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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Worst weather I've gotten so far occurring as I type. Torrential rain with a gust of 43kts right now...you can hear the winds howling. Twigs flying around outside of the window and the TV signal flickering. Very bad weather right now. Extremely dark as well.

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Quoting KEEPEROFTHEGATE:
OFFICIAL FORECAST



Houston is in the cone? Really? there goes my week
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Improving all the time according to Key West Radar.

Link
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headed.boc?
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http://tidesandcurrents.noaa.gov/cgi-bin-mp/data_ plot.cgi?mins=&datum=0&unit=1&stn=8724580&bdate=20 120825&edate=20120826&data_type=wind&shift=&metint erval=&type=Meteorological%20Observations

Steep drop in winds at key west.
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a nother car accident near hialeah
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Tropical Storm ISAAC Forecast Advisory

Home Public Adv Fcst Adv Discussion Wind Probs Graphics Archive

US Watch/Warning

000
WTNT24 KNHC 261457
TCMAT4

TROPICAL STORM ISAAC FORECAST/ADVISORY NUMBER 22
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
1500 UTC SUN AUG 26 2012

CHANGES IN WATCHES AND WARNINGS WITH THIS ADVISORY...

THE HURRICANE WATCH FOR THE FLORIDA EAST COAST FROM GOLDEN BEACH
SOUTHWARD TO OCEAN REEF IS DISCONTINUED.

THE HURRICANE WATCH IS EXTENDED WESTWARD ALONG THE LOUISIANA COAST
TO JUST EAST OF MORGAN CITY...INCLUDING METROPOLITAN NEW ORLEANS
AND LAKE PONTCHARTRAIN.
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What do we have here coming out???

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668. CJ5
Quoting Maineweatherguy20023:

N/A last i say radar looks good though


Thanks, yesterday they indicated some eyewall but open North to West. I was looking for improvement but it never happened. Hopefully, we will have a new report soon. Radar indicates it is open on the East and South side.
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looks like a wooble to the north
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OFFICIAL FORECAST

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I believe inflow to the southern side of the circulation has been re-established. Huge infulx of moisture on the last radar frame..
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Quoting RickWPB:

Largo is from Largo, FL... not Key Largo.
right ty
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Quoting photomunkey:
Commenting on a few posts...
Yes, weaker ridge and Bermuda High, but this weaker TS doesn't have the power yet to break through it and go north, so the tracks keep readjusting west.

You shouldn't be saying that Isaac is going to go through "reintensifiication", or "RI" because it has not yet intensified past TS stage.

I posted Friday night that I thought Isaac would have a difficult time intensifying, and because of that would go west to the TX/LA border. Models have consistently overintensified storms over the last two years in the Atlantic and the Gulf. There is simply no evidence that the models have been corrected for this, so every 12 hours their tracks "self-adjust" for the updated position of the weaker storm.

In about 4 more hours, the northwestern feeder band should be clear of FL and the Keys, and the undisturbed t-storms in it should finally be successfull at wrapping up the eyewall. Look for intensification to hurrican status to begin then.


RI=Rapid intensification.

While I respect your opinion, you are clearly misinformed.
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Webcam is live an I'm on the way to N Gulf Coast
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Quoting BahaHurican: Yeah, you guys are likely to get rain and intermittent heavy winds all afternoon....


Most likely, started here early this morning. I think it will last until tomorrow though. Big system.
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Would the mesocyclone to the NW of the center cause for confusion in the models?
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To top it all off the system that is to precede 97L is supposed to be a big issue in the carribean next week and if it can avoid fronts and thread the needle it could also be a problem.
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Quoting washingtonian115:
Don't forget 97L!.


It's a fish.
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Quoting TomballTXPride:

Nice graphic. Very clear and concise. Nice work.

Thanks

Quoting washingtonian115:
This is all your fault!.

I know, we are getting our bad Isaac.
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BTW, clickBOOM, welcome to the blog... hope you hang around for the rest of the season!
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Quoting flcanes:

well, there is largo
speaking of him, is he on the internet, or has he lost power

Largo is from Largo, FL... not Key Largo.
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Still some radar clutter in the center. That seems to indicate Isaac has a bit of work to do in order to spin up in intensity. The 5PM Advisory will be the one to watch.
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Well if there really are any surface winds supporting a 65 mph storm in Isaac, they are probably going to be in the band lashing Key West.

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Quoting Grothar:



There have been numerous storms that have taken this similar path from Florida to New Orleans. This is not unusual.


Hi Grothar, been "liking" your posts all weekend but have not given you a shout out yet.
So how are things on east side of town?
Bills, said her power actually flickered a few times but no really stormy weather over there.
We are getting a lot of rain here.. over 3" so far.
ARe you getting rain???



I must pull myself off this computer for a while and go do a few things inside....

will be back later.

Stay safe!
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Quoting WxGeekVA:


Hmmm....
Convection seems NW of COC/LLC


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Alright so back and a bit calmed down from the frenzy of earlier.


