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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1576. sar2401
Quoting HurricaneDean07:

Mayor of New Orleans said Superdome and other large buildings are not going to be used as shelters... This system is very unpredictable, and if Mandatory Evacuations are ordered. HEED THEM, there will not be any shelters in the case Evacuations are ordered.

Hurricane Warnings being issued from Baton Rouge to Pensacola going into affect at the 5pm Advisory.


Say what? Where did you hear hurricane warnings are being issued for such a large area?
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Quoting HurricaneDean07:

Mayor of New Orleans said Superdome and other large buildings are not going to be used as shelters... This system is very unpredictable, and if Mandatory Evacuations are ordered. HEED THEM, there will not be any shelters in the case Evacuations are ordered.

Hurricane Warnings being issued from Baton Rouge to Pensacola going into affect at the 5pm Advisory.
Now the fun of the tracking became serious now.
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1574. gator23
Quoting uncwhurricane85:
if it wasnt for cuba and hispanola its safe to say that US homeowners insurance in the south and southeast coast would basically not be available! they sure have helped us out! the geography of the North American Cont. is so good but so bad


Homeowners insurance in Florida wasnt available. The state had to create a state funded one
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tomorrow is going to be a very busy monday from nawlins to talahaseeeee
Quoting HurricaneDean07:

Mayor of New Orleans said Superdome and other large buildings are not going to be used as shelters... This system is very unpredictable, and if Mandatory Evacuations are ordered. HEED THEM, there will not be any shelters in the case Evacuations are ordered.

Hurricane Warnings being issued from Baton Rouge to Pensacola going into affect at the 5pm Advisory.
where did you here that
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1572. 892mb
Apologies if this is a repost - I just left EOC meeting - Grand Isle has issued mandatory evacuation for non-residents effective immediately and mandatory evacuation for residents beginning at 9:00 am Monday. No EMS or fire on the island after 12:00 tomorrow.
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1571. yoboi
Quoting Bluestorm5:
Mayor Landrieu: In the event an evacuation is called, everybody needs to make preparations. Superdome will not be shelters of last resort



there will be no shelters of last resort includes convention center....i hope isaac does not hitt nola.....
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Quoting GTcooliebai:


Quite a cone.
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Oh my...

At present speed the storm can reach NOLA so fast...

I think just under 40 hours from the current center fix.
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Quoting leelee75k:
afternoon thunderstorms popping now? second clap of thunder, no rain at the moment but another band seems to be coming my way


I have cabin fever in Oakland Park, FL. It's so bad that I'm almost excited about going to work tomorrow.
Member Since: May 20, 2009 Posts: 0 Comments: 205
1566. HrDelta
Quoting MAweatherboy1:

Biggest was a 5.3


Good, at least no serious damage.

We need FEMA to be prepared for Isaac. Having a jacka-- earthquake would screw up preparations.
Member Since: October 4, 2007 Posts: 0 Comments: 451
1565. ITCZ
Quoting TallyWeather15:
According to TWC in Tallahassee we have thundershowers and wind from Monday through Thursday



Yes,that's what i am saying -it sure beats the previous forecast from yesterday when it was tropical storm conditions. That's all i am saying, everybody. I promise I'm not wishcasting, forecasting or typecasting. Lordy lord. :)
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Quoting MississippiWx:


A little worried about Isaac getting strong like K too.
I think Isaac could almost make landfall at the same strength(120mph).Now this is only speculation is going to change.
Member Since: August 14, 2010 Posts: 10 Comments: 17592
1563. robj144
Quoting KeyWestbeachcomber:


no it did not..unless you were filming a blackout


I see it here. No one else sees it?
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Quoting bigwes6844:
athome i hope this doesnt scare n.o. people but isaac is almost in the same postion as k 7 years ago. time frame of landfall same time as K. just smh!
That's insane. What day did K hit ? Good grief. I hope it goes to the East of ya's in N.O.
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1561. vince1
Quoting pottery:

Well, yes!

But we don't want to go there......

:):))

Maybe we should. EPA strongly advised against the use of Corexit, yet BP flipped them the bird. Hooray for fascism lite.
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Quoting bigwes6844:
athome i hope this doesnt scare n.o. people but isaac is almost in the same postion as k 7 years ago. time frame of landfall same time as K. just smh!


