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 Levi32:


After the last couple of days of attempting to forecast Isaac's track, I certainly am.


If you would listen to me a little more, you wouldn't have to go through all of this. :)
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Quoting Stormchaser121:

Im sure SE Texas will be under watches soon




There are TS Warnings for coastal EDIT: (off-shore) waters south of LA, so I wouldn't be surprised if they pop up watches here too by the end of the day.
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12Z CMC now is slightly east of GFS, roughly at Biloxi, MS. From Pensacola WC can follow the storm on I-10 down the coast.
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407. Isaac is HUGE. It would take an incredible amount of total oceanic heat content to support a hurricane of this size.
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522. HCW
Quoting sgarver:

I wouldn't call it safe with him around. He was at the Va hospital between Gulf Port and Biloxi during the "K" storm. Sunk his sat truck and a lot of the crews gear while helping the hospital move the patients to higher floors. Both Gulf Port and Biloxi got washed away during that storm.


He has been at TWC for 20+ years and Katrina was his only direct hit. So I would say that he is the storm shield. I don't know anybody that would have better odds than not getting a direct hit than 4'9 Jim C
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942mb

Left/west hook scenario, and barely moving.


66hrs
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Quoting Gorty:


But look, as Issac is moving away from Cuba, more and more banding features are seen. What do you think about that?


It is constantly improving for sure, but gradually. We're not seeing bombing out yet.
Member Since: November 24, 2005 Posts: 635 Comments: 26548
At this point with what information we have what are the chances for landfall within 50 miles of the following locations..

Pensacola

Mobile

Gulfport

New Orleans

Houston..

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Now it's intensifying. 995.0mb.

170600 2353N 08106W 8438 01457 9950 +214 +185 163018 019 030 002 00
170630 2352N 08108W 8426 01468 9950 +212 +186 171017 018 029 002 00
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Quoting flcanes:
idk, must be looking at old forecast maps

With the Air Force base there the shelters are probably good, too, in case the HWRF verifies. Sorry was thinking that was the UKMET path.
Anyway, the fact is nobody can say where Isaac will landfall yet with any certainty. And the models are farther apart now than they were yesterday and even the day before.
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Quoting caneswatch:
I can't believe Palm Beach County waits until the very end of the weekend to announce closures when every single county around us cancels everything way ahead of time. They'd do this for a hurricane too!


Personally, I think they are doing it the smart way. Conditions are bad here where I am in Broward and by the sound of things, will clear up sooner rather then later. Why close if no need?
Member Since: May 20, 2009 Posts: 0 Comments: 205
Quite a bit of misinformation flying around today, so I will try to assist in helping people understand. I hope this doesn't come off rude or anything, but it does get annoying seeing people post that somehow this storm is badly organized.

A) This system appears to be developing a decent central core per recon.
B) The system is not weakening, the last VDM that came in is showing 65mph with a *slightly* higher pressure of 996mb. NHC will keep it 995 for consistency.
C) The system is on a steady course.
D) Dry air intrusion is happening, but is no where near as bad as to too badly inhibit intensification in the future.
E) Inflow is being restricted from Cuba, once we have it clear Cuba intensification will probably resume.
F) Satellite presentation isn't too badly disorganized, noted spiral banding improving.

G) Large pressure gradient tells me that Isaac will probably gain size as well as intensity. Could be another Ike/Alex situation at peak. We will see.
H) The models are extremely consistent with forecast intensity and track, at least the global models. Also the ECMWF has a good initialization , showing a slightly elongated low from the NE-SW, still bombs it out in the Gulf. Current disorganization seems to be only a temporary thing.

I) The NHC has had the intensity forecast pegged for a long time as the models shifted north, they called for most of the time a 75-80mph hurricane hitting Haiti, Isaac hit as a 70mph TS.

There's a lot more technical aspects to it, but I wanted to cover the basics to help people who are new here. Hope this helps.
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Citation?
Quoting VR46L:
I remember seeing a comment earlier in the past week where there was a poster who appeared to be gloating and full of glee ,when it was predicted DOOM for Tampa and the Panhandle region . Wonder how that person is feeling now that the models have shifted west and focus on NOLA now ?

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Quoting RTSplayer:
935mb

centered on southern NOLA coordinates.


hwrf is out of it's mind
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Quoting CoastalAlabama:
Eastern Shore Alabama:

Gas stations about 75% fuel pumps still good with less than 10 min wait, Walmart full but all registers open so not too crazy, bread and hot dogs and hamburger meat gone but stocking shelves continually.

Marinas locking down today, moving trailers etc.

Weather absolutely beautiful today! Expecting about 4-5 ft surge on Tuesday afternoon.

Looks like eye wall forming right now, anticipate strengthening throughout afternoon, will be interesting to see final growth through landfall.

Looks more like NOLA now but Eastern Shore Alabama watching closely, especially as we will be on the dirty side.


Where on the eastern side are you?
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What would happen to NOLA if they have effects? Is everything up to code? Not saying it will or won't go there. Just concerned.
Member Since: August 16, 2007 Posts: 0 Comments: 397
Quoting nola70119:


Go to you local weather site and see if a hurricane watch is issued for your location.....your local forecaster knows best how to interpret the data. If you are not under a hurricane watch check back every 6 hours or so....
LOL, my local met right now is AL, MS and LA.. I work in Pass Christian MS as a truck driver delivering to NOLA and then got to get back to home in obile AL.
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Quoting nola70119:


Go to you local weather site and see if a hurricane watch is issued for your location.....your local forecaster knows best how to interpret the data. If you are not under a hurricane watch check back every 6 hours or so....

