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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Viewing: 1976 - 1926

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1976. yoboi
Quoting weatherganny:

Will this cause the models to move east or west??? I am confused...LOL


look at the latest gfs....i think it picked all this up...reed levi drake hope ya'll study the layers now might help ya'll down the road...
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Quoting AtHomeInTX:
TWC just said entire state of Louisiana state of emergency.


I think they meant "anarchy"
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12Z Nogaps just west of New Orleans at 62 hours.
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25Aug.06pmGMT: 20.8n75.3w (308.1*NWest@17.9knots) 50knots, 997millibars, TS
26Aug.12amGMT: 21.7n76.7w (304.7*NWest@15.9knots) 50knots, 997millibars, TS
26Aug.06amGMT: 22.8n78.2w (308.5*NWest@17.7knots) 50knots, 995millibars, TS
26Aug.12pmGMT's numbers are below, before 26Aug.6pmGMT's
Derived from NHC_ATCF data for TropicalStormIsaac for 26August6pmGMT
MinimumPressure decreased from 995millibars to 994millibars
MaxSusWinds decreased from 55knots(62mph)102km/h to 50knots(58mph)93km/h
Vector changed from 293.2*WNWest@20.7mph(33.3km/h) to 286.5*WNWest@16.5mph(26.5km/h)

7FA1-SugarloafKey :: MTH-Marathon :: CCC-CayoCoco :: PST-Preston

The easternmost dot on the westernmost line is TS.Isaac's most recent position

The westernmost line is a straightline projection through TS.Isaac's 2 most recent positions to its closest approach to an inhabited coastline
25Aug.6pmGMT: TS.Isaac had been headed for passage over Marathon,Florida (left,MTHblob)
26Aug.12amGMT: TS.Isaac had been headed for passage over SugarloafKey
26Aug.6amGMT: TS.Isaac had been headed for passage over GrassyKey (right,MTHblob)
26Aug.12pmGMT: TS.Isaac had been headed for a 1:25pmGMT passage 2.9miles(4.7kilometres) SSWest of CaySal,Bahamas (unlabeled dot above the 1st line right of the straightline projection)... followed by a 9:32GMT passage 4miles(6.5kilometres)SSWest of FortJefferson,Florida
(top, blob above the straightline projection)
26Aug.1:45pmGMT: TS.Isaac passed 6.1miles(9.8kilometres)SSWest of CaySal
26Aug.6pmGMT: TS.Isaac was heading for an 11:52mGMT passage 24.8miles(39.8kilometres)SSWest of FortJefferson,Florida in ~3hours from now (when this was posted)

Copy&paste cun, 24.626n82.873w-24.571n82.897w, fmy, 24.621n81.517w, 7fa1, 24.722n81.015w-24.747n80.969w, mth, 23.703n80.41w, ccc, pst, 20.8n75.3w-21.7n76.7w, 21.7n76.7w-22.8n78.2w, 22.8n78.2w-23.5n80.0w, 23.5n80.0w-23.9n81.5w, 23.5n80.0w-24.279n82.977w, 24.626n82.873w-24.279n82.977w, 23.5n80.0w-23.618n80.437w, 23.703n80.41w-23.618n80.437 into the GreatCircleMapper for a larger map and other information
The previous mapping for comparison
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3.6" of rain today.. wind gusts upwards of 40mph with that last band.. just let up...
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Quoting Civicane49:
Isaac:




Barometer is falling rapidly here -3.3 1007.7mb
Member Since: December 18, 2006 Posts: 7 Comments: 2686
Quoting AtHomeInTX:


Starts about 430 central for 18z. 1030 for 12z. 1030 p.m. for 0z, 430 a.m. for 06z

Thank you so much!!!!Really!!!!
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When the levees where re-built, after Katrina, what were they built to withstand? Also, did they install a better pumping system, to withstand a Katrina strength storm? Anyone know? I hope they planned for a worse case situation. . . and hope they don't have to hope it works.
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So things are looking better for Panama City Beach????
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1965. yoboi
Quoting Joanie38:


Where do I go once there????


the blog is moving so fast press conference...
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Samian1234 the reason why people in SETX and Houston are starting to panic is because the models are trending west just like what happened with Ike. Also, Rita I believe was supposed to go into northern Mexico or corpus Christie and it came into SETX. We've seen this a few times were a storm is supposed to go to one area and ends up somewhere else. So that is the reason why SETX people are starting to get concerned.
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Quoting sunlinepr:

dry area filling in...
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Quoting Charmeck:
Quoting violet312s:
My goodness I would not want to be in Brawley, CA. The little quakes keep coming. That would freak me out.
There have also been several small quakes all week in the Puerto Rico and Virgin Island area - but I guess they are so use to it that they don't think much about them.



