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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interesting, plaquemines parish president - levees are 30ft, surge predicted 5 to 10ft. Decision on schools this afternoon...
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Here is my Afternoon Update video blog Link
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Quoting Civicane49:



That's a great pic. Looks like he's thrown off the confining shackles of Cuba and Florida and is now going to show the world what he's made of. He's a big boy and getting bigger, and I fear he's also going to be bad. Maybe only a cat 2 at landfall, but pushing an Ike sized wall of water in front of him. Major disaster looming.


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Anyone else see some weirdness with the CoC on radar?

Something is changing, and I'm not talking about intensity.

Maybe it's stalling or changing directions, I'm not sure.
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Quoting wxgeek723:
Jim Cantore just got to Tampa and was there for maybe a few seconds and now he's already getting shipped out to New Orleans.


So they're sending "The Shield" to NO? Hope his protection continues to work for them.
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2118. deb1
Quite eerie, watching this. Some of the forecasts are showing Isaac hitting New Orleans 7 years to the day after Katrina.
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Jim Cantore is going to be in NOLA after SNF. As you were...
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Quoting AtHomeInTX:
Not to us. Gotta makes some calls. :)

we arent out of the woods yet!
Member Since: September 2, 2011 Posts: 0 Comments: 1148
Pascagoula, MS here....what do you guys think?
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Quoting Tazmanian:




are you trying too get bannd
Stay weather related.
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Quoting WxLogic:
All right...

Currently the main convection is displaced from the center to the N of it. Pressure appears to be up 1MB which should be expected due to the loss of convection.

Dry air still being an issue so Isaac still has to work on that core more.

Up a millibar? It's down to 992 millibars.
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No one's bashing anyone. This is a blog where we share our opinions and that's exactly what we're doing.
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Quoting Axelia:
Kctinney, regarding Orlando area, I'm an hour west and all we've had thus far is one strong rain/wind band earlier today. Since it's moving so far to the west, I'm not expecting that much of an event in this area. Just some more rain and wind but not as fierce as was projected 24 hours ago.


Thank you
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Quoting CybrTeddy:
Not entirely sure I agree with the intensity forecast, I mean, it doesn't look like Pasch thinks that Isaac will strengthen for a day almost.




that will turn out too be wrong
Member Since: May 21, 2006 Posts: 5091 Comments: 115080
I'm not going to spend ages bashing the NHC over their intensity forecast, but there is no way this is taking 36 hours to hit hurricane status.
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Quoting Tazmanian:



Pasch is a nut head what he trying too pull


Following protocol? I wonder how concerned the NHC and governmental agencies are about a panic situation--even if it is just a "preceived" threat? I'm so glad I don't have to make these calls.
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So Pasch writes a two paragraph discussion for a system that could be a major hurricane making landfall in the next 3 days. Wow. Where is Stacy Stewart???
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Quoting Jedkins01:



No, a PWAT of 2.0 or greater is deep tropical moisture. The PWAT to the southwest is 2.5 inches, that's still excessive moisture. If you don't believe me, ask a real meteorologist. Isaac has a large swath of extremely high moisture, the reasons for why some regions of the storm don't have rain can't be blamed on dry air, it's more complex than that.


What do you expect for us in St Pete/Tampa based on the most recent track?
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2101. WxLogic
All right...

Currently the main convection is displaced from the center to the N of it. Pressure appears to be up 1MB which should be expected due to the loss of convection.

Dry air still being an issue so Isaac still has to work on that core more.
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kudos the NHC... the current center fix is only 100 miles to the west south west of the original forecast point 5 days out when the storm first formed. They have done a great job with this storm!
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Not entirely sure I agree with the intensity forecast, I mean, it doesn't look like Pasch thinks that Isaac will strengthen for a day almost.
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Quoting wxgeek723:
Jim Cantore just got to Tampa and was there for maybe a few seconds and now he's already getting shipped out to New Orleans.


Cantore has been in Tampa for a few days.
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Brawley CA another 5.4 quake a few mins ago.
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2095. yoboi
Quoting CatfishJones:
Let's see your marvelously insightful, educated - and what's more, flawless - analysis of the system... not bashin' ya, just sayin'.


surface maps tell the story....they change if ya follow them they will tell ya where the storm will go...not good for predicting the future but it will tell ya where the storm will go..
Member Since: August 25, 2010 Posts: 7 Comments: 2335
went to the store just now shelves are empty......
come back now im under hurricane warning and the doom cone is about 30 mins east of my house. ( the center of the cone)
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Quoting LBAR:
Another huge "win" for the GFS. I stand corrected!


No...GFS did not have this solution until yesterday

Most of the time the GFS was off, nrtl posted the margin of error for the models and the GFS was way down the list...The Euro had this solution way before any other model swung west

but of course the GFS will continue to get the praise it does not deserve.
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Quoting huntsvle:


I agree...PWats less than that cause flooding rains on systems onland...even when it's the season for that type of moisture.

I concur.



Correct!
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Quoting Joanie38:


He is an EXCELLENT Governor!!! He takes good care of his State!!!!


I'm in Texas..I wouldn't know an excellent governor if he fell in my lap (and with our gubenetorial track record...that's actually a drunken possibility)
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Quoting Stormchaser121:
Still barely shifted....man. What in the world.


They're not buying the GFS.
Member Since: August 24, 2010 Posts: 0 Comments: 120
If Isaac doesn't intensify now the forecast could be right.I do think a major is still possible.
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Quoting Maineweatherguy20023:

5:00 PM EDT Sun Aug 26
Location: 24.2°N 82.3°W
Moving: WNW at 16 mph
Min pressure: 992 mb
Max sustained: 60 mph


The initial forecast advisory put out said 994mb, but it has been changed. Just go back and look at it as it was when TropicalAnalyst posted it here.
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Damned if you do...Damned if you don't.
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Questionable intensity forecast at best there... He's the expert, but a hurricane Tuesday morning is way conservative.
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Jim Cantore just got to Tampa and was there for maybe a few seconds and now he's already getting shipped out to New Orleans.
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Quoting Abacosurf:
to the SW there a little.



No, a PWAT of 2.0 or greater is deep tropical moisture. The PWAT there is 2.5 inches, that's still excessive moisture. If you don't believe me, ask a real meteorologist. Isaac has a large swath of extremely high moisture, the reasons for why some regions of the storm don't have rain can't be blamed on dry air, it's more complex than that.

If you go far enough southwest yes, but that isn't part of the storm, you will always have sharp drop off in moisture as you get that far away to the southwest of a tropical cyclone.
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Quoting NICycloneChaser:
Hurricane warning issued for Gulf coast from Morgan City, LA, to Destin, FL.
So have the tracks changed any? I see the new warnings up for the 5:00 but can anyone advise on the tracks?
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2081. flcanes
Quoting Bluestorm5:

Old... Get a new one before going against people. Geez...

what does" s s" mean
Member Since: August 20, 2012 Posts: 13 Comments: 1254
2080. breald
Warnings up for LA to western FL Panhandle. That is a large area. It looks like LA/Ms will get the landfall.
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Quoting NICycloneChaser:


He's put the pressure at 994, when recon clearly shows it to be 992.

5:00 PM EDT Sun Aug 26
Location: 24.2°N 82.3°W
Moving: WNW at 16 mph
Min pressure: 992 mb
Max sustained: 60 mph
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Quoting Maineweatherguy20023:

The last time i am posting this image


stil the 11am...
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Quoting Maineweatherguy20023:

The last time i am posting this image
That's 11 am, dude...
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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.