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 samiam1234:
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

TWC advised earlier that west all the way to Houston should be on alert and watch Issac closely!
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1924. TxKeef
Quoting JGreco:



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


I'm no expert so take this advice for what it's worth:
This storm has a rather large wind field, 205 miles for TS winds last time I read(could have changed). Either way nobody on the gulf coast is in the clear. If that track moves 50 miles to the east, that would put the western florida panhandle right in the path of the "dirty" side of the storm. Be prepared and keep an eye out is the best you can do.
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1923. yoboi
Quoting samiam1234:
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


beacause the track been shifting west...
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Quoting Joanie38:



Are you Serious??? I am in Central LA...this can't be good...:(


Yes, governor is on all local channels with a press conference.
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Quoting Stormchaser121:

So its just inching towards us huh??


Maybe so.
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Quoting WetBankGuy:


I just want to debunk an old Katrina myth before it's starts getting legs. There are not enough buses in the NOLA area to evacuate the number of people who do not have cars.

That 400 bus figure was high and pre-Katrina. RTA runs 32 lines, many only two buses at a time.

http://wetbankguide.blogspot.com/search?q=buses


What about school buses? I've read that due to a mix up school buses weren't used during Katrina.
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Quoting FatPenguin:


thanks for info. sounds very likely that Isaac is at least CAT 2 by this time tomorrow.


Dry air has really kept him suppressed today. The worst squalls are over for the Keys already. Not much at all on the right side (dirty) side of the storm at this time. The sun may even peak out there before the day is done.
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Quoting TomballTXPride:
Is it me or does the eye appear to be shifting/jumping a bit to the south?

Link


Issac has not fully formed an eye wall yet but he is trying.......Hard to tell as to jogs as the circulation is so large.
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Quoting AtHomeInTX:


Oh. Ok. I know she started round there somewhere. Long day.

That state of emergency seems to be inching our way...
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Quoting GTcooliebai:
Isaac getting huge coverage wise again..
Member Since: August 14, 2010 Posts: 10 Comments: 16437
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 :)
I might have misunderstood but I thought I just saw on TWC that Pensacola to NOLA would be under a warning at the 5:00 update. I could be totally off here and Dr. Masters was standing there on this update.
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Quoting AtHomeInTX:


Was this on Facebook?

Been at work.. are these models suggesting Isaac won't recurve at landfall? But hang a left?
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1912. KEEPEROFTHEGATE (Mod)
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1911. Sasha
I've been glued to this board all day...  all I've done is snack endlessly and hit F5 to refresh!!! 
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Quoting AtHomeInTX:
TWC just said entire state of Louisiana state of emergency.



Are you Serious??? I am in Central LA...this can't be good...:(
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Quoting Sfloridacat5:


I dought it because the Euro model is still to the East. The NHC puts a lot of weight into that model.


I'm not wishing harm to LA, but gosh I Hope the Euro follow the rest soon. Even better if it would just fizzle and bring rain or something less dangerous.
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1908. mfcmom
Okay this is really aggravating they are doing mandatory evacuations in the county east of us (Appalachicola) and west of us (Walton County) but not here yet. What to do I just don't know. One school is closed for special needs shelter and the others are probably open. Whew!
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Quoting airmet3:

Actually, Rita was already a Cat 2 in the Florida straits


Oh. Ok. I know she started round there somewhere. Long day.
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Quoting AtHomeInTX:
TWC just said entire state of Louisiana state of emergency.

WOW!
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TWC live report on beach with *gloves* on. WTF? Must be sweating like a SOB. lol
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Quoting 1900hurricane:
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:







Gonzo is out on a mission and went through the central gulf






You can access the dropsonde sounding on tropical atlantic
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Quoting TomTaylor:

Strengthening very well may become explosive tomorrow if Isaac can rid himself of dry air and build up solid core today, allowing him to take full advantage of the ideal upper level environment.


thanks for info. sounds very likely that Isaac is at least CAT 2 by this time tomorrow.
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Quoting weatherman410:
The 12z GFS and EURO are about 350 miles apart at landfall. I'm thinking the NHC will split it down the middle at 5 PM near New Orleans.



This.
Member Since: September 16, 2007 Posts: 0 Comments: 6188
1898. BZCTX
What does the big retreating west mean?

Texans are worried because Ike wasn't supposed to hit Texas either
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Quoting CaneHunter031472:
By looking at the GFS and now the other models following suit I am sure that the 5pm advisory will significantly be shifted to the west once again. This is starting to look alot like Gustav instead of Georges.


I dought it because the Euro model is still to the East. The NHC puts a lot of weight into that model.
Member Since: September 16, 2007 Posts: 0 Comments: 6188
Quoting wxgeek723:
Masters on TWC.
He's a lot taller than Bryan Norcross.
He sounds like such a dork too. In like a cool dork way, of course. Haha.


I was thinking the same thing ;)
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Quoting yoboi:


go to la.gov it's there...


Where do I go once there????
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For those who want New Orleans info online....

wwlradio.com. > Click listen live

wwltv.com. > the television station website. Newscasts are live.

WDSU and WVUE(FOX8) also have televsion websites with info and live newscasts.
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The 12z GFS and EURO are about 350 miles apart at landfall. I'm thinking the NHC will split it down the middle at 5 PM near New Orleans.
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Quoting AtHomeInTX:
TWC just said entire state of Louisiana state of emergency.

