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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1376. bappit
Quoting HurricaneDean07:
Recon Latest:
992 MB
65 Mph?


Why did the NHC bring Isaac down to 60 Mph? It was at 65 Mph, No reason to bring it down.

Because the winds weren't high enough to keep it at 65 mph.
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Isaac is developing a good central dense overcast.

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1374. pottery
Quoting GBguy88:


http://www.gulfcoastnewstoday.com/area_news/artic le_dc063626-96ce-11e1-bfd1-0019bb2963f4.html

Appreciate that.
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Quoting Thing342:
HWRF is probably overdone. It has a Category 5 Isaac hitting New Orleans.

Yeah, I think it may be overdoing things a bit. Still though, the possibility of that is concerning.
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Going to make a New Tracking Map.
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Quoting GBguy88:
There was a large area of submerged oil found in Perdido Pass in May. I'd post the link, but that's against the rules, yes? Easily found on Google.


Why would submerged oil slicks be against the rules? If one of these is pulled up by a storm it would significantly increase the catastrophe of a storm.
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Quoting RTSplayer:


Takes a long time to initialize and run.

the output will be in the early morning hours, since it only runs once per 12 hours.


I guess it will be a late night again... Waiting.... Hoping.....
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1369. MZT
The convection ball near the keys is becoming more CDO-ish. This is going to be a classic storm by tomorrow AM if he keeps building up.
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1368. hydrus
High winds could reach well inland..
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Quoting LargoFl:
yes it sure looks that way


and the way some of the ensemble members are showing, it's almost an identical track inland too...

that would be a great news for the drought north of here but it would completely drown us down here.
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1366. Drakoen
The TVCN seems to have a good track given Issac's current motion.
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1365. ITCZ
City of New Orleans officials/Mayor Landrieu will hold 2:45p press conference to talk #Isaac via @WDSU
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Quoting TallyWeather15:
What is the farthest east you see schools closing along the gulf coast?

Santa Rosa County, Fl (East of pcola) closed tuesday and probably wed.
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I'm in Lafayette, Louisiana here.

Gotta start the 'cane prep (groan). Bad back, bad hip, bad arm. Gonna take my boat off the trailer and fill it halfway with water for ballast, then strap the trailer to one of the big oaks.

Haven't commented since Rita in '05, IIRC - just been lurking. Maybe this is a newer screen name I got during Gustav? I forget. Going to get gas this afternoon, because Lafayette isn't in high gear yet. Tomorrow will be a different story. My aunt in Thibodaux says the lines are already an hour long at Wal-Mart.
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Quoting HurricaneDean07:
Recon Latest:
992 MB
65 Mph?


Why did the NHC bring Isaac down to 60 Mph? It was at 65 Mph, No reason to bring it down.


And the mets on TWC keep saying the storm isn't intensifying?
Yet everyone in here says the opposite.
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Quoting Drakoen:
Issac is steadily getting better organized. Good central dense overcast, poleward outflow, and evidence that the equatorward outflow is trying to establish itself.

Do you believe this will be a Hurricane Tonight?
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1360. pottery
Quoting guygee:
You are very mis-informed, go take some organic chemistry classes and come back. True much of the crude will break down but the worst most carcinogenic compounds are very persistent in the environment. The cyclic hydrocarbons do not break down, those little molecular rings just clump together to form ever-more complex polycyclic compounds. Even when they are digested by bacteria or anything else, the metabolites are just other dangerous polycyclic hydrocarbons.

Just because we buried the problem in the deep ocean doesn't mean it went away...as usual we just put the load on future generations.

OK. Totally agree, but...

This thread has taken off on a Tangent.

The original point was the possibility (raised by Dr. M. ) of the storm raising the oil from the seabed, affecting the coastline.

Do you think that could happen ?
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1359. Grothar
Illustration only.


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Quoting muddertracker:
Hurricane watches extend well outside the "cone." Isaac's three days out...that's a wide spread.


Even if it landfalls in Mobile, Baton Rouge will get TS force winds. That's how big the wind field will be at landfall.
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Recon Latest:
992 MB
65 Mph?


Why did the NHC bring Isaac down to 60 Mph? It was at 65 Mph, No reason to bring it down.
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1355. ITCZ
Pressure has been falling since hunters got in the storm from 996 to 992 mb.
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1354. bappit
Quoting Ameister12:
Starting to look very dangerous.

Looks like a large tropical storm. Oh yeah, that's what it is!
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1353. Levi32
Quoting HrDelta:


The map does show winds between 65-70 on there. They will have to up the strength just a touch.


Those winds are at flight-level, 10-20% higher than at the surface. This plot here shows SFMR-estimated surface winds still 45-50kt for the moment.

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Quoting 1900hurricane:
Is it just me, or are some of these model runs starting to look a bit doomsdayish? Particularly the HWRF.



