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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We just have to wait till the 18z and 00z runs come out to see where his eye will make landfall...
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925. IKE

Quoting nrtiwlnvragn:


SFWMD has the 12Z UKMET Office track, you may not like it.
Thanks. no...dont need Isaac here. Rainfall may be a big issue.
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Quoting Bamatracker:


ukmet staying east as well.


Because of the ECMWF and UKMET models staying east (Mobile/Pensacola), I don't think the NHC will adjust the track further west at this time. They will split the middle until a better agreement is found.

The NHC has said previously however that the GFS had a better handle of the overall pattern of the atmosphere yesterday. That said, all these models will have new G-IV data in time at the 18z and 00z models so we shall see (apparently some even had info in the 12z models).

I would put my bet on a east Louisiana or Mississippi landfall... but that is JUST MY OPINION.
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Quoting washingtonian115:
So both the ECWMF and GFS sends Isaac into M.S and A.L?.Not good at all.Definitely something to keep a eye on as that part of the coast is vulnerable to storm surge.Especially since Isaac is suppose to be large.


I'm still going with a Mississippi landfall.
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922. Kumo
Quoting nofailsafe:




There are TS Warnings for coastal EDIT: (off-shore) waters south of LA, so I wouldn't be surprised if they pop up watches here too by the end of the day.


Even though the current models don't have this thing making landfall in TX, the power grid just north of Houston is pretty shoddy. We always lose power up here during a severe T-Storm. IF the current models pan out, looks like we're going to be getting TS force winds up here in the Spring/Woodlands area. Things might get miserable for a couple of days. Our local HEB has a sale on the 3L bottles of water, just got back from picking up a few cases of the stuff.
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Quoting masonsnana:
Did you board up the house?
no, just took in the lawn stuff that could blow around,dont think i'll be getting much sleep tonight lol..if this changes course and comes closer, i can board the windows pretty quickly if need be, years ago for charley when we WERE supposed to get hit, i had the bolts cemented in the walls around the house and had the plwood cut to order..all i need to do is put the wood on and secure it..my neighbor loves the idea
Member Since: August 6, 2011 Posts: 4 Comments: 33368
Quoting TropicalAnalystwx13:
The 12z ECMWF has a borderline Category 4 hurricane at landfall. More importantly however, it did not shift east and remains at the same position it was at 00Z.
Someone said cat 2.
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Quoting ltlurker:
that was from an archived site , not the 2012 Isaac
\



708. phenomenoloj... as a neophyte here also, hope you saw the above post
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Quoting IKE:

Thanks....uncertainty in track...continues.


I wonder what the NHC will do.
Member Since: August 24, 2010 Posts: 0 Comments: 459
Quoting Chiggy:
12Z Euro @ 72 hrs - exactly the same position at the 00Z run BUT much further east than 12Z GFS..


949? Wow.
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Quoting Clearwater1:
Yes, you are right, but not must closer, maybe 120 miles or so?


maybe but with TS force winds up to 210 miles from the center, there is a very good chance we see that here
Member Since: March 10, 2010 Posts: 1 Comments: 7233
Quoting TomballTXPride:
11 AM he was at 23.9N
2 PM has him at 23.9N

3 hours of moving due west I guess....from 80.8W to 81.5W.


from the Radar, it all still appears to be moving west northwest, but the center was 35 miles wide and then it tightened to 15 miles wide. When it did this the smaller circle formed in the southwest of the larger one, moving the center about 30 miles WSW in less than an hour.

Now, the latest Buoy observation shows that the new smaller center is continuing to move Northwest. Pressure is dropping again, at Key West.
Member Since: December 28, 2010 Posts: 0 Comments: 134
">

p'cola still for euro. Sorry folks....these computers aren't going to make it easy on us.
Equal anxiety distributor ......
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More of the southern feeder band is emerging over water off from Cuba.

South side convection is firing for the first real time in a while.



and



GFDL 90hrs

Somewhere between Rita and Gustav.

A bit more conservative than HWRF.



A compromise between the two would still be in the 940mb range...
Member Since: January 25, 2012 Posts: 32 Comments: 1502
911. IKE

Quoting WDEmobmet:
CMC shifted back west

You mean east? It went the other way from the 00Z run.
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Quoting MiamiHurricanes09:
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.



