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 DougKahn:
Storm surge in Englewood, Florida concerns me right now. Can anyone figure how far the storm's center will be from Englewood, at the closest point?


I think they're estimating 3-5 ft for pretty much all of SWFL...
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Here's a little broader view:

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Quoting flcanes:

well, there is largo
speaking of him, is he on the internet, or has he lost power


Largo is from Largo FL near Tampa
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so much energy within in this band! I'm on the safety on my front porch watching it
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Dropsonde dropped in eye brought back an MSLP of 995mb plus 21kts of surface winds, which factors down to about 993mb. The splash location was 23.83N 81.3W.

Isaac appears to be intensifying.

000
UZNT13 KNHC 261725
XXAA 76178 99238 70813 08131 99995 26822 33021 00546 ///// /////
92644 25037 33007 85387 214// 28507 88999 77999
31313 09608 81709
61616 AF309 2109A ISAAC OB 11
62626 EYE SPL 2383N08130W 1711 MBL WND 34016 AEV 20802 DLM WND 32
511 994843 WL150 34020 084 REL 2384N08130W 170929 SPG 2383N08130W
171119 =
XXBB 76178 99238 70813 08131 00995 26822 11878 23643 22850 214//
21212 00995 33021 11973 35017 22950 34514 33936 34007 44913 32506
55898 30011 66882 29514 77874 30011 88866 28514 99857 28508 11843
28505
31313 09608 81709
61616 AF309 2109A ISAAC OB 11
62626 EYE SPL 2383N08130W 1711 MBL WND 34016 AEV 20802 DLM WND 32
511 994843 WL150 34020 084 REL 2384N08130W 170929 SPG 2383N08130W
171119 =
;
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Quoting flcanes:

what are you're conditions


At the moment -- very dark, looks like a band is getting ready to come in. Been windy, some strong gusts, and fairly steady rain.

Estimating that the gusts have been around 40-45 mph and steady approx 20.
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Quoting WxGeekVA:


Hmmm....


Yep once clear of the landmasses Bamm

Florida is Ok but at landfall this will be another creature


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Quoting TreasureCoastFl:
WPBF First Alert Weather
26 minutes ago via SocialNewsDesk
Major feeder band about to come ashore in Key West. Our Randy Gyllenhaal says winds are already picking up there. According to doppler radar, winds are at 60-65mph in this band... -Eric

Anyone on from Keys?

well, there is largo
speaking of him, is he on the internet, or has he lost power
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Storm surge in Englewood, Florida concerns me right now. Can anyone figure how far the storm's center will be from Englewood, at the closest point?
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Quoting flcanes:

stay safe


Thank you flcanes. Still not sure its coming here but watching it closely. :)
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Quoting nola70119:


You have to be really vigilant.
thanks nola
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Quoting tropicofcancer:

If you think this is howling winds then you are either young or new to the area. I'm way south of you in the Redlands and have been here for 30 years , this is what a light TS feels like down here! I guess is just a matter of perspective!
I gotta admit the winds weren't howling... though they will in a strong TS.
Quoting clickBOOM:
Getting some pretty strong winds and rain here on Grand Bahama. Impressive for a storm that's so far away.
Yeah, you guys are likely to get rain and intermittent heavy winds all afternoon....
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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.


The images you are showing us has Isaac running up the Florida coastline the entire time.

Wrong images?
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Quoting charlottefl:

nnw jog?
o_o
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WPBF First Alert Weather
26 minutes ago via SocialNewsDesk
Major feeder band about to come ashore in Key West. Our Randy Gyllenhaal says winds are already picking up there. According to doppler radar, winds are at 60-65mph in this band... -Eric

Anyone on from Keys?
Member Since: August 18, 2008 Posts: 0 Comments: 474
Quoting Gorty:


What does that mean?
It could mean a more westerly component to Isaac just before it hits. Changes thing for folk inland also who may be flooded.
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Quoting TierraFirme:


Levi, when do you think it will bomb out? Thanks in advance.


If it strengthens rapidly I would expect it to be mid-way between the north gulf coast and the Florida peninsula, out in the middle of the Gulf of Mexico.
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Quoting zoomiami:


Hi Gamma! Getting weather your way?

what are you're conditions
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Intense rainfall right now. Winds are moderate with the occasional 30-35kt gust.
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please post link to 12z gfdl and hwrf
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Quoting nola70119:


You have to be really vigilant.

ok
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Quoting seflagamma:


Ditto for me!


Hi Gamma! Getting weather your way?
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Quoting HurricaneHunterJoe:


Good Afternoon Gro


Yo, Joe!
Member Since: July 17, 2009 Posts: 71 Comments: 27125
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Quoting nola70119:
Gonna take a swim and back in a little, don't panic over the intensity yet.

where?
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Quoting drs2008:
how about me.im in biloxi.


You have to be really vigilant.
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Quoting AtHomeInTX:
I'm hopelessly behind on everything. Levi, great job. Thank you.

stay safe
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Gonna take a swim and back in a little, don't panic over the intensity yet.
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Not that the Weather Channel now owns this site, but they have always done a good job. They have always had good talent and are adding even more. I have been an ardent watcher since Rick Griffin was on.

The only real problem I have is that they have too much information on the screen.

1. They have two people on the left you can hardly see.
2. The have the hurricane track on the right (which is minimized.
3. They have the local radar on the bottom left, which no one can really see.
4. Then on the bottom they have message which take up the whole bottom screen.

5. Then when they do show a full map, someone stands directly in front of it so you can't see what is behind them.

Pretty soon the only thing visible will be someone's eyeball poking through a little slit in the screen.

There is a very useful Yiddish word for this-ungepatchke.

Member Since: July 17, 2009 Posts: 71 Comments: 27125
Quoting MississippiWx:
Core is tightening up as seen on Key West radar:




Agree: feeder band taking shape on the SE quad...
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Quoting MississippiWx:
Core is tightening up as seen on Key West radar:


W eyewall??


The WORST Intensitly model... is not a weak TS and it will NOT stay that way (i dont think)
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Quoting mobilebayal:


Where on the eastern side are you?

what do you mean?
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I'm hopelessly behind on everything. Levi, great job. Thank you.
Member Since: August 24, 2010 Posts: 0 Comments: 253
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Quoting TexNowNM:


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.


Sorry, meant KFDM, KFDA is the Amarillo station we watch here in NM.
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Quoting CybrTeddy:


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.


I've always wondered if the SFMR in the plane has more data, since they report a 10 sec average every 30 seconds. What about the other 20 seconds?
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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.
how about me.im in biloxi.
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I see alot of IMBY forcasts like usual. Kinda weird seeing people hope for a major and it also hitting them. Not sure what to make of it, but it happens here all the time.
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Quoting zoomiami:
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.


Ditto for me!
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Quoting TexNowNM:


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.


KBMT met on Facebook said we're not out of the woods yet. Last time I looked.
Member Since: August 24, 2010 Posts: 0 Comments: 253
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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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