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 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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Quoting AtHomeInTX:
N.O. met on TWC Get ready Louisiana
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!
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press conference no mayor now weather channel
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Repost From Previous Page.
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Quoting GTcooliebai:
EGR2 is that the European or UKMET?



UKMET Office interpolated back 2 cycles.
Member Since: September 23, 2005 Posts: 14 Comments: 11215
1521. yoboi
Quoting AtHomeInTX:
N.O. met on TWC Get ready Louisiana


people in la are starting to flood the stores....atleast the word is getting out...
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Quoting MAweatherboy1:
Several decent earthquakes in California.
How big?
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Quoting Levi32:
Here's the GFDL. It has Isaac taking its time strengthening as a still broad circulation in the central gulf, only going sub-970mb and becoming a Cat 2 just before running into Louisiana. This is the kind of solution that would pan out if Isaac takes a while to mix out the dry air still in his circulation.

What we're watching closely for now is the formation of a tight core with consistent convection at the center today or tonight, with hurricane force winds in the eyewall. If it can accomplish that tonight and be a strengthening hurricane going into the middle of the gulf, it could easily become a major.



Levi, what is the difference between the Gfs and the Euro. Is it the timing, strength.... At what point and when you think the crunch time is? Tia
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Quoting yoboi:



good chance....lilli part 2

oh Lili....that was not a fun one. 5 days no power, in the heat
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1517. guygee
Quoting floridabuckeyes:


Wow...thats a strong post....looks like somebody studied in their Organic Chem class. Id give it a 1 if I understood what you were sayin!!! :) I have to give you the "W" in that battle!
There are quite a few publications in the scientific literature describing studies made after major oil spills all over the world. These studies cover many years after the spills. All of that science is out there and well-known by the people who want to know the truth. I wish the greedy publishers didn't have such a lock on some of the information, but most any college student can access the information and a lot of it is in the public domain.
(OK, back on topic for me now)
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Several decent earthquakes in California.
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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.

My honeysuckle vine has dropped half its leafs today. Was a beautiful leafy green plant this morning. It is in a protected greenhouse type area. What's up with that?
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Quoting sar2401:


This is your fifth post and you're predicting a monster cat 3 to hit NOLA, based on some kind of prediction you made for Europe last year? If you're not a troll, you might want to not use terms like "monster" when there's absolutely no evidence to support such a claim.

It is within the rules, its on-topic so...
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1513. sar2401
Quoting robj144:
Here's a squall that I recorded a few minutes ago. I'm in south Palm Beach County, FL:

Link


Private videos are hard to watch on YouTube. :)
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Just in case...

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Pressure offshore Marathon, Middle Keys is 29.52 in Pressure Tendency (PTDY): -0.01 in ( Falling.

Pressure offshore Key West is 29.65. Interesting.

Link

Link
Member Since: August 18, 2008 Posts: 0 Comments: 682
1510. ITCZ
Quoting sar2401:


First, 40 mph gusts don't qualify as TS force winds. A TD maybe, but not TS. Second, this is based on several east model runs. There are several reliable models that want to takes Issac about 400 miles to the west, over LA, in which case you will see nothing but some showers. Until all the reliable models begin to converge, just pay attention to what's actually happening and not model runs.



Yes I realize the wording isn't perfect but -either way what I'm saying is WHEW.
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1509. Grothar
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?






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.
Member Since: July 17, 2009 Posts: 71 Comments: 26499


.....
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Quoting Stormchaser121:
TWC just said all the way to HOUSTON needs to start preparing.

lol. Don't worry about it yet... If Isaac continues WNW for the next 48 Hours... THEN PREPARE.
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1506. GatorWX
Quoting MississippiWx:
Impressive how much Isaac has organized since this morning.



After nearly two weeks, agree 100%! Not that it took me that long to agree with you, but Isaac sure took his sweet time. There is certainly something we don't understand as to why some storms organize rather fast and others don't. I know Isaac started with a strong mid level circulation, but I can't imagine that's the only factor. There's certainly much more to it than that. The upper atmosphere so far this season hasn't been overly conducive, but he's never seemed to truly live up to his expectations until now.
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Quoting robj144:
Here's a squall that I recorded a few minutes ago. I'm in south Palm Beach County, FL:

Link
You have the video set to private.
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1504. pottery
Quoting melwerle:


BIG roller just went through. Whole floor just waved out.

YIKERS !!

Hope that was the last one !!!!
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Quoting snow2fire:
Computational Models: when models are working ok, doing more computations with more data results in predictions that converge on “correct” solution. When model runs are diverging, something is not working like it’s supposed to.