*DISCLAMER* This is just one model run and not the official forecast to those lurking.
HWRF is probably overdone. It has a Category 5 Isaac hitting New Orleans.
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1350. LargoFl
Quoting huntsvle:


That is damn near an Ike track......just sayin.
yes it sure looks that way
Member Since: August 6, 2011 Posts: 4 Comments: 42277
1349. KEEPEROFTHEGATE (Mod)
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Pressure now rising at Key West buoy... as of the 3pm observation. A dry slot may stop the rain in Key West for a few minutes. Windows are expected to still pick up as the storm departs to the west.

http://www.ndbc.noaa.gov/station_page.php?station =kywf1


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Isaac is beginning to look like he did just a few hours prior to crossing Haiti/Cuba.
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Quoting LargoFl:


That is damn near an Ike track......just sayin.
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1345. GBguy88
Quoting pottery:

Not against the rules in this case.
Dr. Masters has voiced concerns about it.

I would appreciate the link.


http://www.gulfcoastnewstoday.com/area_news/artic le_dc063626-96ce-11e1-bfd1-0019bb2963f4.html
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1344. WxLogic
Getting there, but it needs to get that inner core going to fend off the dry air in the Gulf:

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Quoting Drakoen:
Issac is steadily getting better organized. Good central dense overcast, poleward outflow, and evidence that the equatorward outflow is trying to establish itself.


what do you think about the track?
Member Since: March 10, 2010 Posts: 1 Comments: 7874
1342. CJ5
The drop in pressure is a good sign. It hasn't had that kind of drop in the past 72 hours. There is some convection surrounded all of the center so perhaps he will start to ramp up now.
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1341. HrDelta
Quoting Grothar:
Isaac already has that classic look, which is remarkable from just a few hours ago.



He's fixing himself up well. I would guess Hurricane by 11pm.
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Quoting bowldwelller:
Jefferson Parish Councilman Chris Roberts reported, "Surf already way up on Grand Isle beach, with it is tar balls.'

So Isaac has balls O'tar..Here in K.W. Isaac has no balls..Yet. So far he is just blowing everyone's mind.I call much ado about NUTHIN..thank you Isaac.
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1339. Drakoen
Issac is steadily getting better organized. Good central dense overcast, poleward outflow, and evidence that the equatorward outflow is trying to establish itself.
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1338. pottery
Quoting yoboi:


yep i fried some shrimp the other night ya don't even need any cooking oil just throw them in the pot and bamm.....fried shrimp

LOLOL
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1337. bappit
Quoting Grothar:
This will not take as long to intensify as they currently believe. As it begins to move away from Florida.....




How long do you think that they think it will take to intensify? And what intensity do you think that they think it will reach when it does intensify as you think that they think?
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18z TVC consensus model forecasting a direct hit on New Orleans. Preparations for a major hurricane along the northern Gulf coast should be underway.

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Quoting SELAliveforthetropic:
When does the Euro run again?


Takes a long time to initialize and run.

the output will be in the early morning hours, since it only runs once per 12 hours.
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1334. guygee
Quoting cajunkid:

Dude...Microbes have eaten all that oil.

Light crude does not last long outside a container, much less in salt water.
You are very mis-informed, go take some organic chemistry classes and come back. True much of the crude will break down but the worst most carcinogenic compounds are very persistent in the environment. The cyclic hydrocarbons do not break down, those little molecular rings just clump together to form ever-more complex polycyclic compounds. Even when they are digested by bacteria or anything else, the metabolites are just other dangerous polycyclic hydrocarbons.

Just because we buried the problem in the deep ocean doesn't mean it went away...as usual we just put the load on future generations.
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Jefferson Parish President John Young has declared a state of emergency for Jefferson Parish in anticipation of the effects of Tropical Storm Isaac. Meanwile, Gov. Bobby Jindal and New Orleans Mayor Mitch Landrieu have called press conferences this afternoon to update the status of emergency preparations. A mandatory evacuation from Grand Isle has been ordered, beginning at 9 a.m.
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1332. hydrus
Quoting Grothar:
This will not take as long to intensify as they currently believe. As it begins to move away from Florida.....



True..I also believe at some point it will rapidly intensify...An eyewall replacement cycle right before landfall would be the only chance of the the northern gulf coast not getting a major strike.
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1331. pottery
Quoting GBguy88:
There was a large area of submerged oil found in Perdido Pass in May. I'd post the link, but that's against the rules, yes? Easily found on Google.

Not against the rules in this case.
Dr. Masters has voiced concerns about it.

I would appreciate the link.
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1330. dmh1026
Quoting HurricaneHunterJoe:


Hows it doing? I got a friend who lives west of 41 and south of corkscrew.
Wind picked up, and so did the rain...eased off now...it seems to be coming in waves...no lightning or thunder.
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Quoting E46Pilot:


That Heather girl has got to go though, she is just getting under my skin for some reason.
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Hurricane watches extend well outside the "cone." Isaac's three days out...that's a wide spread.
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Quoting gustaveye:


Norcross on twc stated that all of the EURO runs were not complete, but from what he was seeing they were still vastly different than the GFS


You'd think they almost have to go with the EURO because it is closer in time. But Rick Nabb said a lot of people are going to have to prepare. So who knows?
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TS Isaac about to become a big threat. Storm is growing.

Link
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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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