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 RTSplayer:


It's Mobile Bay/Pensacola at 949mb, which corresponds to about 125 to 130mph.


Important thing to note here, is the intensity is actually splitting the GFDL and HWRF.

It's probably going to be category 3 or 4 at landfall, and not category 2.


You got the most recent runs each for the "best" global model and 2 hurricane models calling for cat 3 or cat 4 landfall.

That's a strong indicator that all of the models find room for lots of intensification, regardless of the exact track it takes.
Those pesky I storms.I expect landfall in Alabama/Mississippi(no offense to anyone living there).
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Good to see Dr. M on TWC
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Quoting weatherman12345:
Did it calm down miami?



We have a bit of a lull right now I noticed that the rain bands are training all around my area I am just lucky it's not over me. Was pretty nasty this morning.

Let's see later this after noon.
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Quoting tennisgirl08:


I know. Very strange. Could the Euro be showing a stronger storm which would naturally move more poleward?


Possibly. Having a strong storm doesn't seem to be effecting the other models though.
Member Since: July 15, 2006 Posts: 17 Comments: 10284
1022. A4Guy
Quoting Grothar:


No, we need a little laugh sometimes. Did you just get that big band that just passed?


Yes...it wsa really dark and gusty...but no worse than any of the others, I don' tthink.

I am in north Ft Lauderdale...just off US1. Where are you?
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1021. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 42029
1020. ITCZ
Dr Masters is on! Dr Masters is on :-)
Member Since: September 5, 2008 Posts: 0 Comments: 47
Dr. Masters on TWC!
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Best case scenario for us in MS Isaac follows the Euro's track, Worst case scenario it follows the GFS track or the NHC track. Remember what DR. Master states in his post, The current official track is an average of the models used by the NHC it does not necessarily mean it will actually take that path it just the most likely path. Expect corrections to either the right or the left.
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1017. IKE

Quoting Bamatracker:


Nogaps 96 hrs. 925 vorticity.

Shifted east. Thanks.
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Dr. Masters is live on TWC!
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Jeff Masters on TWC now
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1014. robj144
Quoting robj144:
I just heard thunder from Isaac. I'm in Palm Beach county, by the way. I don't recall hearing thunder in a tropical system before.
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Quoting BahaHurican:
I thought it was a pretty cool photo, but it just didn't have the look of an arriving tropical storm... and I've seen one or two in my time...

Hey, Ike. IF my hypothesis about when each of these is accurate has any validity, I'd have to go with the GFS also... GFS didn't start calling this scenario until a few days ago. Now it matches, more or less, what the Euro was saying 7 days out...


I agree the GFS has a decent track record
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Quoting TomballTXPride:

I agree. I'm not saying I see what they see either, just posting what NHC had him down for because it doesn't seem right. Oh well.


Well NHC said he was heading 300 degrees which is north a bit from the 292.5 heading of true WNW.
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Dr. Masters is on The Weather Channel.
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Doc's on now!
Member Since: February 11, 2012 Posts: 84 Comments: 8032
1009. robj144
I just heard thunder from Isaac. I'm in Palm Beach county, by the way. I can't recall hearing thunder in a tropical system before.
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Quoting LargoFl:
yes we will see what happens, hope you come thru it ok also
Thank you Largo, you also
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Quoting IKE:
What about the 12Z NOGAPS?

Hello to everyone.


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Quoting MississippiWx:
Well, the European models want Isaac to make landfall around Gulf Shores, while the North American models/hurricane models want Isaac to make landfall in SE Louisiana. You have to wonder what the difference is between the models since they should be receiving mostly the same data.


I know. Very strange. Could the Euro be showing a stronger storm which would naturally move more poleward?
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Im not totally trusting GFS b/c after all it didnt have Isaac going into the Gulf like the Euro did.. So GFS has been totally right about this storm like some ppl claimed it has.. I know about what it has done the last few years but lil things can be missed and can result into a big error.. IMO
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Quoting IKE:
What about the 12Z NOGAPS?


Nogaps 96 hrs. 925 vorticity.

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Just turned it to TWC. OMG this Heather person is rather dense.
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Quoting StormPro:

Hey....I'm 3 blocks from the lake...where are you?


5 blocks in mandeville
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Quoting IKE:

Good question. Flip a coin? GFS is what I would trust....until it switches...IF it ever does.


Quite interesting: after several days of Euro model bashing most models switched to the western solution while ECMWF now is one of the eastern outliers against the GFS.
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I told you guys Isaac had weakened :P

Definitely temporary though.

Member Since: February 11, 2012 Posts: 84 Comments: 8032
998. Kumo
Quoting daddaddad:
Dr. Jeff Masters on TWC?!?


Aye, WU and TWC are now owned by the same company. Seems like its turning out to be a pretty good deal for both networks. :)
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Well in any event, over all westward shift.


