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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Quoting Levi32:
Interesting that the group of CMC members, of all things, don't strengthen Isaac beyond Cat 2 in general.

god i hate the cmc model lol almost as much as the nogaps. first it was going west of bermuda than east coast of florida and now mississippi. so what are your thoughts on isaac right now levi?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting washingtonian115:
If the clouds have cleared up look for a beautiful sunset.

"red shy at night, sailor's delight"...mmmm, don't think we will be in for a beautiful sunset and I would not expect all the clouds to clear out. Winds will still be very gusty tomorrow over the marine areas. Depending on the track and how well Isaac builds/rebuilds, we still have the "back side" or the southeast quadrant to deal with...
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I am here in Baton Rouge, watching not looking forward to no power for 5 days.
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Quoting Bootsie1:
How come no one ever mentions MS when they talk about this storm or any storm. MS took the brunt of Katrina but seems everyone thanks about LA.


Ive been saying a Mississippi landfall for a while now. I don't know why everyone focuses on NO so much . I guess cuz of the levees. Mississippi got worst of Katrina directly, I agree.
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So my girlfriend is in West Palm Beach for the week on vacation, and her hotel is a block from the water. She just sent me this photo of the current conditions from Isaac:



As you can see, heavy rain is preventing you from really being able to see the ocean, and the winds are blowing the trees around. She estimates the winds at about 30 MPH with 50 MPH gusts.
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Proud of TWC's coverage, they sure are pulling in all the resources they can get.
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Evacuation orders will likely be needed for portions of the northern gulf coast
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1169. ITCZ
Louisiana preps: Mandatory evacuation for Grand Isle called immediately for campers and tourists, residents tomorrow at 9 a.m.
Member Since: September 5, 2008 Posts: 0 Comments: 47
Quoting MAweatherboy1:
Rick Knabb just said warnings will be going up for the Gulf coast soon.


I hope that means no school。
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1167. Levi32
Quoting MississippiWx:


The CMC operational has only shown around a 985mb system.


A few days ago some of the CMC members were showing a sub-920mb Isaac near Florida.
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Quoting MarkTodd2233:


How is the traffic there, I am wondering if I need to go get my 5th wheel at the Rigolets,, I am in Zachary, La


I-12 is clear, no traffic but that could change quick if people start to evacuate. took 16 hours to get to Florida during "K".
Member Since: July 6, 2010 Posts: 0 Comments: 50
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Quoting WhoDat42:


Forest Brooke


Cool! Ur not far from me!
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Mandatory Evacuations for all residents of Grand Isle, La. Starting tomorrow morning at 9AM. Per WWL. So it begins.
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Quoting Hurricanes101:


Please enlighten us with your thinking or are you capable of that?



very capable. so much arguing over which model is doing what, it's coming here, no it's coming here.
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Quoting Levi32:
Interesting that the group of CMC members, of all things, don't strengthen Isaac beyond Cat 2 in general.



The CMC operational has only shown around a 985mb system.
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Id say one of the strongest lines I've seen all day moving through Boca Raton right now.
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Quoting LSUCaneGirl:


Yep! Awaiting the next advisory..
Quoting yoboi:


yep are ya ready????
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Rick Knabb just said warnings will be going up for the Gulf coast soon.
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very dangerous Hurricane expected to affect the northern gulf coast , i am very concerned for people in New orleand and point east to pensacola,fl very likely a cat 3
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1154. LargoFl
just had a sprinkle of rain and the breezes have died down for the moment..looks like the very outer rains are starting to reach up here to central florida..here's orlando..ST2K you online?........
Member Since: August 6, 2011 Posts: 4 Comments: 36653
Quoting louisianaboy444:


Euro was the same and shifted east lol...Why the dislike for the GFS all of a sudden?


not saying I dislike it, but everyone is praising the GFS and pretending it has been right all along when in fact for most of the duration of Isaac, it has been wrong; the Euro gets the victory on this one because it was the first to note the westward trend and was the first one to suggest this could be a GOM storm. The Euro was also the furthest west of the major models until yesterday
Member Since: March 10, 2010 Posts: 1 Comments: 7337
Quoting highstepper:



Don't think the Lake is up 2 ft. Yet.

Link

I'm on it right now it's up a bit
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Quoting Neapolitan:
BP says there is no oil leftover from the spill; it's all simply vanished with the wave of their magical wand. And when has BP ever lied to us?

Oh, wait... :-\
Then they have the nerve to show commercials saying how tourism along the Gulf states are better than ever and everything is back to normal.We all no it's not,and won't be for years and years to come :(.
Member Since: August 14, 2010 Posts: 10 Comments: 16327
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It's fairly interesting to watch the moisture envelope increase as Isaac's center pulls away from Cuba. Moving away from the mountainous terrain is limiting the downsloping-induced dry air and thus the cloud coverage has increased.

