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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1076. maeko
TWC said Dr. Masters would be on for the 'duration'. Wow! Is this the first time he's done this?
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Quoting robj144:
On the TWC radar, there seemed to be some lightning strikes within Isaac. Isn't that kind of strange?


no, tropical systems often have lighting, and intensifying ones often have a lot more.
Beryl had tons of lightning before landfall
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This storm is looking like a combination of Ike, Gustav, and Katrina. If it strengthens to at least a strong category 2, look for storm surge similar to the storm surge Ike produced.
Member Since: June 18, 2010 Posts: 0 Comments: 528
You know what...I like the NHC track. Models are just a guide. NHC uses the guide, but adds a human element.

MS landfall likely, in my opinion.
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1072. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 36855
1071. robj144
Quoting Charliesgirl:
Yes.


Did they? My impression was they just fixed them, but did not improve them.
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Nearing the circulation; 998.0mb.

183800 2350N 08211W 8426 01481 9980 +181 +174 338034 037 041 002 00
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Quoting MAweatherboy1:
UKMET- I think its been best and most consistent so far.



No way.......GFS the best so far
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Quoting hurricanehanna:
didn't know if this had been posted yet...

NEW ORLEANS - Offshore oil and gas operators in the Gulf of Mexico are evacuating platforms and rigs in the path of Tropical Storm Isaac. The Bureau of Safety and Environmental Enforcement (BSEE) Hurricane Response Team is activated and monitoring the operators' activities.

There's going to be an horrible mess on those North Gulf beaches when this thing finally blows over.
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Quoting Chiggy:
18Z BAM suite models moved further West - these models seems to be doing well of late and their tracks have been closely following the GFS runs...



thats because the Bamm suite is initialized off of the GFS, so they will almost always mirror it
Member Since: March 10, 2010 Posts: 1 Comments: 7365
Quoting JasonRE:
Just missed Dr. Masters on TWC. What did he have to say? Still keeping an eye on Isaac here in Lafayette LA. Anyone else here in LA?

Hi Jason - Lafayette here too. I miss Doc's appearance.
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So we're going to get hit a hurricane and "Deepwater" (pun intended) oil.
Member Since: August 16, 2007 Posts: 0 Comments: 397
Quoting schistkicker:
Did NOLA ever upgrade those levees after 2005? If those models putting the storm ashore right on the Miss. River delta pan out, they might get a pretty stern test...
Yes.
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Isaac's convective structure has improved considerably over the past 6 hours. As it continues to distance itself from the Cuban mainland, equatorward inflow should becoming more vigorously established and lead to some gradual intensification throughout the majority of today and into tomorrow.

Conditions here in southeast Florida have begun to clear; winds are rather calm with the occasional 10-15kt gust. Rain is no longer falling.

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1061. Chiggy
18Z BAM suite models moved further West - these models seems to be doing well of late and their tracks have been closely following the GFS runs...

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Quoting Hhunter:


good job Dr. Masters.. the storm is gonna do what? to the oil. someone tell me what that word he used was?

nice... but scary


mobilize, i believe
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Quoting SrChiefFan1:


Always support American over European...;). Btw...beach was super in SoWal today.


Im going with GFS as well.
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1057. robj144
On the TWC radar, there seemed to be some lightning strikes within Isaac. Isn't that kind of strange?
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1056. 900MB
Yeah! Dr. Masters on TWC, maybe this merger will work out okay after all!
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Great job Doc!
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ut oh....we lost Dr. Masters to TWC....

I guess TWC thinks...."If you can't beat em, invite em to join you in Atlanta...."


lmfao....
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Quoting weatherh98:


5 blocks in mandeville

I'm on Carroll in mandeville...you?
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1052. ITCZ
TWC now has credibility in my book. Go Dr Masters!!!
Member Since: September 5, 2008 Posts: 0 Comments: 47
1051. JasonRE
Just missed Dr. Masters on TWC. What did he have to say? Still keeping an eye on Isaac here in Lafayette LA. Anyone else here in LA?
Member Since: August 6, 2012 Posts: 0 Comments: 122
1050. Hhunter


good job Dr. Masters.. the storm is gonna do what? to the oil. someone tell me what that word he used was?

nice... but scary
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Key word for Isaac: STORM SURGE
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Recon headed back to the center.
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Great Job Dr. Masters! BP oil dredging up from botton. Ugh!
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1045. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 36855
Way to go Doc.
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Quoting IKE:
I would bet on the GFS over the ECMWF on track. Just my opinion.