Until the model runs start converging, there won’t be a good idea of where Isaac will end up. Hopefully, the model runs are showing the possible range of outcomes (assuming nothing big was missed).

Isaac is starting to look more like a hurricane: I hope that helps – sooner than later.


Based on the steering situation, intensity barely matters before category 3.

In fact, presently it's just about the same, give or take, for everything before cat 5.

That will change again over the next several hours, but I'm just making the point.

If anything, a category 1 will go farther west in the short term, because the bridge between the ridges is stronger at that steering layer.
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Quoting Doppler22:
Isaac+Gulf= Uh Oh

x 120000 he is HUGE!
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Quoting Grothar:

dang Gro...just not likin' this one bit.
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1500. sar2401
Quoting matara28:


I am a meteorology passionate and i made some accurate forecasts for last winter for S-E Europe. I read this blog from 2008 since Ike and i assure you i am not a troll. The conditions in the gulf are appropiate for a rapid intensification and for the last 3 days the majority of weather models shifted Isaac track westerly from Florida every day.


This is your fifth post and you're predicting a monster cat 3 to hit NOLA, based on some kind of prediction you made for Europe last year? If you're not a troll, you might want to not use terms like "monster" when there's absolutely no evidence to support such a claim.
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1498. yoboi
Quoting Articuno:
This is going to Vermillion Bay I can just tell.



good chance....lilli part 2
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1497. robj144
Here's a squall that I recorded a few minutes ago. I'm in south Palm Beach County, FL:

Link
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Quoting quebecjimmy:


hahaha you from canada eh?


lol, right now, I wish!
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Quoting MississippiWx:
Impressive how much Isaac has organized since this morning.


Isaac will have 3 days to continue to do so, and...
Isaac is Officially in the Gulf Of Mexico...


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TWC just said all the way to HOUSTON needs to start preparing.
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mayor of N.O. is now on air. www.wdsu.com. HERE WE GO!
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Quoting shfr173:
don't say the "N" word, people on this blog tends to get PO'd


If he strengthens faster than expected, he likely would, right?
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Rainbands starting to come into Tampa:

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Quoting MississippiWx:
Impressive how much Isaac has organized since this morning.

Almost looks like "K" when she exited Florida..
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1489. Grothar
Member Since: July 17, 2009 Posts: 71 Comments: 26499
1488. yoboi
Quoting AtHomeInTX:
Guy on TWC just said people as far west as Houston need to pay attention.



will the high split like rita???in swla pressure been rising all day....
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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
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Quoting pottery:

Yeah, I am sure there are tar-balls around.
Probably always have been some, and likely there will always be some.
But the way the thing was quoted it sounded like thousands of barrels of oil are going to come washing over the beaches and into the ecology....

That is a very unlikely scenario.
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?



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1485. pottery
Caicos.. post 1430.

Nasty stuff.
Major issues to deal with that kind of deposit.

The article does not say where it came from or how long it's been there.
But a strong storm could surely expose that kind of thing through erosion.

Dread !
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This is going to Vermillion Bay I can just tell.
Member Since: October 22, 2011 Posts: 0 Comments: 2465
According to Jefferson Parish Facebook: Grand Isle will be declaring a mandatory evacuation for campers and tourist today and will expand that to a total mandatory evacuation of residents tomorrow at 9AM.
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Quoting RTSplayer:


quit being silly.

The winds dropped 5mph and TWC reported that.

The pressure dropped 2mb and they reported that.

They mentioned that the winds came down so it "weakened" but that it probably didn't mean much at that time.


What else are they supposed to do?

It was the NHC that dropped the wind speed by 5mph, not TWC. TWC just reported what NHC says.

If you don't like it, blame NHC.


Really was a personal decision, I have nothing against them. Just decided I had watched enough recycled loops of the same info and way to many "WTF" moments. I doesn't take a rocket scientist to see that Isaac is a much more formidable storm now than earlier today. Pressures have been falling, not rising. Winds will respond soon enough.

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1481. Grothar
Member Since: July 17, 2009 Posts: 71 Comments: 26499
Quoting SykKid:


convection is waning. what are you talking about


If your'e watching the visible.. it looks that way as the angle of the sun changes towards sunset... Used to catch me out!
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Quoting hurricanehanna:


Wow...and to think we just knew this was going to stay in Fl. Never can tell eh?


hahaha you from canada eh?
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Quoting dmh1026:
Try the Lee County Emergency Operations Center....


the evacuations are for
Bonita Beach
Fort Myers Beach
Big & Little Hickory Islands
Black Island
Lovers Key
San Carlos Island
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Quoting huntsvle:


Hello athome...always good to see you!!


Hi hntsvle. Good to see you too. :)
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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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