Yeah, but this was a swarm of small and moderate ones over the course of just an hour. There were dozens it looks like, and numerous were +4 and a +5 and loads of +3's. I grew up by Palm Springs and lived in San Diego just as long, and not seen a swarm quite like that out there. I'd be way creeped out. Some of my friends in San Diego are feeling them as per their FB posts. I just hope it's relieving pressure and not a precursor.

Anyway, I agree with Wash, Gordon's an excellent example of strengthening without high heat content! So Isaac is in a much sweeter place without deep content
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Convection had been warming a little, but he's getting another good burst going.



Recon just got 58mph winds, so 60mph should stay the intensity, though 55 may be a little more accurate.
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This has been pretty much a non-event in Orlando, so far. Occasional light rain and breezy. Seems with Isaac's WNW movement we probably won't see much more than that? Thoughts?
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992.3 mb is the pressure now. I was off by .1 mb.
Member Since: August 1, 2011 Posts: 28 Comments: 8009
Quoting yoboi:


just look at the texas high going west amazing....

Will this cause the models to move east or west??? I am confused...LOL
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Quoting Catfish57:
What an assinine comment.

Blog, I see I offended some because of my mistyped word in my comment. I was typing and talking with my 12 year old son, who has shown great interest in tracking this storm. So my apologies go out to any that I offended.
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1956. Levi32
Quoting TomTaylor:
No kidding, that's why I specifically said consolidated and closed circulation. Ernesto was flirting with an open circulation the entire time, even declared an open wave by some, including Bastardi. And of course, the reason it remained open was the fast trade wind flow. Failure to consolidate and get a closed circulation from accelerating trade winds prevented the storm from deepening inspire of convection.

All I'm saying is I don't understand how a downward MJO can prevent deepening and vertical alignment when convection was clearly present over Isaac the whole time. Convection is rising air.


It's all connected Tom, that's my point. The accelerating trade winds ahead of the storm cause the sinking air ahead of the storm, which offsets the pressure rises from the convection in the storm. The misaligned center and degeneration into an open wave can follow as a result of all of that. The downward MJO only enhanced the situation with Isaac.

I'm not saying there was sinking air directly above the circulation. A TC in the middle of a strong downward MJO pulse is like an overshooting thunderstorm in the stratosphere. Air is rising there, but sinking elsewhere, and if that sinking outside the storm is strong enough, such as it was west of Ernesto due to accelerating trade winds, it can offset the rising air in the storm and result in no net change in air pressure, or even a rise. That's how you can get powerful convection but the net air mass flux out of the storm column is zero or negative.
Member Since: November 24, 2005 Posts: 635 Comments: 26647
Quoting nrtiwlnvragn:


Gonzo is out on a mission and went through the central gulf






You can access the dropsonde sounding on tropical atlantic

Excellent! Thanks for that.
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Quoting weatherganny:

At what times do the GFS model runs come out?


Starts about 430 central for 18z. 1030 for 12z. 1030 p.m. for 0z, 430 a.m. for 06z
Member Since: August 24, 2010 Posts: 0 Comments: 160
1953. Michfan
2nd Recon just took off from MacDill AFB in Tampa. Looks like NHC wants to get as much data as possible on this.
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Isaac:

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Quoting FSUstormnut:
getting hammered in west boynton area.

Stay safe and keep us informed...please?
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50 percent of oil wells being shut down as of now. Sounds like Warnings for NOLA at 5PM.
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Quoting Bobsled27:


Actually, you are judging me. I judged noone. It was an unprecedented and horrible situation compounded by an incompetent mayor who left hundreds of transit buses to drown when they could have evacuated people. I am a native of New Orleans who had to deal with Katrina, and I truly resent your comment. Can we get back to weather please?