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Quoting MiamiHurricanes09:
Without a doubt we have an intensifying cyclone with an improving core. New pass down to 992.8mb; waiting for the dropsonde. Isaac on the way of becoming a hurricane.

184630 2401N 08143W 8430 01440 9928 +203 +180 282002 007 024 002 00


Dropsonde will likely be 993mb. 2mb in 30 mins or so ain't shabby.
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1146. Levi32
Interesting that the group of CMC members, of all things, don't strengthen Isaac beyond Cat 2 in general.

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1145. aquak9
Quoting Masquer08er:
Any comment on Isaac churning up the oil from BP? Dr Masters mentioned this.


saymoBEEL???
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Quoting weatherh98:


Woodlands?
Quailcreek?


Forest Brooke
Member Since: July 6, 2010 Posts: 0 Comments: 50
How come no one ever mentions MS when they talk about this storm or any storm. MS took the brunt of Katrina but seems everyone thanks about LA.
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Quoting Hurricanes101:


GFS had this over Florida for well over a week, only in the last 24 hours has it shifted west



Euro was the same and shifted east lol...Why the dislike for the GFS all of a sudden?
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Quoting WhoDat42:



I'm just south of I-12 off 1088.


How is the traffic there, I am wondering if I need to go get my 5th wheel at the Rigolets,, I am in Zachary, La
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Quoting Hhunter:
He stated a big stomr like Isaac can send currents all the way to the ocean floor, this the ploughing affect of the ocean floor and possibly pulling up some oil...


Get the buckets, time to load up the truck for the refinery.
Member Since: June 18, 2010 Posts: 3 Comments: 941
Why doesn't everyone think Isaac will be a Major Hurricane? That's a bit bizzare after looking at the conditions it will have the next few days. My prediction is a Category 3-4 at peak.

Have we not learned after storms like Katrina, Rita, Tomas, Earl, ect?

Storms will take advantage in RIPE conditions.. What makes Isaac different?
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Quoting franklincounty:
never seen so much wishcasting in my life


Please enlighten us with your thinking or are you capable of that?
Member Since: March 10, 2010 Posts: 1 Comments: 7337
Without a doubt we have an intensifying cyclone with an improving core. New pass down to 992.8mb; waiting for the dropsonde. Isaac on the way to becoming a hurricane.

184630 2401N 08143W 8430 01440 9928 203 180 282002 007 024 002 00
Member Since: September 2, 2009 Posts: 130 Comments: 21091
Quoting WhoDat42:



I'm just south of I-12 off 1088.


Woodlands?
Quailcreek?
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Great post by the science/weather guy at the Houston Chronicle on Isaac. Good stuff for people in Texas, but also includes commentary from former NHC director Bill Read that everyone on here should see.

http://blog.chron.com/sciguy/2012/08/dont-look-no w-but-forecast-models-continue-to-trend-west-with- isaac/
Member Since: June 18, 2010 Posts: 0 Comments: 528
Quoting JasonRE:


Thank you. Appreciate the update.
u welcome but he also did mention it could be a big storm surge to the gulf coast
Member Since: July 25, 2010 Posts: 0 Comments: 2510
1133. Hhunter
Quoting Masquer08er:
Any comment on Isaac churning up the oil from BP? Dr Masters mentioned this.
He stated a big storm like Isaac can send currents all the way to the ocean floor, this the ploughing affect of the ocean floor and possibly pulling up some oil...
Member Since: August 19, 2007 Posts: 0 Comments: 2973
Quoting SykKid:
im notice some dry air in the gulf.

maybe that why he isnt forecasted to become stronger then cat2.
You should contact the NHC immediately so they can adjust their forecast!
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never seen so much wishcasting in my life
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Quoting Masquer08er:
Any comment on Isaac churning up the oil from BP? Dr Masters mentioned this.
BP says there is no oil leftover from the spill; it's all simply vanished with the wave of their magical wand. And when has BP ever lied to us?

Oh, wait... :-\
Member Since: November 8, 2009 Posts: 4 Comments: 13442
Quoting ITCZ:


(sheepish grin,toe in dirt) well, you know, it's DR. MASTERS, for petie's sake.

Stay tuned! ~:)

he's a rock star :-)
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1127. yoboi
Quoting presslord:
Stormjunkie enroute to the Gulf coast for Portlight can be seen here Link it's pretty boring right now...but he's kinda fragile...and his ego would eat up the attention if y'all wanna check it out....


i sent him an e-mail earlier.....looks like he needs to head to la...
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Quoting WhoDat42:



I'm just south of I-12 off 1088.
Im surprised the Weather Channel has Tropical Updates usually they have re runs of Ice Pilots
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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.