Looks awfull, really! Stay safe.
http://www.earthcam.com/usa/florida/miamibeach/?c am=miamibeach3
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UKMET 12Z




Very similar to Euro run...
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Quoting Hurricanes101:


thats not true, tomorrow Isaac will still have a decent northerly movement to him, tomorrow midday will be the closest he gets to us
Yes, you are right, but not must closer, maybe 120 miles or so?
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Anybody have the 12z GFDL or Hwrf?
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Quoting eyewall99:
Soon FL will be in the rear view mirror and all eyes will be looking at the north central gulf coast. Wonder if this blog will slow down slightly then????
I think it will be the talk of the town with the possibility of a strong storm heading to the northern gulf coast... Please keep informed
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Quoting MiamiFL305:
Key West


AWESOME PICTURE! Thanks for sharing. I am loving the new technology. StormJunkie has a cool webcast going. Here is the link: http://www.stormjunkie.com/live.php
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Key West radar is showing something that looks like beginnings of eye... cant see it on mia radar
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Quoting TropicalAnalystwx13:



UKMET I take it? Looks to be around Orange Beach.
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Quoting WDEmobmet:
CMC shifted back west



cmc shifted east, not west.

It didn't shift by too much though
Member Since: March 10, 2010 Posts: 1 Comments: 7233
Quoting IKE:
Euro sticking to its eastern solution. Anyone have the 12Z UKMET?


SFWMD has the 12Z UKMET Office track, you may not like it.
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12Z Euro 96 hours. No help for drought in the plains:

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O.O... yikes.
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If anything the Euro staying east will make for a dangerous situation..the NHC track will probably stay east of Louisiana because of it giving Louisiana folks a sense of security...
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So both the ECWMF and GFS sends Isaac into M.S and A.L?.Not good at all.Definitely something to keep a eye on as that part of the coast is vulnerable to storm surge.Especially since Isaac is suppose to be large.
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Quoting LargoFl:
so there is no chance issac comes up the coast of florida..for sure he goes westward, we dont wake up tomorrow and he's close to the coastline, making for the panhandle of florida?
I don't think that will happen. 3 days out, the nhc is pretty mostly right.
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893. IKE

Quoting Bamatracker:


ukmet staying east as well.
Thanks....uncertainty in track...continues.
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Quoting Levi32:


It matters here though. With a storm like Isaac, you can't separate out the Caribbean portion of the track because it is the steering currents that brought it out of the Caribbean that are still affecting it now, and it is that pattern that makes this path so improbable, even though it is occurring before our very eyes. You can't just look at the storm every day, take the new position, and draw up a new historical map. The past track matters.


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?
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891. 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!
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Quoting IKE:
Euro sticking to its eastern solution. Anyone have the 12Z UKMET?

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Euro still east, around Pensacola or so. It nudged a little west, but not much.
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Quoting TallyWeather15:
Is this the only model that shows significant FL panhandle impacts? And is it reliable
dont really know, its not a hurricane model..just rainfall
Member Since: August 6, 2011 Posts: 4 Comments: 33368
CMC shifted back west

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Quoting LargoFl:
thats what my local met says as well tomorrow and possibly tonight is our stormiest days
Did you board up the house?
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ukmet staying east as well.
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I really think Isaac is going to pull a Georges track and eventual landfall point.

Georges track was also uncertain. Landed along MS/LA line. I think this seems reasonable at this point.
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882. IKE
I would bet on the GFS over the ECMWF on track. Just my opinion.
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yea euro is eastern... but looks like cat 3
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Had only 1 1/2 inches of rain here today in Palm Beach county... I don't know how accurate that is with the rain blowing sideways during the feeder bands. I think it looks worse than it is with the sideways rain.
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What, if any impacts do you see Tallahassee getting? And when?
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Quoting flhurricanesurvivor:
Has the new ECMWF come out yet and if so did it track West with the GFDL?
Wow! September of 2002? You're a ten year Wunder weather vetter!
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Quoting Chiggy:
12Z Euro @ 72 hrs - exactly the same position at the 00Z run BUT much further east than 12Z GFS..

DOOOOOOOM!!!!!!!!!!!!!!
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Soon FL will be in the rear view mirror and all eyes will be looking at the north central gulf coast. Wonder if this blog will slow down slightly then????
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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.