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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226. wpb
Quoting gulfshoresAL:
Excellent explanation...appreciate all you do and totally respect your input.
Look forward to your next tidbit. Thanks...have a great day.
he is very very good plus he effort is aaa
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Quoting WxGeekVA:
Seven years ago today, to the hour:



Le Deja Vu, anyone?



Latest compter model runs have shifted significantly west in the past six hours, and the threat of a strike on New Orleans by Katrina as a major hurricane has grown. The official NHC forecast is now 170 miles west of where it was at 11am, and still is to the east of the consensus model guidance. It would be no surprise if later advisories shift the forecast track even further west and put Katrina over New Orleans. Until Katrina makes its northward turn, I would cast a very doubtful eye on the model predictions of Katrina's track. So much for the model prediction being high confidence, as I was surmising at 8am this morning! Recurvature is a difficult situation to forecast correctly.

The pressure of Katrina has continued to slowly drop, to 965mb. Dry air on the northwest side of the hurricane has interfered with the strengthening process, and may continue to do so over the next day. I still expect Katrina to attain Category 3 status Saturday, but Category 4 is looking less likely due to the dry air to the north. As one can see from the latest long range radar out of Key West, the northwest side of the eyewall is fragmented.

Some fairly prodigious rain amounts fell in the Miami area today. Homestead south of Miami measured 13.2 inches, and isolated amounts of 15 - 20 inches were observed between Homestead and Miami. The 7.55 inches at Key West was its 10th heaviest rainy day in history.

Jeff Masters
Member Since: July 6, 2010 Posts: 113 Comments: 32531
Quoting FLWeatherFreak91:
Isaac getting stronger faster and being influenced directly by the trough retreating slowly to the NE


thanks.
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Quoting Levi32:
Good afternoon.

Blog update:

Tropical Tidbit for Sunday, August 26th, with Video


Thanks Levi.
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Quoting KeyWestwx:
woohoo! eyewall (or close to it) should be here soon!

why do you want an eyewall?
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Plenty of tropical storm-force gusts on a consistent basis. Rain is very light though.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
Link


Eye is elongated and winds have fallen some.


Time: 16:08:00Z
Coordinates: 23.3167N 79.1167W
Acft. Static Air Press: 842.5 mb (~ 24.88 inHg)
Acft. Geopotential Hgt: 1,541 meters (~ 5,056 feet)
Extrap. Sfc. Press: 1005.0 mb (~ 29.68 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 166° at 40 knots (From the SSE at ~ 46.0 mph)
Air Temp: 17.9°C (~ 64.2°F)
Dew Pt: 16.2°C (~ 61.2°F)
Peak (10s) Flt. Lvl. Wind: 41 knots (~ 47.1 mph)
SFMR Peak (10s) Sfc. Wind: 42 knots (~ 48.3 mph)
SFMR Rain Rate: 1 mm/hr (~ 0.04 in/hr)
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217. jpsb
Quoting flcanes:

ouch, very ike-ish
Stormchaser121 is good, which is why I am now worried here in Galveston.
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Quoting newportrinative:


Max Mayfield is saying the opposite (on local news now). SE quadrant of storm is dry....The worst weather he is saying is happening now.


You can understand what Max Mayfield is saying? Wow! He just mumbles along. For me, it's like listening to that teacher from the Peanuts cartoons.
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Quoting jpsb:
I am not expert on the models and really only lurk when a storm is heading towards the USA. But my recollection is that the New Improved GFS is the more reliable model. It certainly was with Debby earlier this year. And one other storm whos name I forget (right after Debby).

Ernesto?
Member Since: August 1, 2012 Posts: 0 Comments: 269
Again, for the people that keep calling this storm a "bust."

Quoting WPBHurricane05:
Perspective for the South Floridians that aren't impressed- this is only a 65 mph tropical storm over 100-200 miles away. We aren't talking a cat 2+ hurricane.
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Quoting PalmBeachWeather:
Do you thin the people in the northern gulf think it is a bust 145?


They said for Ft Lauderdale....read before being rude.
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Quoting PalmBeachWeather:
Do you thin the people in the northern gulf think it is a bust 145?

if they learned their lessons, no
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woohoo! eyewall (or close to it) should be here soon!
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Quoting newportrinative:


Max Mayfield is saying the opposite (on local news now). SE quadrant of storm is dry....The worst weather he is saying is happening now.


Maybe down south however, this for Palm Beach County

This Afternoon Tropical storm conditions possible. Rain and possibly a thunderstorm. Some of the storms could produce heavy rainfall. High near 84. East wind 28 to 33 mph, with gusts as high as 47 mph. Chance of precipitation is 100%.