I know Wes. It's stirring up some memories. Hope it won't be bad. You take care. :(
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afternoon thunderstorms popping now? second clap of thunder, no rain at the moment but another band seems to be coming my way
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1558. sar2401
Quoting ITCZ:



Yes I realize the wording isn't perfect but -either way what I'm saying is WHEW.


I'd say WHEW along about Wednesday. :)
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Quoting Bluestorm5:
New Orleans under State of Emergency...

Mayor of New Orleans said Superdome and other large buildings are not going to be used as shelters... This system is very unpredictable, and if Mandatory Evacuations are ordered. HEED THEM, there will not be any shelters in the case Evacuations are ordered.

Hurricane Warnings being issued from Baton Rouge to Pensacola going into affect at the 5pm Advisory.
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Isaac really looks good, much more symmetrical than it ever has. Winds are really low though, probably 55mph, but since the NHC doesn't use that they'll probably keep 60... They'll come up though.

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Mayor Landrieu: In the event an evacuation is called, everybody needs to make preparations. Superdome will not be shelters of last resort
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Quoting Grothar:



Did I just have deja vu, or did you post this exact blog before verbatim? last year? It is word for word what you posted before. I just happen to have a memory that remembers every word I read. Please tell me.
Just sat here and typed it while listing to Isaac's winds. It was not copied and pasted. It would have been something I would have posted last year as well regarding this subject. Guess I'm consistent.
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Quoting Beachfoxx:
Hanna
I just filled hummingbird feeder... They are swarming.

Lots of boats coming into bayou & dropping anchor...

Wish the models were more in agreement!

I hear ya. I think by tomorrow we'll see better agreement..at least I hope so.
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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.
Not sure about the large scale sinking in the western Caribbean preventing the storm from being vertically stacked. We often see large areas of sinking air behind or even in front of an intensifying storm (ULL retrograding to the SW). The fact remains that a large amount of convection (upward motion) was present over Isaac while he was in the eastern and central Caribbean. The environmental problems I see are a broad circulation failing to consolidate due to fast trade winds. This kept storm motion fast, preventing significant consolidation, and additionally, the maintenance of strong trade winds ahead of the storm further prevented consolidation. As a result, we saw multiple vorticies swirl around each other at times and numerous center relocations. Dry air intrusion was also an issue and sinking air ahead of the storm isn't helpful, but the root of the issue (issue being the failure to become vertically stacked and consolidate under over LLC) seems to be the broad size and fast trade winds flow.
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1550. pottery
Quoting LightningCharmer:
Been finding tarballs on beaches for over 20 years. They are not all from sunken ships (some from WWII u-boats), dirty bildging, tanker spills, or platform spills, etc. Oil seeps from the sea floor naturally; that's why people starting drilling offshore in the first place. This not to dismiss, there may still be quite a bit of oil below the surface from the BP spill that has not broken-down, and is shallow enough for storm surge to dislodge.

Has Isaac officially made his first U.S. Landfall?




Well, that's a serious concern, then.
Hopefully, it will not come to that.

Isaac looks really ominous right now, and with the models going west, could spend more time getting stronger.

It's a HUGE system, that has the potential to continue to do weird things....
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Quoting Drakoen:
Looks like Issac may want to start taking that turn towards the NW.


I agree. I know many of the people here LOVE the GFS and HWRF models because they spell doom and gloom for New orleans and the rest of SE Louisiana.

But, if you look at the most recent radar and watch the eye, it does look as though, as stated by Drakoen, that Isaac may be making a move in the N/NW direction. This would mean, if Isaac is, in fact, making that move, that the European model has held that position true.

As a New Orleanian, I certainly hope the European model is correct. Plus, as stated by meteorologists here in New Orleans, with such uncertainty, watch the storms movement, not the models.

