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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Soon FL will be in the rear view mirror and all eyes will be looking at the north central gulf coast. Wonder if this blog will slow down slightly then????
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Texas coming into play???
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bout to leave cocoa beach right now to get in the chase...wheres everyone this is a good place to meet this thing on the west coast of florida..im thinking near tampa? im trying to get into the worst condidtions possible
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Quoting watchingnva:


Hey tf...

Yeah it was nice I enjoyed it... lost power in the enon areas for a around an hour or 2 ... even the storms last night around 830 were nice too...


Yeah our tent on the deck succumbed to the weight of the water because it poured for so long. What a loss!
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12Z Euro @ 72 hrs - exactly the same position at the 00Z run BUT much further east than 12Z GFS..
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Quoting TropicalAnalystwx13:
The 12z ECMWF has a borderline Category 4 hurricane at landfall. More importantly however, it did not shift east and remains at the same position it was at 00Z.


It didnt shift west either lol
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12Z ECMWF- 72 hours

Still pretty much in line with the last run, maybe a smidge west.

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Quoting MiamiFL305 [#727]:
Key West
Please don't falsely label images. That is a well-used picture of a decidedly non-Isaac event that can be found all around the interweb. Such as here, and here, and here, and here, and here, and here...

If you're unsure of the provenance of a particularly stunning image, use caution with posting and labeling. Facebook, for one, is full of falsely label weather photos that are trotted out with each severe weather event.
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Quoting ringeaux:


Jindal has a press conference later this afternoon. You can also go to the state site:

http://www.gohsep.la.gov/


Thank you SO MUCH for your reply...:)
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Quoting Hurricanes101:


thats not true, tomorrow Isaac will still have a decent northerly movement to him, tomorrow midday will be the closest he gets to us
thats what my local met says as well tomorrow and possibly tonight is our stormiest days
Member Since: August 6, 2011 Posts: 4 Comments: 38194
Technology is so amazing...

from Twitter:

@weatherchannel

Check out @RedCross #HurricaneApp. Track storms~use "I'm Safe" button to update fam/friends & more http://rdcrss.org/PSKPnr
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p'cola still for euro. Sorry folks....these computers aren't going to make it easy on us.
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863. IKE
Euro sticking to its eastern solution. Anyone have the 12Z UKMET?
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Bp in Texas city tx is going in hurricane prep 1
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The 12z ECMWF has a borderline Category 4 hurricane at landfall. More importantly however, it did not shift east and remains at the same position it was at 00Z.
Member Since: July 6, 2010 Posts: 113 Comments: 31911
Link


These models are driving me nuts here in LA. This photo reminds me of how the models have been.
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Quoting MiamiFL305:
Key West


WOW is all I can say! (------I apologize for acknowledging this picture d/t I guess this is NOT from the current situation.
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Quoting tropicfreak:


Hey watch... buddy long time no see!

Really interesting weather we had yesterday morning. As far as I've heard it was somewhat subtropical in nature? Interesting.


Hey tf...

Yeah it was nice I enjoyed it... lost power in the enon areas for a around an hour or 2 ... even the storms last night around 830 were nice too...
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Quoting eyewall99:
2pm EST Mon
a)still tropical storm
b) 75mph
c) 80mph
d) 85mph
e) 90mph
f) Cat 3
g) Cat 4
h) Cat 5
i) Jupiter hurricane!!!!
j) open wave

11 AM Monday open wave, 2 PM Monday Jupiter cane. In all seriousness though, probably cat 2 100 mph, which is not an option though I see...
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12Z Euro 72 hours:

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Quoting Clearwater1:
Largo, do you think that will happen? I don't, if it stays on the current track. In fact, it is about as close to the TB area now, as it will ever be. Since it's moving away, my bet is very little impact, unless the track move closer to us, but the cone has really narrowed. We will know soon enough.
yeah we'll see after this passes thru the keys and comes up the coast a lil, say ft meyers before heading west..we have to watch because its a huge storm in area..remember..we lose power in a thunderstorm..some of those squall lines are pretty strong thunderstorms..we'll see what happens..im just a bit nervous today..dont trust this storm at all
Member Since: August 6, 2011 Posts: 4 Comments: 38194
Quoting MiamiFL305:
Key West


Wow, thats amazing!
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Quoting Clearwater1:
Largo, do you think that will happen? I don't, if it stays on the current track. In fact, it is about as close to the TB area now, as it will ever be. Since it's moving away, my bet is very little impact, unless the track move closer to us, but the cone has really narrowed. We will know soon enough.


thats not true, tomorrow Isaac will still have a decent northerly movement to him, tomorrow midday will be the closest he gets to us
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Quoting Hurricanes101:


thats not right, I think they had him too far north at 11am, it is clear on radar he has been moving between WNW and NW since then


This...