As far as I can tell a few noteworthy updates:

1. Recon not finding much over 60 mph at this time, and its hard to tell why, sat and radar imagery continues to improve. In any event, the storm will be more solidly built going into tonight and tomorrow with its new re-formed core (post land interaction).



2. Guidance is still a big ??? But am expecting another shift west of the official track to be more in line with the envelope if nothing changes by 5 o-clock with the 18z run.


3. Possibility of additional westward shifts in the models themselves. The cyclone continues to wobble between a westward and wnw motion, and due to the fact that its not gaining strength, this is just going to make it easier for it to move west with Low Level steering patterns as opposed to more northerly with larger patterns.So I would not be at all surprised if we see the models shift even further west towards Galveston/Houston following the GFS.


4. RI tommorrow, tuesday? Big question and largely depends on where it goes.

Of note seems like the rest of the models have sort of "followed" the gfs on this one so far. The GFS led the charge on the eastward shift to Florida and it has lead the charge back to the westward NOLA track, and it continues to want to favor a Galveston-Beaumont type scenario with a stall at the end.


While this seems a bit far-fetched, its worth noted that so far, where the gfs has gone the other models of soon followed, albeit not quite as far.


With GFS now wanting to potentially go even further west (if 18z follows the trend) this means that anyone from Biloxi to Matagorda bay is in the proverbial watch and wait danger zone for this cyclone.


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Quoting wxchaser97:
My forecast, not much has changed.
This is all your fault!.
Member Since: August 14, 2010 Posts: 10 Comments: 15710
I am in central Palm Beach County. (KFLWELI6) and we have gusty winds (30mph) and almost 1.5 inches of rain. All is fine for now. Keep safe!
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There have been numerous storms that have taken this similar path from Florida to New Orleans. This is not unusual.
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Quoting Hurricanes101:


Largo is from Largo FL near Tampa
Im here
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Slowly getting his act together.. I expect tonight, botta beam um botta boom!
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64 mph wind at Port Everglades (reported by TWC)
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My forecast, not much has changed.
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Where are the New Orleans DOOM Casters at?
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GFS continuous shifting west......IF the trend continuous this will be a Mexico storm XDD
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Quoting StormHype:


Pensacola is a good vantage to get the most dramatic onshore RFQ effects of the storm if it goes directly into the mobile area.
Note to the media: If you want sensational TV, Dauphin Island gets beat up no matter where a GOM storm goes. Always rough water shots. Plus it's west end is no more and they have a pier completely over land. It reminds me of an old boxer. Beat up, but still hanging in there. Love the place.
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Quoting zoomiami:


Hi Gamma! Getting weather your way?


Yes, now in another "quite" time...rainy but no wind...

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After reading Dr. Masters blog it sounds as if he is saying one of the two models consensus will be correct and the NHC will have a large correction to make instead of blending the two solutions and splitting it down the middle. Being that a Hurricane warning has to be issued 36 hours ahead of the onset of tropical force winds I would think if this model difference continues the NHC will have to flip a coin or so to speak. Anyone have a take on this?
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Quoting Levi32:


It is constantly improving for sure, but gradually. We're not seeing bombing out yet.


That will most likely happen tonight once Isaac passes through the diurnal minimum and had to time to develop his inner core. By later this evening he should be much more organized and heading into diurnal maximum and to add to that he will have gained separation from landmasses. This should will allow him to be able to intensify rather moderately at the very least tonight.
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993mb now slow streghthining.
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Live mobile chaser video driving the streets of Key West:
Live mobile Key West stream
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Don't forget 97L!.
Member Since: August 14, 2010 Posts: 10 Comments: 15710
Commenting on a few posts...
Yes, weaker ridge and Bermuda High, but this weaker TS doesn't have the power yet to break through it and go north, so the tracks keep readjusting west.

You shouldn't be saying that Isaac is going to go through "reintensifiication", or "RI" because it has not yet intensified past TS stage.

I posted Friday night that I thought Isaac would have a difficult time intensifying, and because of that would go west to the TX/LA border. Models have consistently overintensified storms over the last two years in the Atlantic and the Gulf. There is simply no evidence that the models have been corrected for this, so every 12 hours their tracks "self-adjust" for the updated position of the weaker storm.

In about 4 more hours, the northwestern feeder band should be clear of FL and the Keys, and the undisturbed t-storms in it should finally be successfull at wrapping up the eyewall. Look for intensification to hurrican status to begin then.
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Quoting hydrus:
. Notice how the north part flattens out against the predicted ridge.


Very good graphic Hydrus,. This illustrates rather well why the models are calling for this West track in the Northern Gulf which a lot of people are not seeing here. Now, if verifies or not is a totally different story, but, each passing hour it is becoming more believable.
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Been getting some consistent rain in South Dade for about an 1 and half. Not much wind.
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Quoting DougKahn:
Storm surge in Englewood, Florida concerns me right now. Can anyone figure how far the storm's center will be from Englewood, at the closest point?


I think they're estimating 3-5 ft for pretty much all of SWFL...
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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.