Yes let's get back to the weather, I am sorry If I offended you..I may have missed it,you never said Who you were calling MORONS at the dome. That is why I reacted...
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1948. yoboi
Quoting GTcooliebai:


just look at the texas high going west amazing....
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1947. angelafritz (Admin)
Reynolds Wolf is making his debut on TWC! I worked with him at CNN -- really great weather reporter. So great in the field. Fearless.
1946. airmet3
Quoting blsealevel:




Isaac appears to be on the very right edge of the guidance envelope based on the recon data.
Member Since: August 4, 2012 Posts: 0 Comments: 170
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More west again??
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1943. dader
Quoting StormHype:


Dry air has really kept him suppressed today. The worst squalls are over for the Keys already. Not much at all on the right side (dirty) side of the storm at this time. The sun may even peak out there before the day is done.




Just another beautiful day in key largo
Member Since: September 6, 2008 Posts: 0 Comments: 165
Lol! The camera on TWC just came up behind a staff person that had facebook pulled up on her phone... Doesnt make you feel all that safe... Weather okay here for now, just gray.
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Which would be more likely; GFS ensembles or GFS operational? The reason I ask is because Gfs Operational is to the far left of the ensembles while the AEMN (GFS ensemble mean is just to the left of N.O. and tracking NNW.
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Quoting redwagon:

Been at work.. are these models suggesting Isaac won't recurve at landfall? But hang a left?


The GFS is left. The EURO is right/north. this is a mess
Member Since: August 24, 2010 Posts: 0 Comments: 160
Quoting AtHomeInTX:


Maybe so.

At what times do the GFS model runs come out?
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getting hammered in west boynton area.
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1937. shfr173
Probably a stupid question, can someone explain the reasoning behind a storm not wanting to go poleward" Is it a high or a retreating trough? just wondering. Thanks
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Quoting StormHype:


Yeah, I paid $7 each for 12 of them in 2005. Saw them locally in FL for $18 last week *before* the storm was in the news. All this money printing has caught up with us *bad*.

Plastic is an oil byproduct... oil prices have risen dramatically, as have gas prices, and refinery capabilities have been cut. While the effect of printing money has been noticeable it is more the fault of the oil and refinery issues than monetary policy woes.
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1935. Michfan
Link

Convection firing up over the center.
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Isaac this morning:



Isaac now:

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1933. dmh1026
75 degrees in Estero, FL feels great right now. Sitting on lanai following WU and the rain and wind from Isaac...
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You can see the that the core is wetter than before from the rotation of the storm on radar. See the image from 20 minutes ago on Key West radar:


Center due south of Key West national wildlife refuge, and the storm is moving just N of due west. Should fill in rather quickly... now how are the CLOUD TOPS doing?

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unfortunatly he is starting to really seperate from Cuba/Florida, as Cuba cuts to the SW, Florida cuts to the north.
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1929. llpj04
Quoting JGreco:
I feel sorry for New Orleans and S.Mississippi that they're gonna get hit again by a major. We in the western florida panhandle look to dodge another one....hopefully the forcefield over my region can be extended to NO....You really don't need another one there...

Thanks. I wished your forcefield would push it to south Texas. We just put up the window panels, and sit about 45 miles (by road) northwest of New Orleans. Staying for a cat 1 or low 2-praying it will not be a cat 3.

Also wondering about that sinkhole near Bayou Corne, that is 1500 feet from all that butane. ( 60 miles southwest of us). I don't think a bunch of rain will help matters there. Gov. Jindal ordered a worst case scenario, before the threat of a hurricane.
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Quoting StormHype:


Yeah, I paid $7 each for 12 of them in 2005. Saw them locally in FL for $18 last week *before* the storm was in the news. All this money printing has caught up with us *bad*.
I actually asked if that was for the whole box. The guy laughed at me. At that price, it was worth going to my storage building and rooting around. BTW, this was before Isaac.
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Quoting Sasha:
I've been glued to this board all day...  all I've done is snack endlessly and hit F5 to refresh!!! 


We can relate.
Member Since: September 20, 2005 Posts: 0 Comments: 369
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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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