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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Member Since: Posts: Comments:
Quoting Levi32:


It is constantly improving for sure, but gradually. We're not seeing bombing out yet.


Levi, when do you think it will bomb out? Thanks in advance.
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Extrap. Sfc. Press: 994.8 mb (~ 29.38 inHg)

Winds around 15mph.
Member Since: July 15, 2006 Posts: 17 Comments: 10284
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Quoting CybrTeddy:

C) The system is on a steady course.


I've been wondering how much the storm has moved since the last update. I have been looking at the radar the last 3/4 hours and to my untrained eyes it doesn't look to have moved much.

Will the next update show a slower moving storm?

Thanks
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Hmmm....
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Quoting tornadolarkin:


Isaac is looking good. He continues to blow up convection and the banding is developing very nicely. I believe he is almost ready to rapidly intensify.

Hell go through RI...Once he is away from land:) / :(
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994.6mb now from recon, but as MH09 said winds are not impressive, its probably about a 60mph storm.
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Quoting MississippiWx:
Core is tightening up as seen on Key West radar:

Eyewall building.
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Quoting tomas5tex:
5667. weatherganny 4:37 PM GMT on August 26, 2012 +0
Quoting tomas5tex:


Buna here also

Buna texas???
Action: Quote | Ignore User

Yes


I have relatives in Buna! Hope everything works out well for you.
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As for models, warnings, effects etc -- my take: its definitely not nothing -- without the little sw turn, could have been much more. But its not an Andrew either. I think that some people expect every storm to be like the outliers, Andrew, Ike, Katrina, that there is not a realistic expectation of what a tropical storm/hurricane actually is.

I'm quite happy its not any worse, and that I'm not growling because I decided not to do my shutters.
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Quoting RTSplayer:
72hrs.

A slight wobble north after the westward hook earlier.



If this verifies, I will get category 3 force sustained winds in Tangipahoa/Livingston parish border area.


Is that the right graphic?
Member Since: May 31, 2007 Posts: 0 Comments: 1204
Quoting MiamiHurricanes09:
Winds are very lackluster as per Recon though.


Sometimes, often I have seen the recon not report all the data. For example, the VDM showed 65mph on the SFMR.. yet I haven't seen that at all on the public.
Member Since: July 8, 2005 Posts: 259 Comments: 24251
Core is tightening up as seen on Key West radar:

Member Since: July 15, 2006 Posts: 17 Comments: 10284
Levi who is a bigger pain to track? Debby or Isaac? Also, it seems that every I storm is notorious for what it does. (Iris(Belieze)Isidore(Yucatan Lousiana)Isabel(North Carolina) Ivan (Alabama/Mississippi) Ike (Texas/Cuba) Igor (Bermuda/Canada) Irene(bahamas, Carolinas, East coast) now Isaac?
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Quoting oreodogsghost:
Just heard my first TV met say for Port Arthur/Beaumont to be alert.


Who was it? What channel? Are you in the Golden Triangle? I am really interested in what KFDA says but they don't put the weather cast on the internet anymore like they used to. Thanks.
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Quoting sgarver:

Its a staging point. They can easily go either way from there.


Pensacola is a good vantage to get the most dramatic onshore RFQ effects of the storm if it goes directly into the mobile area.
Member Since: May 31, 2007 Posts: 0 Comments: 1204
Quoting TomballTXPride:

That a shift left or right from the 06Z run??


Its about the same, really.....
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Quoting Levi32:


It is constantly improving for sure, but gradually. We're not seeing bombing out yet.


Isaac is currently undergoing MI -- Messy Intensification.
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Quoting Chicklit:
Wonder why the newsfolk are all in Pensacola rather than Mobile.


They're following Cantore and he hasn't been watching TWC.
Member Since: August 22, 2009 Posts: 0 Comments: 3150
NWS Key West @NWSKeyWest
63 mph wind gust just measured by the ASOS instrument at Key West International Airport. #Isaac
Member Since: August 6, 2011 Posts: 4 Comments: 39690
Quoting Grothar:
.


Good Afternoon Gro
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552. Gorty
Quoting hydrus:
. Notice how the north part flattens out against the predicted ridge.


What does that mean?
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72hrs.

A slight wobble north after the westward hook earlier.



If this verifies, I will get category 3 force sustained winds in Tangipahoa/Livingston parish border area.
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Quoting TomballTXPride:

Yup. LOL


Yeah... I recall less than 24 hours ago, the HWRF popped out a Cat3 offshore Tampa. I'm not saying this won't eventually get it's act together, but it's behind schedule in doing so.
Member Since: May 31, 2007 Posts: 0 Comments: 1204
Quoting BahaHurican:
Why does this sound like a disgruntled parent to me?

;o)Not really. We can take our crow when we have to.

Actually, except for the bust forecasts, I think a huge number of bloggers would be relieved to see a TS rather than a cat 3 at landfall, especially if the surge doesn't get too high as a result. Most of us have been or are currently in the cone for Isaac, and that does not create a particularly happy feeling. So some will be disappointed that their forecasts don't verify, but at the same time most of them will also be relieved that it didn't.

I know it's hard for you to understand the complexity of feeling others may experience.


Baha -- how is the weather over there?
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Winds are very lackluster as per Recon though.
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Quoting RTSplayer:
942mb

Left/west hook scenario, and barely moving.


