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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Recon headed in for another pass at the center of Isaac.
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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!



Link
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15Z steering from CIMSS for 970mb - 989mb - THis is what Issac "will" follow it it was a CAT 1/2 entity.

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Quoting Lizardking1:
The focus is on NOLA of a direct hit, SOFL will be brushed by tropical storm winds and life moves on.
Focus is on others besides new Orleans. This could hit a lot of other spots unless I misinterpreted your quote. I am not in no but In the cone.
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Member Since: May 23, 2012 Posts: 0 Comments: 4511
Quoting WxGeekVA:
Seven years ago today, to the hour:



Le Deja Vu, anyone?



Is there an Isaac link with similar resolution?
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Quoting MiamiFL305:
3 people dead in flordia so far do to driving, so please stay off the roads

oh, even ts can be dangerous( for those who are disbelievers)
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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.
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When is the crunch time going to be on where Isaac will go La or Florida Panhandle?
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Quoting Levi32:


His track would have him crawling along shallow shelf water, so he's likely to miss the deep heat content in the western gulf, but there is plenty for him in the eastern gulf.

interesting levi. this would probably result in him not being able to reach cat 5 intensity. how come in 2005, the water was more warmer and deeper than it is this year? i remember both katrina and Rita both exploded when they crossed the warm eddies
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Member Since: May 23, 2012 Posts: 0 Comments: 4511
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...

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Quoting guygee:
Doc mentions "moderate shear" in his blog above, that usually causes some tilt in the vertical alignment...and that is what I am seeing now as well. Check the composite radar out of Key West and you will see it.

yeah, but he still could strengthen in the gulf
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3 people dead in flordia so far do to driving, so please stay off the roads
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Here is a graph of the pressure in Key West. Should be below 29.50 very soon:

wunderground
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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


NO, not wish casting! GFS keeps showing Issac going more WEST with every run. We need to watch this and possible prepare too..., just like Floridians!
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102 hrs
Member Since: September 2, 2011 Posts: 0 Comments: 1148
285. Southfla

Words to live by this time of year in Florida.
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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.

he should be in new orleans lol
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Quoting TomballTXPride:
I have not had the chance to gaze at any of the CIMSS maps, but I'm under the impression that Isaac is still pretty well vertical aligned through the low and mid levels. Someone correct me if I'm wrong.
Doc mentions "moderate shear" in his blog above, that usually causes some tilt in the vertical alignment...and that is what I am seeing now as well. Check the composite radar out of Key West and you will see it.
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What would the steering look like if this storm intensified more quickly then expected?
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Quoting PalmBeachWeather:
Do you think the people in the northern gulf think it is a bust 145?


You just got to put these people on ignore, My ignore list just keeps growing since yesterday.

Do not quote them or answer them put them on ignore and you don't have to even see their comments unless someone quotes them.

I know you are a nice person and trying to make sure their mis statements are corrected but ignoring them is best.

You are getting much rain in Palm Beach yet???

we are in the middle of a band right now but already over 3"...
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Offshore South Miami-Dade


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Quoting Sfloridacat5:


Oh, it's all about NOLA I see?

And there's a very good chance Isaac will not make landfall in NOLA.

Storm is still a few days out and the track can change (East or West). The track hasn't stayed the same yet, so I don't know why it will now stay in one stop.

NOLA should be prepared, but so should everyone on the gulf coast.

everybody except for mexico LOLOLOL
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350. HCW
Cantori scheduled to arrive at Pensacola Airport at 4:30. Fox News, CNN, ABC, etc all arriving today as well.
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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
Member Since: August 13, 2007 Posts: 0 Comments: 133
Quoting cutlerbay32:

While I am counting my blessings, the rain and howling wind we have been experiencing is anything but a "dud". This weather is exactly what I expected because I do lurk and learn here. Be careful in your generalizations. It is difficult to hear that what I am experiencing now is nothing because I just gotta tell ya it ain't.