So its just inching towards us huh??
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Per the governor: Voluntary evacs for low lying areas of the 15 parishes in the hurricane watch. Those areas will most likely face mandatory evacs tomorrow!
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1890. breald
Quoting melwerle:


BIG roller just went through. Whole floor just waved out.


You guys are getting trembles?
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Just had another squall come through Grand Bahama, probably what West Palm is feeling right now.
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By looking at the GFS and now the other models following suit I am sure that the 5pm advisory will significantly be shifted to the west once again. This is starting to look alot like Gustav instead of Georges.
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Quoting violet312s:
My goodness I would not want to be in Brawley, CA. The little quakes keep coming. That would freak me out.

We, my wife and I, both felt the 5.3 at 12:30 PM, but not any of the others. We're up in Blythe, so we are a little distance from them and there's a LOT of sandy soil out here which dampens the shock quite a bit.

People in Brawley cannot be too happy though as they're occuring right under the northern edge of the city, where it meets the farmland in the valley. This is one really deep rift valley here so these quakes don't worry me very much. They stand almost no chance of the kind of magnitude that a subduction zone quake can generate. You couldn't pay me to live on the Seattle area coast with that massive subduction zone right out there!
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Quoting LargoFl:
Wow... bands already coming in to Orlando and Tampa...
Member Since: October 25, 2005 Posts: 19 Comments: 21549
Quoting FOREX:


so that will pull Isaac more west than Northwest??

Thats right
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1884. LargoFl

Flood Watch


FOR: INLAND COLLIER COUNTY

EFFECTIVE: 4:59AM, AUGUST 26, 2012

EXPIRES: 8:00PM, AUGUST 26, 2012

...FLOOD WATCH IN EFFECT THROUGH MONDAY EVENING... THE NATIONAL WEATHER SERVICE IN MIAMI HAS ISSUED A * FLOOD WATCH FOR A PORTION OF SOUTH FLORIDA...INCLUDING THE FOLLOWING AREAS...COASTAL BROWARD...COASTAL COLLIER...COASTAL MIAMI DADE...COASTAL PALM BEACH...FAR SOUTH MIAMI DADE... GLADES...HENDRY...INLAND BROWARD...INLAND COLLIER...INLAND MIAMI DADE...INLAND PALM BEACH...MAINLAND MONROE...METRO BROWARD...METRO MIAMI DADE AND METRO PALM BEACH. * THROUGH MONDAY EVENING * TROPICAL STORM ISAAC IS FORECAST BY THE NATIONAL HURRICANE CENTER TO MOVE NORTHWEST TODAY AND INTO THE SOUTHEAST GULF OF MEXICO TONIGHT...BEFORE MOVING INTO THE NORTHERN GULF OF MEXICO EARLY THIS WEEK. THIS WILL BRING DEEP TROPICAL MOISTURE TO THE AREA THROUGH MONDAY...INCREASING THE CHANCE FOR TORRENTIAL RAINFALL. STORM TOTAL AMOUNTS OF 4 TO 8 INCHES EXPECTED THROUGH MONDAY WITH LOCALIZED HIGHER RAINFALL AMOUNTS WHERE RAIN BANDS MOVE THROUGH.
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1883. Michfan
Quoting BahaHurican:
Can I get an update on the HHers pass through the heart of Isaac on going?


Just passed by with its 4th center pass. Waiting for dropsonde and 4th vortex message.

Time: 20:24:30Z
Coordinates: 24.0167N 82.0W
Acft. Static Air Press: 842.9 mb (~ 24.89 inHg)
Acft. Geopotential Hgt: 1,437 meters (~ 4,715 feet)
Extrap. Sfc. Press: 992.7 mb (~ 29.31 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 142° at 3 knots (From the SE at ~ 3.4 mph)
Air Temp: 20.0°C (~ 68.0°F)
Dew Pt: 18.5°C (~ 65.3°F)
Peak (10s) Flt. Lvl. Wind: 7 knots (~ 8.0 mph)
SFMR Peak (10s) Sfc. Wind: 25 knots (~ 28.7 mph)
SFMR Rain Rate: 3 mm/hr (~ 0.12 in/hr)
(*) Denotes suspect data
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Masters on TWC.
He's a lot taller than Bryan Norcross.
He sounds like such a dork too. In like a cool dork way, of course. Haha.
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1881. airmet3
Quoting AtHomeInTX:


Rita was just developing.

Actually, Rita was already a Cat 2 in the Florida straits
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1879. reid221
After all these years of following Dr. Masters here .. so cool to see his face on the Weather Channel! Congrats Dr. Masters!
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1878. sar2401
Quoting FOREX:


Where I live in Panama City beach is not in the cone anymore, but maybe we should be.


See what the 5 pm update brings. Remember, there's still a 1 in 3 chance the storm could at least affect you if you are outside the cone.
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Quoting Levi32:


No, no not true. Think about Ernesto. He looked like a near-hurricane on satellite with persistent deep -70C or lower convective tops for over 24 hours. The pressure rose during that time.
No kidding, that's why I specifically said consolidated and closed circulation. Ernesto was flirting with an open circulation the entire time, even declared an open wave by some, including Bastardi. And of course, the reason it remained open was the fast trade wind flow. Failure to consolidate and get a closed circulation from accelerating trade winds prevented the storm from deepening inspire of convection.

All I'm saying is I don't understand how a downward MJO can prevent deepening and vertical alignment when convection was clearly present over Isaac the whole time. Convection is rising air.
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Voluntary evacuations called for all low lying areas in the 15 parishes in the hurricane watch.  As per the Governor. 
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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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