Always support American over European...;). Btw...beach was super in SoWal today.

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drmasters was texting on camera. nice!!! i love masters
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Did NOLA ever upgrade those levees after 2005? If those models putting the storm ashore right on the Miss. River delta pan out, they might get a pretty stern test...
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UKMET- I think its been best and most consistent so far.

Member Since: February 11, 2012 Posts: 82 Comments: 7612
Not sure if anyone mentioned this, but Gonzo is in the air. Dropsonde at 28.8N 85.1W has 500mb height at 5900 meters. Winds out of the northeast. G-IV is now 200 miles south of Pensacola.
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Do you have a link for this?
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Quoting LSUCaneGirl:
Gov Jindal has a press conference scheduled for 3:15..things are headed to a frenzy in SE Louisiana..I'm on edge :/

Mayor of NOLA has one at 2:45. I wonder if they will start towards evacuating NOLA.
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ok thats it i cant even post the current maps right now... im off mentally exhausted
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1034. wxhatt
Dr. Jeff on the Weather Channel Now! Go Dr. Jeff!!!
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Quoting violet312s:
Just turned it to TWC. OMG this Heather person is rather dense.
can I plus this 1000 times???

Member Since: October 25, 2005 Posts: 19 Comments: 21473
ha Masters is on TWC now
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1031. airmet3
Quoting MississippiWx:
Well, the European models want Isaac to make landfall around Gulf Shores, while the North American models/hurricane models want Isaac to make landfall in SE Louisiana. You have to wonder what the difference is between the models since they should be receiving mostly the same data.


In the whole scheme of things, the distance between the two is not that great.

While models generally use the same basic equations and algorithms, the finer points are continually updated based on past results and current research findings. Plus, the authors of the two models are different (NWS vs. Bracknell)so there are general differences in opinion and theory in application of data.
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1030. LargoFl
Quoting LargoFl:
..wow is that 20-30 foot waves?
Member Since: August 6, 2011 Posts: 4 Comments: 36855
Quoting Levi32:


This is the one part of Isaac's forecast I did nail perfectly lol.

The reasons Isaac did not attain hurricane status in the central Caribbean are not dissimilar to why Ernesto didn't. Isaac came into the islands with a larger circulation than Ernesto, and was better able to survive the fast trade winds in the central-eastern Caribbean. However, as the upward motion pulse of the MJO moved away and the trade winds remained strong, large-scale sinking settled into the western Caribbean and offset any pressure falls Isaac was attempting to make.

This meant that the circulation could not get vertically-stacked because without pressure falls, there is nothing to glue them together. The other part of this was the monsoonal nature of Isaac's development, which limited convection to the southern side for a long time, and made the development of the storm very gradual, like all monsoonal cyclones.

For these reasons which I discussed extensively before-hand, I had Isaac as a 60mph tropical storm at Hispaniola 4 days out.


You did that. Thanks again.
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Dr Masters on TWC now
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1027. maeko
Watching Dr. Masters on TWC!!
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Quoting RTSplayer:


It's Mobile Bay/Pensacola at 949mb, which corresponds to about 125 to 130mph.


Important thing to note here, is the intensity is actually splitting the GFDL and HWRF.

It's probably going to be category 3 or 4 at landfall, and not category 2.


You got the most recent runs each for the "best" global model and 2 hurricane models calling for cat 3 or cat 4 landfall.

That's a strong indicator that all of the models find room for lots of intensification, regardless of the exact track it takes.
Those pesky I storms.I expect landfall in Alabama/Mississippi(no offense to anyone living there).
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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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