I agree. While Isaac may not be directly hitting SoFla, it is still affecting us. Many of us are in areas where even a strong afternoon storm will knock out the power. No reason not to be prepared when the track was so inconsistent! With the ground being so wet, and the possibilities of tornadoes, I'm glad I took the proper precautions. We may not be dealing with hurricane conditions, but it's still an issue.
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Quoting Chicklit:
Okay, I'm late to the blog, but see that Dr. Masters will be on TWC on Sunday, Monday, and Tuesday afternoons and/or evenings. That's cool.

true
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Quoting Lizardking1:
The focus is on NOLA of a direct hit, SOFL will be brushed by tropical storm winds and life moves on.


Oh, it's all about NOLA I see?

And there's a very good chance Isaac will not make landfall in NOLA.

Storm is still a few days out and the track can change (East or West). The track hasn't stayed the same yet, so I don't know why it will now stay the same.
Models will change.

NOLA should be prepared, but so should everyone on the gulf coast.
Member Since: September 16, 2007 Posts: 0 Comments: 6792
Quoting CaicosRetiredSailor:


14:47 GMT



Bingo at last
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jb.tweeted..fill.up
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Okay, I'm late to the blog, but see that Dr. Masters will be on TWC on Sunday, Monday, and Tuesday afternoons and/or evenings. That's cool.
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3,900 customers without power in Miami-Dade county.
Member Since: September 2, 2009 Posts: 130 Comments: 21193
The life of this storm, huff and puff but couldnt maintain it....since it developed it was suppose to be stronger by NHC but never could....even with ideal conditions. Seems that has been the story for this hurricane season so far...even in ideal conditions, storms have just not strengthened as expected.
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WX station in the central florida Keys:

Station SMKF1
NDBC
Location: 24.628N 81.111W
Date: Sun, 26 Aug 2012 16:00:00 UTC
Winds: ENE (60°) at 35.9 kt gusting to 40.0 kt
Atmospheric Pressure: 29.54 in and falling
Air Temperature: 78.1 F
Dew Point: 77.7 F
Tide: 1.08 ft (above MLLW)
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Quoting eyewall99:
Watch it stay a TS until landfall....man, talk about overhyped, still looks ragged and all and it is flying....will be alot of sad sad bloggers if it does not turn into a major before it hits land....

don't underestimate storms
remember allison(2001)
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Any chance of Issac slowing down in before landfall somewhere along the gulf coast?
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334. A4Guy
Quoting flcanes:

well, i've got to agree with you too


Quoting Tazmanian:




no the nhc made the right call by put a hurricane watch up for LA has its forcast too be come a hurricane later too day or tonight


"TOO" reference was probably lost....
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Quoting flcanes:

can you post that again


you mean you're sick of it?
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Quoting floridaboy14:
if he heads farther west, is there a better chance he gets to a warm eddy?


His track would have him crawling along shallow shelf water, so he's likely to miss the deep heat content in the western gulf, but there is plenty for him in the eastern gulf.

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Quoting Articuno:


It can't be weakening...
If it is, then it's like ernesto, good satellite presentation but bad winds.

Maybe I should call this Trollsaac?


Ernesto didn't even have a closed circulation most of the time in the Caribbean, the fact that it has an eye in the first place is telling that it is developing a good central core.
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330. CJ5
LOL slap down from and Admin, nice! You don't see that often.

The storm has struggled for the past 96 hours. There have been times he looked like it was about to fire off but it didn't. We just have to keep watching and analysing. I said yesterday morning when things where coming together I though by this am he should be a cane. Still didn't happen. This morning I think he looks pretty good and should start to ramp up. Shear is low, water is hot, nice anti-cyclone, building convection, still have a great spin, etc. That being said, he still is strugging to pull good convection all the way around and establish a broad full CDO. I look for that to begin happening over the next 12 hours but, like I said at the start, we have been waitin 96 hours for that to happen.
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Quoting GetReal:

Wow you can clearly see the eye.
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327. 7544
looks like most of the convection is streaming north over south fl anyway
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Quoting trHUrrIXC5MMX:
LATEST UPDATE ON ISAAC



new looking graphic

can you post that again
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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.