I believe that it is ever so slightly west of the last run...
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Quoting Sasha:
Nice...!!! Felt just like I was in the car driving thru KW!!!  I need to make another trip down there sometime...  Stay safe!

Thanks, put another one when ever you can , I appreciated, you put us in the center of the action...
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Gov Jindal has a press conference scheduled for 3:15..things are headed to a frenzy in SE Louisiana..I'm on edge :/
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941. IKE

Quoting muddertracker:


Good Afternoon, Ike. Welcome back to the flock :) It's so good to see you blogging!
Good afternoon to you and all.
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Quoting IKE:

Good question. Flip a coin? GFS is what I would trust....until it switches...IF it ever does.

I trust GFS
Member Since: September 2, 2011 Posts: 0 Comments: 1149
Isaac is getting the look... really hope this doesn't pull RI.
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Quoting mobilebayal:


Where on the eastern side are you?



Fairhope... BP at 44/98 has some empty tanks... Walmart between foley and Fairhope busy but not out of control... BP by that new Publix had tanker refilling.

Fly Creek Marina is looking for some surge regardless so they just moved all trailers up to entrance. Not worried about boats at lowest level of stackable...

I was on Ono Island yesterday and nobody was boarding etc, just some larger boats coming off front to back inlets and folks taking patio furniture off their docks and piers.
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Wierd...my ears are popping.
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Lake pontchartrain up 2 feet. Not tlfrom isaac but will allow for more surge than previously thought
Member Since: June 17, 2011 Posts: 11 Comments: 6539
Quoting IKE:

Thanks....uncertainty in track...continues.


Good Afternoon, Ike. Welcome back to the flock :) It's so good to see you blogging!
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UKMET 12z
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Quoting midgulfmom:
">

p'cola still for euro. Sorry folks....these computers aren't going to make it easy on us.
Equal anxiety distributor ......


Yep
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931. IKE

Quoting AtHomeInTX:


I wonder what the NHC will do.
Good question. Flip a coin? GFS is what I would trust....until it switches...IF it ever does.
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Quoting snow2fire:
Key West radar is showing something that looks like beginnings of eye... cant see it on mia radar





Looks like it's about to pass just south of Key West, Winds are out of the north-north-east at 23kts.
Member Since: June 18, 2010 Posts: 3 Comments: 946
Quoting TierraFirme:


Gro, what you say sounds right but what Levi is saying almost makes this storm human (don't mean to be deep Levi). You have to see where it came from to understand where and why it is now. The thing that I question is that Isaac had all the opportunity and enviorment to become a huricane quite a while back, but i didn't, IMO, and I can most certainly be wrong. Any answers, opinions or guessing?


Perhaps we can agree with Levi that this is the first CV storm since 1842 to take this particular track, while not diminishing Grothar's observation about tracks of storms from Isaac's current location.
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Quoting Joanie38:


Thank you SO MUCH for your reply...:)


Your Welcome--ASH '77 ;-)

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Im still going with the westward trend regardless of what the ECMWF said.
Member Since: September 2, 2011 Posts: 0 Comments: 1149
We just have to wait till the 18z and 00z runs come out to see where his eye will make landfall...
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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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