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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Voluntary evacuations called for all low lying areas in the 15 parishes in the hurricane watch.  As per the Governor. 
Member Since: Posts: Comments:
Quoting BahaHurican:
Can I get an update on the HHers pass through the heart of Isaac on going?


Just got a 992.3mb extrapolated pressure reading. Waiting for dropsonde...
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Quoting KeyWestbeachcomber:

Before you start judging what those scattered,shattered frightened people did..Float a mile in their coolers..Then tell me how you feel.


Actually, you are judging me. I judged noone. It was an unprecedented and horrible situation compounded by an incompetent mayor who left hundreds of transit buses to drown when they could have evacuated people. I am a native of New Orleans who had to deal with Katrina, and I truly resent your comment. Can we get back to weather please?
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Unfortunately, soundings don't get released in the center of the Gulf, but conditions look pretty favorable from places along the Gulf's rim. A couple of examples:





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Quoting Articuno:

It's about Getting started, maybe midway.


Its about over for south east Florida accoording to radar.
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WOW!!

Member Since: May 23, 2012 Posts: 0 Comments: 4207
Louisiana Gov. on channel 6 news out of new orleans
Calling for volintary evac. right now
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Can I get an update on the HHers pass through the heart of Isaac on going?
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that last squall has my rain totals close to 3.5 inches now. We're fortunate in South Florida that Isaac didn't have time fill in that south eastern side, though it seems he wants to make an attempt at it now.
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Quoting Qball561:


In Lake Worth. A few more like that and flooding will definitely be a widespread issue in PB County.


that was a crazy band of rain... need the break to help things drain a little now....
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1866. ITCZ
He's on he's on Dr. Masters is on TWC :-)
Member Since: September 5, 2008 Posts: 0 Comments: 47
TWC just said entire state of Louisiana state of emergency.
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Quoting KeyWestwx:
we are still in the wall of the 'center' of Isaac. We are still experiencing heavy rains and strong winds. It's like we are in the 'creation' of the forming of eye over us. I mean that in the literal sense,,,back to movie and lots of food!


Stay Safe Yo!
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Can someone please explain me why is Houston worried.. They are 200 miles out of the cone and probably won't even get a sprinkle out of this.. Yet a third of these post are of texans worrying about the storm coming towardsnthem. Please don't hype Texas. Thismis still alabama, mississippi and very eastern Louisiana event
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1862. airmet3
Quoting JGreco:



k.....so western florida panhandle in clear then..


No, especially if you are prone to surge. Why the concern. Do you live there or are you travelling there?
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1861. LargoFl
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992.3mb on the extrap. instrument. Let's see what the dropsonde brings back.


202330 2403N 08158W 8430 01437 9923 +207 +182 133014 019 026 003 00
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Quoting Maineweatherguy20023:

I dout it will make landfall there but he is a HUGE storm. impacts are likely to affect you. But dont listen to any of us, listen to the nhc pros :)


Yes, despite what many might think. This is not the best site for accurate information.

Half of the stuff posted in this blog is pure imagination.
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Quoting JGreco:



This sucks...really need to know if pensacola-Fort walton Beach-Destin out of the woods....


I am in NOLA, and, unfortunately, you guys are in the same boat as we are. Good luck to you! And, God bless us, everyone.
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Quoting JGreco:



k.....so western florida panhandle in clear then..

No, they're still showing close Destin in the cone.
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1852. yoboi
Quoting FOREX:


so that will pull Isaac more west than Northwest??


that's what gfs was looking at...look at current surface map looks like it retreating farther west...
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Quoting Masquer08er:
Has anyone noticed plastic gas cans have more than doubled in price since Katrina? I think it was the 5 or 6 gal (whatever that larger one is) that was $17 and change at both Home Depot and Lowes. My gas cans have done better than any other investments.


Yeah, I paid $7 each for 12 of them in 2005. Saw them locally in FL for $18 last week *before* the storm was in the news. All this money printing has caught up with us *bad*.
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Nice response time on that banning blog admin, i reported that person like 10 times.
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Quoting catastropheadjuster:
So has everything changed since this morning? Is it going to NO or TX? I've been out getting a few things and haven't been able to hear anything. I live in Mobile,Al our we out of it know?

sheri

We are right in the middle of the cone still.
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Quoting JGreco:



This sucks...really need to know if pensacola-Fort walton Beach-Destin out of the woods....

I dout it will make landfall there but he is a HUGE storm. impacts are likely to affect you. But dont listen to any of us, listen to the nhc pros :)
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1843. yoboi
Quoting Joanie38:
Ok, dad can't find the briefing with Gov Jindal...is it on TV????


go to la.gov it's there...
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Quoting angelafritz:
I like the new music choices on TWC for Local On the 8s. 24 hour coverage of an approaching hurricane is very difficult, especially 3-5 days in advance of landfall, when the only changes we really see are from Hurricane Hunters and every three hours in the NHC advisories. Personally, I've enjoyed watching the TWC coverage of Isaac. I like seeing the conditions on the beaches where the reporters are located, and I like seeing the different ways that Bryan and his team explain current observations and forecasts to the viewers.

By the way, Doc is on around 4:30 EDT, so tune in! :)



now if you can get them too get rid of all the TV ads or find some new ones that would be nics
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1841. JGreco
Quoting sunlakedude:

The latest European model did budge. Previous run showed Pensacola. Lastest run shows Mobile.



k.....so western florida panhandle in clear then..
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1837. FOREX
Quoting yoboi:


texas high is retreating...


so that will pull Isaac more west than Northwest??
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1836. airmet3
Quoting JGreco:



This sucks...really need to know if pensacola-Fort walton Beach-Destin out of the woods....


Use the NHC and local forecast and make your decision. Start some of the basic preparations now.
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Quoting angelafritz:
I like the new music choices on TWC for Local On the 8s. 24 hour coverage of an approaching hurricane is very difficult, especially 3-5 days in advance of landfall, when the only changes we really see are from Hurricane Hunters and every three hours in the NHC advisories. Personally, I've enjoyed watching the TWC coverage of Isaac. I like seeing the conditions on the beaches where the reporters are located, and I like seeing the different ways that Bryan and his team explain current observations and forecasts to the viewers.

By the way, Doc is on around 4:30 EDT, so tune in! :)


Going to guess you like the old Van Morrison songs?
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Quoting PalmBeachWeatherBoy:
Thunder in a tropical system rain band is an indication of strengthening right?


Curious about that too. First thunder I've heard all day.
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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.