Tonight Tropical storm conditions possible. Rain and possibly a thunderstorm. Some of the storms could produce heavy rainfall. Low around 82. Southeast wind 28 to 33 mph, with gusts as high as 47 mph. Chance of precipitation is 90%.

Monday Rain likely and possibly a thunderstorm before 2pm, then a chance of rain and thunderstorms after 2pm. Some of the storms could produce heavy rain. Cloudy, with a high near 88. Heat index values as high as 100. Windy, with a southeast wind around 23 mph, with gusts as high as 32 mph. Chance of precipitation is 70%.
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Quoting atmosweather:


Everyone in the entire southern half of the peninsula will see major impacts from Isaac today. I fear that tornadoes could be pretty widespread given the very unstable airmass present and huge directional shear down through the lower levels.

this is getting really ugly here, stay safe please
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Quoting 145:
Not to be rude, but This 'Isaac' storm is a bust. Here in ft. Lauderdale its just a thunder storm. Nothing more. A boring ole thuderstorm.
Do you think the people in the northern gulf think it is a bust 145?
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Quoting Levi32:
This storm is a huge win for the ECMWF, redemption for its performance with Debby, as it led the models into the Gulf of Mexico from the very beginning.

It is so ridiculous to have two storms like this in one year.


Yet the Euro had Isaac going through the Yucatan Channel or south of Cuba. A long range win for the ECMWF, a medium range win for the GFS.
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After Isaac....

JMasters: "...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".




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Quoting TXCWC:


YOUR NOT GOING TO LIKE IT :) Landfall on TX/LA border FOLLOWED BY CONTINUED WEST MOVEMENT RIGHT OVER HOUSTON LOOKS LIKE then goes north



Link


Thanks I didn't know how west it got. sigh...guess EURO'S next.
Member Since: August 24, 2010 Posts: 0 Comments: 253
Quoting Bluestorm5:
And we got convection booming to south of the center in Cuba... this could get bad.



o_o
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Quoting Levi32:
Everyone in south Florida please stay safe today.


Everyone in the entire central and southern parts of the peninsula will see major impacts from Isaac today. I fear that tornadoes could be pretty widespread given the very unstable airmass present and huge directional shear down through the lower levels.
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Quoting newportrinative:


Agreed.


Yeah FOR YOU! LOL
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201. txjac
Quoting Skeptic33:


Probably trolling at Godlike Productions...


I think that I remember seeing a post a while back saying that Patrap would be on vacation?
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Quoting Levi32:
Good afternoon.

Blog update:

Tropical Tidbit for Sunday, August 26th, with Video
Excellent explanation...appreciate all you do and totally respect your input.
Look forward to your next tidbit. Thanks...have a great day.
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Quoting AtHomeInTX:


OK Everyone. It's called the Missouri Dome this year.

?
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And we got convection booming to south of the center in Cuba... this could get bad.


Member Since: August 1, 2011 Posts: 28 Comments: 8041
Quoting 145:
Not to be rude, but This 'Isaac' storm is a bust. Here in ft. Lauderdale its just a thunder storm. Nothing more. A boring ole thuderstorm.


Agreed.
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Quoting SykKid:


why? isaac is struggling greatly right now everytime he's forcasted to get stronger he never does. im just saying he looking really ragged right now jeeze why is everybody so harsh for me being excited that he isnt becoming strong? :( i just want whats best for everybody in the gulf.

I feel your pain. I have to admit, even I get a little fed up for the bashing people get when they make an observation that doesn't indicate strengthening. I consider myself to be pretty conservative when it comes to topical forecasting, and I often get criticism for saying I think a storm looks weak or is weakening. If you think about it, Isaac has been a huge bust so far from a numbers standpoint. The first forecast indicated a 110mph hurricane at this point. Of course you can't call him a bust because he's already claimed lives, but still, I'd ask a little more respect for opinions on the blog, even if you don't agree with them.

Data from recon still indicating a slightly weaker storm, but satellite keeps looking better. It may take the winds some time to catch up.
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Quoting SykKid:


why? isaac is struggling greatly right now everytime he's forcasted to get stronger he never does. im just saying he looking really ragged right now jeeze why is everybody so harsh for me being excited that he isnt becoming strong? :( i just want whats best for everybody in the gulf.
The first sentence is kinda true.
Member Since: August 14, 2010 Posts: 10 Comments: 17487
Quoting angiest:


I don't think it's much stronger than normal, and has spent much of the summer further NW than normal, I think.
The ridge is forecast to move more towards central Texas mid to late week. Depending on the speed of Isaac, and if the track stays WNW or wobbles W, a Texas impact is possible. Right now, there doesn't seem to be enough consensus as to where this thing is going. I am thinking somewhere just west of the Mississippi River Delta, but not as far west as say Lake Charles area. If Isaac doesn't quickly strengthen it could track further west... This one is just tricky as hell.
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Quite a change since this morning.


