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 newportrinative:


LOL! I do have to turn the volume up signifantly to hear him and then down again after he's done!

Could be the microphone they gave him.
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Quoting RTSplayer:
Incredible...

Even if you add 20mb to correct for the over aggressiveness in the first 12 to 18 hours, it's still a category 3...




thats likey a strong cat 4 or low cat 5 with a 934mb
Member Since: May 21, 2006 Posts: 5091 Comments: 114719
Quoting angelafritz:


SST modeling doesn't exist in the public domain, that I'm aware of. What the SST's are yesterday are what they are today. It changes very little. The currents change very little as well.

The SST maps on Wundermap:
http://wxug.us/r806

The SST maps on Tropical:
http://www.wunderground.com/tropical/

Both have Anomaly to select if you wish.

There's also some products from AOML but their site is running very slow today.
http://www.aoml.noaa.gov/phod/cyclone/data/go.htm l


Is there a particular dataset they all run off of? I've been meaning to make a delta_T map to show set-to-set variation.
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UKMET 12Z



Latest UKMET run has come further west to Mobile, AL.
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Guess ussems in Fla can expect Isaac rainbands for a couple of days. And more if the storm decides to landfall at or near the Panhandle.

Melbourne Tropical Storm Watch
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420. ITCZ
nwskeywest: Lower Keys be ready! Winds already gusting to 50 mph ahead of the squall approaching Key West
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http://www.trafficland.com/city/EYW/camera/12464/
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Incredible...

Even if you add 20mb to correct for the over aggressiveness in the first 12 to 18 hours, it's still a category 3...

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Quoting nola70119:
Cone has already moved left after the 5am update and New Orleans is now under a hurricane watch......as expected. But obviously everyone on the Gulf Coast west of the storm's current position should be alert....

don't forget about east, just goes to show about debby
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Did the track models shift more West? They were around Mobile Bay or Pascagoula, MS when I left the hotel a couple hours ago and now everyone is talking about the other side of NOLA. What gives?
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Quoting ITCZ:



AWESOME. Thank you so much for taking the time to do this! I started a "detailed" action plan last night, but you have some wonderful ideas (necessity=mother of invention) I wouldn't have dreamed of in a million years. Kudos to you and glad you survived.


A cast iron skillet works great on the grill. It takes a little getting use to though. You might want to try it a couple of times before the power goes off so you don't waste your time and food. Once it gets hot you can remove it from the grill and it will stay hot for awhile without getting hotter which is why you removed it from the grill. Easy way to make eggs without melting the handle of a regular skillet.
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Quoting Chiggy:


IT initializes at 987mb which is about 10mb lower than Issac is currently - HWRF intensity in most cases is always overdone - take about 15mbs off - my rough guess!
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Quoting samiam1234:
If anyone outside the cone (ahem Texas) keeps calling this to be a lousiana/Texas border event they are just wishcasring and taking the Ttention of places like Florida which could still get hit by a hurricane. That's very irresponsible. I believe NHC has this pretty well pegged if Nythinf it may move slightly to the right
Or they are looking at the models. I think the NHC is closer to the true track, but there is HUGE uncertainty in their discussions. Tampa should be happy about the uncertainty of the forecasted path a couple of days ago.
Member Since: August 16, 2007 Posts: 0 Comments: 397
412. HCW
Quoting Chicklit:

rut ro...the dreaded Cantori sighting


Why is it dreaded ? If I lived in Pensacola I would be happy that the Storm Shield was in my city cause you know that you are safe from Isaac when 4'9 Jim C is near
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Quoting washingtonian115:
Now that's just ridiculous.


IT initializes at 987mb which is about 10mb higher that Issac is currently - HWRF intensity in most cases is overdone - take about 15mbs off - my rough guess!
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410. angelafritz (Admin)
Quoting FSUGradWxMan:
Does anyone know where I can find current SSTs, SST Anomalies, and ocean current analysis for the Gulf? I will settle for just images, but I really would like to see some models. Everywhere I have checked has old data. Thanks!


SST modeling doesn't exist in the public domain, that I'm aware of. What the SST's are yesterday are what they are today. It changes very little. The currents change very little as well.

The SST maps on Wundermap:
http://wxug.us/r806

The SST maps on Tropical:
http://www.wunderground.com/tropical/

Both have Anomaly to select if you wish.

There's also some products from AOML but their site is running very slow today.
http://www.aoml.noaa.gov/phod/cyclone/data/go.htm l
Quoting weatherman12345:
Just because we are giving an opinion about the current conditions at our location??? We never even talked about conditions for NOLA down the road and the way the track is going it may go even farther west. Alot of unknowns. Dont be so quick to put people on ignore just because of a simple opinion.