Also, has anyone else noticed that spinning low diving down through Tennessee?
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00Z


06Z


12Z


Member Since: September 2, 2011 Posts: 0 Comments: 1149
1547. yoboi
shelter in place for nola too late if they even wanted to evac...hope isaac does not get strong...
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Quoting ITCZ:



Yes I realize the wording isn't perfect but -either way what I'm saying is WHEW.
According to TWC in Tallahassee we have thundershowers and wind from Monday through Thursday
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Quoting uncwhurricane85:
if it wasnt for cuba and hispanola its safe to say that US homeowners insurance in the south and southeast coast would basically not be available! they sure have helped us out! the geography of the North American Cont. is so good but so bad
Good point.....but realistically, if it wasn't for Cuba and Hispaniola Florida would likely be as desolate as the Baja Peninsula. Place never would have had much of a population.
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Quoting robj144:


Thanks. Let me try this:



no it did not..unless you were filming a blackout
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1543. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 176 Comments: 55503
4.5 Earthquake: Westmorland, CA (12 minutes ago) 4.1 Earthquake: Brawley, CA (13 minutes ago) 4.9 Earthquake: Brawley, CA (19 minutes ago) 5.3 Earthquake: Brawley, CA (21 minutes ago) 5.3 Earthquake: Brawley, CA (21 minutes ago) 4.7 Earthquake: Brawley, CA (31 minutes ago) 4.6 Earthquake: Brawley, CA (32 minutes ago) 4.0 Earthquake: Brawley, CA (35 minutes ago) 4.0 Earthquake: Brawley, CA (36 minutes ago)
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Quoting washingtonian115:
Almost looks like "K" when she exited Florida..


A little worried about Isaac getting strong like K too.
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Quoting yoboi:


people in la are starting to flood the stores....atleast the word is getting out...


Yeah husband's Walmart is waiting on me to tell them what the NHC says. Lol. Sure NOW they want to talk to the weather nerd. :)
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Quoting Grothar:

wow!
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Shelter in place for NOLA SMH 
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Smaller towns seem to have their priorities in order. While returning to Polk County from Miami earlier today I drove through Clewiston. Upon entering the town I noticed the church parking lot was full. Further into town were two gas stations across US-27 from each other. Both had cars at the pumps but there was no line waiting. Next to one gas station was a grocery store and the lot was about a third full. Next to the other gas station was one of those drive through places where you can purchase cold beverages without getting out of the car. Here is where I found the line, mostly consisting of pickup trucks. I figured most Sunday morning breakfast conversations in Clewiston this morning concerned the husband convincing the wife that he needed to stay home to prepare for the storm while the wife took the kids to church. And as soon as the family car was out of sight he was off in his pickup truck to join the line at the cold beverage drive through. Also noticed as I was driving through the ran that the wind was blowing from the North as the sugar cane was bent over towards the south. A kittle later I saw the can bending to the North indicating wind fron the south. Finally it started blowing more to the west indicating a wind from the east. When I stopped for a late lunch in Highlands County the wind was definately coming from the east.
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Just heard thunder in West Broward! Is that wierd? First for the day!
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Hanna
I just filled hummingbird feeder... They are swarming.

Lots of boats coming into bayou & dropping anchor...

Wish the models were more in agreement!
Quoting hurricanehanna:

you know - funny you should say that. I haven't seen hummingbirds in a a while. But, had one at my home today as did my Mom.
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Quoting MAweatherboy1:

Biggest was a 5.3
Thanks
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1533. Walshy
Cobb Island, MD

Link
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Quoting nrtiwlnvragn:


UKMET Office interpolated back 2 cycles.
Thanks
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1443
Quoting gbreezegirl:
The ants are going crazy in my back yard now. Just sayin. Also have a bunch of hummingbirds at my feeder that I never had b4. Strange days.


Oh, Lordy - I remember way back when LaDobe Lady had ant issues - just before the storm we will not name. Have been wondering when/if someone would post what's going on in the animal/insect world.
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As of now, NOLA is saying prepare to shelter in place until more information on path is available.
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Quoting Bluestorm5:
How big?

Biggest was a 5.3
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New Orleans under State of Emergency...
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1527. robj144
Quoting GTcooliebai:
You have the video set to private.


Thanks. Let me try this:

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Quoting Civicane49:


gettting that outflow going in almost every quad...water is basically the warmerst anywhere in the world...this thing is about to take off! the spin has tightened and increased considerably also! Im kinda feeling a destin/ft.walton landfall since i see it getting much stronger a lot faster..if not more of a ms/al/la landfall
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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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