I've been following the center on radar since about 9am and it has been moving WNW/NW all day. It was a broad center earlier that has since tightened up a little bit. Could be why the coordinates aren't 100% accurate.
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Quoting MiamiHurricanes09:
Power back! I'm thankful LOL. No wi-fi or satellite signal on the TV, but at least we have power. Rain has subsided, but the gusts still appear relatively intense. Will try to get pictures for you guys of the neighborhood a little later.
Congrats for surviving Isaac :)
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Quoting DataNerd:
New ECMWF To Join GFS:



No. Further east than GFS
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Holy you know what the Euro BOMBS the last 24 hours before landfall. Yikes!
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Quoting Stoopid1:


Most likely a wobble.


Northward movement.. REALLY? LOL...
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Euro is still farther east but its trending more and more west every run...this run looks like Mississippi....Euro is usually stubborn but its slowly coming around to the pretty consistent GFS
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Quoting LargoFl:
.........................rainfall model
Is this the only model that shows significant FL panhandle impacts? And is it reliable
Member Since: June 26, 2012 Posts: 0 Comments: 39
Quoting MiamiFL305:
Key West


that's awesome
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Quoting weatherman12345:
Did it calm down miami?
Less audible gunfire? (That comes from talking to my Miami friends, not me)
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Quoting Hurricanes101:


???

Doesnt look like it to me


No it's NOT, Euro much more EAST... might now be the most eastern outlier among the major models...
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Quoting shfr173:
What's with that northern movement??? temporary jog or beginning of something more???


Most likely a wobble.
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Power back! I'm thankful LOL. No wi-fi or satellite signal on the TV, but at least we have power. Rain has subsided, but the gusts still appear relatively intense. Will try to get pictures for you guys of the neighborhood a little later.
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Quoting LargoFl:
guess this is what we can expect tonight or tomorrow
Largo, do you think that will happen? I don't, if it stays on the current track. In fact, it is about as close to the TB area now, as it will ever be. Since it's moving away, my bet is very little impact, unless the track move closer to us, but the cone has really narrowed. We will know soon enough.
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Quoting shfr173:
What's with that northern movement??? temporary jog or beginning of something more???


I'll explain it for you very quickly.

There is no northern movement. just WNW/NW. Recon confirms this.
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Quoting ringeaux:


Jindal has a press conference later this afternoon. You can also go to the state site:

http://www.gohsep.la.gov/

Thanks Ringeaux.

I was going to say this is probably not the best site to get the local government and media advisories for your area. Plenty of other local news and .gov sites will have it.
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Quoting justsouthofnola:
euro very close to the gfs


???

Doesnt look like it to me
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.........................rainfall model
Member Since: August 6, 2011 Posts: 4 Comments: 38194
Quoting watchingnva:


I call bs...that water is too calm..this is a supercell elsewhere...hate what people lie about photos...
Miami news time tweeted that picture
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Quoting Stormchaser121:

My forecast


Not that I wish a hurricane on anyone or location but I hope you're wrong.

Over the past few days I've been watching the steering currents closely as well as the various systems around the storm and also believe at this point it may move more west. Not an expert by any stretch of the imagination, like others here, but trying to do some analysis.

Time will tell, won't it.

Be safe
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Has the new ECMWF come out yet and if so did it track West with the GFDL?
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Quoting TreasureCoastFl:
https://sphotos-b.xx.fbcdn.net/hphotos-ash4/29529 2_ 432818426759459_977235953_n.jpg


oops not sure how to do images

Link
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Quoting CybrTeddy:
Euro starting, again with good initialization.


Please post more images of Euro?
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What's with that northern movement??? temporary jog or beginning of something more???
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New ECMWF To Join GFS:

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Euro has to flip back west for the 12Z doesn't it? It'll make the NHC's 21Z updated track a lot easier if it does. Even if not, if it still shows the FL solution, supported only by the UKMET, doesn't the NHC finally have to stop splitting the baby and buy into its own GFS's western solution?

My pure WAG, following many here, is that lightly populated South-Central Louisiana will be ground zero. Not to minimize Isaac's effects on those many tens of thousands of residents who would be near there, or the storm surge to NOLA. And a felix culpa could be the rain benefit to AR and maybe OK down the road.
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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.