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Isaac is looking better and better. He is actually trying to build storms on his southside now, which is good for him and bad for everyone else.

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Quoting AtHomeInTX:


Lol. Guilty as charged. :)


Where are you located (roughly) in TX?

I am in Sugar Land but work at NASA-JSC - Will have to watch this carefully as even though we may likely to be on the West (weak) side of Issac (IF GFS pans out) NASA will probably begin to shutdown from Tues/Wednesday....!?

GFS keeps moving that little bit West over the past three run. Consistency is there..
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188. 145
Not to be rude, but This 'Isaac' storm is a bust. Here in ft. Lauderdale its just a thunder storm. Nothing more. A boring ole thuderstorm.
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Quoting PalmBeachWeather:
flcanes.What did I tell you about that?

sorry, it's just the coincidence with the "K" storm
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Quoting nofailsafe:


We don't have a death ridge this year. A smaller ridge by far.


OK Everyone. It's called the Missouri Dome this year.
Member Since: August 24, 2010 Posts: 0 Comments: 253
Quoting treehuggingsister:


Appreciate that, you all! Anything WE'VE learned that helps someone else, I'm glad to pass on! I'm reposting at intervals through-out the day and it's ON my WunderBlog, if you need to print it to check stuff off or add your own good ideas.


Just link your wunderblog with a small excerpt, not the whole thing. It's a really long post to have to scroll through on a busy blog when you're looking for other info. Good stuff btw :)
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Ah the gfs......she is a cruel mistress but an honest one.
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183. ITCZ
Quoting Tazmanian:




can i jest you guys not qute treehuggingsister post when he post that has its takes up hafe the blog page it is a vary long post


Yes so sorry Taz, I'm still learning how to work this quoting-posting-blogging thing. :-) To that end--is there anyway other than refreshing the site to move smoothly among the comments? I feel like I must be missing a button somewhere...
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Quoting ProgressivePulse:


Now through the overnight hours will be worst for us. Especially as Isaac builds in intensity and beefs up these rain bands. Heavy rain just started in Jupiter.


Max Mayfield is saying the opposite (on local news now). SE quadrant of storm is dry....The worst weather he is saying is happening now.
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Winds appear to have dropped some

Time: 16:08:00Z
Coordinates: 23.3167N 79.1167W
Acft. Static Air Press: 842.5 mb (~ 24.88 inHg)
Acft. Geopotential Hgt: 1,541 meters (~ 5,056 feet)
Extrap. Sfc. Press: 1005.0 mb (~ 29.68 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 166° at 40 knots (From the SSE at ~ 46.0 mph)
Air Temp: 17.9°C (~ 64.2°F)
Dew Pt: 16.2°C (~ 61.2°F)
Peak (10s) Flt. Lvl. Wind: 41 knots (~ 47.1 mph)
SFMR Peak (10s) Sfc. Wind: 42 knots (~ 48.3 mph)
SFMR Rain Rate: 1 mm/hr (~ 0.04 in/hr)
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Quoting SykKid:
isaac pressure 996? wow he really struggling im so happy that he cant get his act together. look like any chances of him ever becoming a major hurricane are dreasing by the hour! :)))))


It is clearly building an eyewall and strengthening. I am going to minus every post along these lines you put up.
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179. TXCWC
Quoting AtHomeInTX:


I think my model site crashed. Can anyone tell where this makes landfall first. Looked like SWLA to me. If this plays out theyll have to move the cone again.


YOUR NOT GOING TO LIKE IT :) Landfall on TX/LA border FOLLOWED BY CONTINUED WNW MOVEMENT RIGHT OVER OR NEAR HOUSTON LOOKS LIKE then goes north



Link
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Quoting TierraFirme:


Hi seflagamma, thanks for the info. I'm on the opposite side, Collier County, and although the radar looks really bad, we've had very little of anything here, west of US 41. Still holding my breath. Stay safe.

we've got on and off rain showers where i live, so far maybe only equivalent to the outer bands of fay i stood up through
ernesto was way worse, in the dark for maybe a day or two
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Quoting WPBHurricane05:
I think STORMTOP hacked the computer models.

Where is Patrap? Haven't seen him on in a while.


Probably trolling at Godlike Productions...

He signed up there... http://www.godlikeproductions.com/members/94761/pr ofile
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Quoting flcanes:

full DOOM ahead
flcanes.What did I tell you about that?
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About

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