66hrs
What model is this?
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Quoting caneswatch:


True, but why sorry rather than safe though?


Well, all signs are pointing to this not being a bad/damaging storm of the mainland of FL especially the farther north you go. Why not plan to open and if for some wierd reason, things change, you can cancel but I really doubt we are getting anything worse then what we already have now.
Member Since: May 20, 2009 Posts: 0 Comments: 205
Quoting CJ5:
Does anyone have the last recon eye characteristic report?

N/A last i say radar looks good though
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As I said earlier, winds will take some time to catch up to the improved sat appearance, especially in a big storm like Isaac. He's not perfectly healthy, but he's improving.

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Quoting newportrinative:


This is what Broward should have done!
Why does this sound like a disgruntled parent to me?

;o)
Quoting eyewall99:
Watch it stay a TS until landfall....man, talk about overhyped, still looks ragged and all and it is flying....will be alot of sad sad bloggers if it does not turn into a major before it hits land....
Not really. We can take our crow when we have to.

Actually, except for the bust forecasts, I think a huge number of bloggers would be relieved to see a TS rather than a cat 3 at landfall, especially if the surge doesn't get too high as a result. Most of us have been or are currently in the cone for Isaac, and that does not create a particularly happy feeling. So some will be disappointed that their forecasts don't verify, but at the same time most of them will also be relieved that it didn't.

I know it's hard for you to understand the complexity of feeling others may experience.
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Quoting ugajag:
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..



Just my guess:

Pensacola 25%
Mobile 35%
Gulfport 55%
New Orleans 40%
Houston not enough data

Shift those odds if you want an east bias and watch the radar.
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Quoting ugajag:
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..



About the same for all of them?
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the conditions in South Florida really depend on where you are. I'm near the falls area, and definitely been feeling the wind, sideways, rain, etc. Swimming pool has waves! I always think thats so funny.

We have had a lot of rain in the last 30 minutes, been fairly steady.

But someone else who is in a different area 15 miles away may be having a completely different experience. Depends on where the band is.

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539. VR46L
Quoting NEPASteve:
Citation?



It would be to hard to find it ... It just stuck in my mind thats all ..
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Quoting KEEPEROFTHEGATE:
Is that the "tail" of Isaac still over Haiti? If so, are they still getting squalls now and will this "tail" be over SE FL and Keys tomorrow?
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On Key West radar, a tighter area of circulation about 20 miles in diameter appears to be forming on the SW core of what was the center before. It isn't rotating very fast yet, but the core is starting to moisten a lot.

This will probably cause the storm to take a jog west and miss Key West completely.
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. Notice how the north part flattens out against the predicted ridge.
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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?


FWIW, TWC's Brian Norcross just interviewed NHC live trying to hype it, and the NHC was down playing it saying they can't find anything suggesting even 60mph winds right now. Dry air taking it's toll in the short term, and that KW probably just going to get a TS. Looks like the worst squall it has to offer is going through KW right now.
Member Since: May 31, 2007 Posts: 0 Comments: 1204
Member Since: August 6, 2011 Posts: 4 Comments: 39690
WPBF First Alert Weather
FPL is reporting 2,000 customers without power in our area... -Eric

Also, saying lots of tornado alerts. And this is still just a tropical storm! People to the east side of where this goes in are gong to get the worst of it. Latest GFS according to my local news shows it going right in to New Orleans!
Member Since: August 18, 2008 Posts: 0 Comments: 474
Quoting guygee:
Well stated gamms. I just cannot bring myself to put anyone on ignore, personally, but that is probably why my hair is going gray and I hit the keys on my computer way too hard...


Thanks! but you should try it.. it is so liberating and makes this blog so much better.


I do, every January clean out my "ignore list" and give everyone a clean slate for next season.
Some I have come to realizes actually grow up after a year or two! but some, alas, never do!

We are doing good right now, light rain in between the bands. This is a typical tropical storm...
and many are going to lose their power because of this tropical storm...

Hope you and your area come thru this ok.
Thanks again.
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Quoting Stormchaser121:

Im sure SE Texas will be under watches soon


I know you have said, but once again where are you? My interest is Orange County south of IH 10. (I'll have to run down there to get family members if needed.)
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Quoting newportrinative:


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?


True, but why sorry rather than safe though?
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Quoting Levi32:
Isaac still has to overcome a large amount of dry air that has become inseparably integrated into its circulation. One can see the center is of banded nature with an "S" shape, not a CDO, indicative of dry air entrainment. The only way this can be overcome is for the storm to perform lots of convective mixing to re-saturate the air inside it. It will accomplish that eventually, but it is large, and it will take a little while yet. Rapid strengthening is not likely until it is out in the open water of the gulf.


I read your blog and have a question. After reading Dr. Masters blog it sounds as if he is saying one of the two models consensus will be correct and the NHC will have a large correction to make instead of blending the two solutions and splitting it down the middle. Being that a Hurricane warning has to be issued 36 hours ahead of the onset of tropical force winds I would think if this model difference continues the NHC will have to flip a coin or so to speak. What is your take on this?
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Isaac is looking good. He continues to blow up convection and the banding is developing very nicely. I believe he is almost ready to rapidly intensify.
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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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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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