Im sure SE Texas will be under watches soon
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Quoting GetReal:
Even lower now 929 mb!!!





this is now getting out of hand, and has to be over-done...
wish that model could tell us how close to the coast it is
Member Since: August 6, 2011 Posts: 4 Comments: 36929
Quoting Chicklit:
Wonder why the newsfolk are all in Pensacola rather than Mobile.

Its a staging point. They can easily go either way from there.
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995mb from recon, wind data supports lowering the intensity to 60mph at 2 but I don't know if they will or not.
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Quoting Grothar:


I'm working on it now. I really don't think the Gulf Coast really needs this, or anyone else for that matter.


None of us need it, but someone's gonna have to eat it.
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503. ITCZ
63 mph wind gust just measured by the ASOS instrument at Key West International Airport
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Quoting guygee:
From now on refer to him as "The Cantore", if you please!


ROFLMAO - Guygee - you are just tooo funny! Of course, they did buy out the WU.....
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I can't believe Palm Beach County waits until the very end of the weekend to announce closures when every single county around us cancels everything way ahead of time. They've done this for a hurricane too!
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Quoting VR46L:
I remember seeing a comment earlier in the past week where there was a poster who appeared to be gloating and full of glee ,when it was predicted DOOM for Tampa and the Panhandle region . Wonder how that person is feeling now that the models have shifted west and focus on NOLA now ?


These are the comments that just shouldn't be allowed.....What's the purpose?
Member Since: May 20, 2009 Posts: 0 Comments: 205
Quoting GetReal:


Don't really like the look of that....
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Quoting HCW:


Why is it dreaded ? If I lived in Pensacola I would be happy that the Storm Shield was in my city cause you know that you are safe from Isaac when 4'9 Jim C is near

I wouldn't call it safe with him around. He was at the Va hospital between Gulf Port and Biloxi during the "K" storm. Sunk his sat truck and a lot of the crews gear while helping the hospital move the patients to higher floors. Both Gulf Port and Biloxi got washed away during that storm.
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Quoting Chicklit:
Wonder why the newsfolk are all in Pensacola rather than Mobile.
idk, must be looking at old forecast maps
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935mb

centered on southern NOLA coordinates.

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Why are you all posting the images from yesterdays run?
Member Since: August 14, 2010 Posts: 10 Comments: 16421
1PM Video Update from Sugarloaf Key.

The wall is coming through now. Better quality recorded video with the GoPro camera, but the audio is a bit hollow.

Video available here.
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Isaac looks excited about getting into the Gulf.

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Quoting MiamiHurricanes09:
Catastrophic hurricane headed towards Louisiana on the 12z HWRF in 48 hours.



And the ridiculous HWRF intensity forecast strikes again! Like I said...same way for every single storm that has remotely favorable conditions in front of it.
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491. VR46L
I remember seeing a comment earlier in the past week where there was a poster who appeared to be gloating and full of glee ,when it was predicted DOOM for Tampa and the Panhandle region . Wonder how that person is feeling now that the models have shifted west and focus on NOLA now ?
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Quoting Chicklit:
Wonder why the newsfolk are all in Pensacola rather than Mobile.
May need to head west.
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Quoting GetReal:
Even lower now 929 mb!!!





this is now getting out of hand, and has to be over-done...
If that were to happen, I'm pretty sure it would mean automatic retirement for Isaac.
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Quoting Chicklit:
Wonder why the newsfolk are all in Pensacola rather than Mobile.


They are not reading this blog!
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Well the HWRF has lost its mind. 920's mb. insane. isaac needs to pass the keys before he rapidly starts to stregthen. he has been very vigorous after surviving the greater anitilles and should become a beast in the gulf
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Quoting aislinnpaps:


Gro, that is one ugly track! It puts me just east of Issac. I don't like it, can you make a new one for me?


I'm working on it now. I really don't think the Gulf Coast really needs this, or anyone else for that matter.
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Member Since: July 4, 2005 Posts: 204 Comments: 8806
Quoting ITCZ:



Nicer beaches? :-)



And nicer airport!!
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Quoting GetReal:
Even lower now 929 mb!!!





this is now getting out of hand, and has to be over-done...
That model had always overdone the storms...
Member Since: August 1, 2011 Posts: 28 Comments: 7906
seems to me that Isaac may take a shot at the middle keys
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Quoting msgambler:
So we are looking prolly well West of NOLA?


Go to you local weather site and see if a hurricane watch is issued for your location.....your local forecaster knows best how to interpret the data. If you are not under a hurricane watch check back every 6 hours or so....
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I Truly Hope all of the Florida people and anyone else that may end up having to deal with Issac stays safe!!!
Member Since: August 8, 2008 Posts: 0 Comments: 220
No way...

another RI even up to landfall...this is like Camille or Labor Day.

929 at Plaquemines and still on a blow-through course and speed.

54 hours.

This is probably impossible to evacuate for if it verifies.

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ok...rain starting to pick up, a bit more wind gust and ant crawling up my arm. bad omen.
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476. CJ5
Using Key West radar w/storms on, you can clearly see the center rotation. Still open though. I think one problem he has had is his size. He has a huge broad circulation and I believe it is harder for a broad system to tighten up. He has alot going for him but struggles to tighten up convection.
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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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