Huh??? rofl.. I don't have you on ignore! It takes a lot of stupid statements for me to put you on ignore
but there have been about 18 bloggers in the past 2 days that earned that designation.
I respect different opinions.. just not the stupid posts or trollish posts...
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Quoting RTSplayer:


I know that's just a forecast, but that sucks. That would allow mad strengthening.

it would really hurt someone
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Cone has already moved left after the 5am update and New Orleans is now under a hurricane watch......as expected. But obviously everyone on the Gulf Coast west of the storm's current position should be alert....
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Quoting Grothar:


Gro, that is one ugly track! It puts me just east of Issac. I don't like it, can you make a new one for me?
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Quoting Chicklit:

rut ro...the dreaded Cantori sighting
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Quoting TomballTXPride:

Thank you. I'll reread his blog and check that out. I guess it's not too much of a surprise right now since the 12Z SHIPS had been calling for moderate shear in the first 24 hours of the forecast afterall.

TIME (HR) 0 6 12 18 24 36 48 60 72

SHEAR (KT) 12 13 12 8 5 8 1 8 12

Link


I know that's just a forecast, but that sucks. That would allow mad strengthening.
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Quoting HCW:
Cantori scheduled to arrive at Pensacola Airport at 4:30. Fox News, CNN, ABC, etc all arriving today as well.

rut ro...the dreaded Cantori sighting
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400. ITCZ
Link



Lee County Mandatory Evacs
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Quoting WDEmobmet:


what model is this

12Z GFS
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Quoting gbreezegirl:
Great just what we needed! I was afraid Cantore might show up. I don't dislike the man but its never a good sign when he shows up.
From now on refer to him as "The Cantore", if you please!
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Quoting RTSplayer:
New HWRF is being slightly over-aggressive on early intensification, but I think a compromise in intensity forecast would be good.

It's in the E. central gulf and very powerful on the latest frame I've seen.

959mb in 24 hours.

It's going full RI, which I think it already missed it's first data point, so it should be considered to be slightly high on it's forecast FOR NOW...

Now that's just ridiculous.
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Quoting flcanes:

oh, even ts can be dangerous( for those who are disbelievers)
yea they were blown off the turnpike.
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Quoting flcanes:

goes to show how big his circulation is

.. If that is the case then does NHC have this wrong - it says TS winds out to 205 miles from center!!
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394. CJ5
Does anyone have the last recon eye characteristic report?
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Quoting SFLWeatherman:



And I really wish the "cone" included the surface wind field. People just seem to look at the center track and figure they're out of it. Yet, within the wind field, there's plenty of rain and gusty winds that cause a heck of a lot of property damage and make for dangerous driving conditions - all of which a lot of people, including tv stations - overlook.
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Rain in Naples so far very minimal; we get the sun peeking through the clouds from time to time.
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Quoting divdog:
If I has a dollar for bad shear forecasts over time I would be rich. Almost as hard to project as intensity it would seem

very much true
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Quoting MZT:
I have seen hurricanes before with large dry air quadrants(Gordon in 2000 comes to mind) so I do think it could be reasonable that Isaac is on the verge of cane status already.

(The funny thing is that the weather on the "dry" side of such a hurricane will seem relatively pleasant, if blustery.)

france, 1986
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Quoting TomballTXPride:

Thank you. I'll reread his blog and check that out. I guess it's not too much of a surprise right now since the 12Z SHIPS had been calling for moderate shear in the first 24 hours of the forecast afterall.

TIME (HR) 0 6 12 18 24 36 48 60 72

SHEAR (KT) 12 13 12 8 5 8 1 8 12

Link
If I has a dollar for bad shear forecasts over time I would be rich. Almost as hard to project as intensity it would seem
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I think sometimes we forgot that a lot has to line up
atmospherically - for a intense storm to form. That's why (minus the 2005 season) Cat 5's are rare. And sometimes we go a couple of years without Cat 3's or 4's. Think of all of the waves that roll of the coast of Africa during the season - and only a few spin-up to become tropical entinties.
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Quoting Stormchaser121:

102 hrs


what model is this
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Quoting guygee:
Most definitely! I am just pointing out he is not "perfect" at this time.

that's true
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382. MZT
I have seen hurricanes before with large dry air quadrants(Gordon in 2000 comes to mind) so I do think it could be reasonable that Isaac is on the verge of cane status already.

(The funny thing is that the weather on the "dry" side of such a hurricane will seem relatively pleasant, if blustery.)
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Quoting Chiggy:
15Z steering from CIMSS for 970mb - 989mb - THis is what Issac "will" follow it it was a CAT 1/2 entity. Of course this gets updated every 3 hours

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While I can't see this going too far west, it certainly would be a good drought relief for the Mississippi valley if it did end up on the western end of the cone.
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Quoting HCW:
Cantori scheduled to arrive at Pensacola Airport at 4:30. Fox News, CNN, ABC, etc all arriving today as well.
Great just what we needed! I was afraid Cantore might show up. I don't dislike the man but its never a good sign when he shows up.
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Quoting hulazigzag:
Just to give a size and strength perspective. I live in lake county FL approximately 400 miles from the center of Isaac. We have had two squalls move through this morning with heavy rain and I've measured gusts of 42 and 46 mph. Pretty impressive for a TC.

goes to show how big his circulation is
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Quoting flcanes:

yeah, but he still could strengthen in the gulf
Most definitely! I am just pointing out he is not "perfect" at this time.
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Recon headed in for another pass at the center of Isaac.
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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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