So now we are "officially" anywhere from Galveston to Biloxi

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Quoting WDEmobmet:


00z had Central LA.. 12z had MS/AL state line... I believe that is west

EDIT**********
LOL BEEN UP WAY TO LONg... MY APOLOGIES thats what I mean lol wow i think a nap is in order

Been there....doing that!
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Quoting weatherh98:
Lake pontchartrain up 2 feet. Not tlfrom isaac but will allow for more surge than previously thought

Hey....I'm 3 blocks from the lake...where are you?
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CDO quickly forming ovet the top of Isaac, as the south side of the system has filled in with newly formed convection....
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Quoting Neapolitan:
Please don't falsely label images. That is a well-used picture of a decidedly non-Isaac event that can be found all around the interweb. Such as here, and here, and here, and here, and here, and here...

If you're unsure of the provenance of a particularly stunning image, use caution with posting and labeling. Facebook, for one, is full of falsely label weather photos that are trotted out with each severe weather event.
I thought it was a pretty cool photo, but it just didn't have the look of an arriving tropical storm... and I've seen one or two in my time...

Quoting IKE:
I would bet on the GFS over the ECMWF on track. Just my opinion.
Hey, Ike. IF my hypothesis about when each of these is accurate has any validity, I'd have to go with the GFS also... GFS didn't start calling this scenario until a few days ago. Now it matches, more or less, what the Euro was saying 7 days out...
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Quoting Stormchaser121:



Are you serious? ??? Even more spread now with 18z?


Wtf man just wtf
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Center is about to pass over Key West...

Member Since: December 18, 2006 Posts: 7 Comments: 2687
Mmmmm...isn't MS/AL state line EAST of Central LA?? Unless you mean the 12z from yesterday??

Most consensus models, on the other hand, still say more of a S Cen LA hit, or SE LA fake, then drift W along LA coast to Lake Charles.

It's getting real now.


Quoting WDEmobmet:


00z had Central LA.. 12z had MS/AL state line... I believe that is west

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Quoting CoastalAlabama:



Fairhope... BP at 44/98 has some empty tanks... Walmart between foley and Fairhope busy but not out of control... BP by that new Publix had tanker refilling.

Fly Creek Marina is looking for some surge regardless so they just moved all trailers up to entrance. Not worried about boats at lowest level of stackable...

I was on Ono Island yesterday and nobody was boarding etc, just some larger boats coming off front to back inlets and folks taking patio furniture off their docks and piers.


I am outside of Weeks Bay towards the Bon Secour side. I am watching some of the bigger boats go into Weeks Bay right now. Still trying to decide what to move, or even if I need to move anything. What a mess!
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didn't know if this had been posted yet...

NEW ORLEANS - Offshore oil and gas operators in the Gulf of Mexico are evacuating platforms and rigs in the path of Tropical Storm Isaac. The Bureau of Safety and Environmental Enforcement (BSEE) Hurricane Response Team is activated and monitoring the operators' activities.
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Quoting washingtonian115:
Someone said cat 2.


It's Mobile Bay/Pensacola at 949mb, which corresponds to about 125 to 130mph.


Important thing to note here, is the intensity is actually splitting the GFDL and HWRF.

It's probably going to be category 3 or 4 at landfall, and not category 2.


You got the most recent runs each for the "best" global model and 2 hurricane models calling for cat 3 or cat 4 landfall.

That's a strong indicator that all of the models find room for lots of intensification, regardless of the exact track it takes.
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More of em went west
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Quoting TierraFirme:


Gro, what you say sounds right but what Levi is saying almost makes this storm human (don't mean to be deep Levi). You have to see where it came from to understand where and why it is now. The thing that I question is that Isaac had all the opportunity and enviorment to become a huricane quite a while back, but i didn't, IMO, and I can most certainly be wrong. Any answers, opinions or guessing?


Still trying to figure it out. I saw what it was doing, but it didn't make sense to me either.
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Quoting masonsnana:
I didn't put the shutters up. I hope I don't regret this..like you said, I don't trust this storm
yes we will see what happens, hope you come thru it ok also
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Quoting Joanie38:


ASH 89 :)


Press conference is scheduled for 3:15
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981. IKE
What about the 12Z NOGAPS?

Hello to everyone.
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Quoting A4Guy:
Storm calamity to report here in East-Central Broward (Lauderdale): one of my heavy, large, potted Majesty palms tipped over. Heavy rain band right now...but I gotta get out there when it lets us...as this thing CANNOT roll into my pool or I will never get it out!!!

All kidding aside - this is of course very minor, and does not mean to jest about those that may have more extensive damage froma stronger storm!


No, we need a little laugh sometimes. Did you just get that big band that just passed?
Member Since: July 17, 2009 Posts: 71 Comments: 27208
Quoting Hurricanes101:


??? MS and AL are east of LA


Your right... my bad been a long day lol im going to take a nap..

I FEEL RETARDED AFTER THAT COMMENT--- FORGIVE ME please
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Quoting TierraFirme:


Gro, what you say sounds right but what Levi is saying almost makes this storm human (don't mean to be deep Levi). You have to see where it came from to understand where and why it is now. The thing that I question is that Isaac had all the opportunity and enviorment to become a huricane quite a while back, but i didn't, IMO, and I can most certainly be wrong. Any answers, opinions or guessing?


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.
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Man I hope everyone is ready for this.



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Quoting Hurricanes101:


It didnt shift west either lol

That's okay, it didn't widen the gap we already have, lol.
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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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