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 pottery:

Point taken.
But in the absence of an alternative, what ?

My point was really, that the people had no clear directions and reacted in panic.


Swimming against the tide again I see....
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Quoting KeyWestbeachcomber:


In this case it was..but you had to catch the post
That's a big assumption.
Member Since: August 18, 2008 Posts: 0 Comments: 474
Great! Sun shining and the power flickered off for a few minutes. Doesn't bode well for Tues or Weds.
Member Since: August 16, 2007 Posts: 0 Comments: 397
1723. Levi32
Quoting MississippiWx:
Other than a bit of dry air initially, I'm not sure if Isaac could ask for better upper level conditions. He has an upper low backing away to his SW, another upper low to the SE of him, and an upper low along the Eastern Seaboard giving him excellent poleward outflow. Combine that with upper level ridging in the Gulf and the stage is set.



You're right. You can't ask for better. The only limitations are the dry air and ocean heat content.
Member Since: November 24, 2005 Posts: 635 Comments: 26661
Quoting AtHomeInTX:


Crap! Sorry I'm running out of nicer words.
Quoting AtHomeInTX:


Crap! Sorry I'm running out of nicer words.
No problem, one of my favorite words. Especially since i am babysitting my 5 month old grandson, LOL
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ST. PETERSBURG --
Tropical Storm Isaac has moved to within 50 miles of Key West, and gusts up to 50 mph have been reported in Polk County.

According to Bay News 9 chief meteorologist Mike Clay, there have been many reports of winds to 50 to 60 mph in the Keys and south Florida and winds have increased to 30 to 50 mph on Florida's east coast.

Although the storm is expected to pass well west of the Bay area, its far-reaching bands are expected to impact local weather for the next two days.

Already, radar has shown some fast-moving squalls moving through the area, and a gust of 50 mph was reported with a band of rain in Polk County.
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Quoting duajones78413:
What are the chances Isaac goes far enough west to effect Corpus Christi? That is where I live.


At this point, pretty minimal.
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Quoting BrandiQ:
I have a question... is the storm already done in south Florida or is it just getting started?

It's about Getting started, maybe midway.
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Katrina is nearly 7 years ago, guys. Can we leave it alone?
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Quoting KeyWestbeachcomber:


In this case it was..but you had to catch the post


I did see the original post....
Member Since: May 20, 2009 Posts: 0 Comments: 205
Quoting Stormchaser121:
"Some computer models then take Isaac more westerly with some risk that the storm could move into East Texas Friday. Obviously, a more westerly track will bring us much more of an impact Thursday and Friday with gusty winds and locally heavy rains. By the way, there is the chance that Isaac could become a CAT2 or CAT3 at landfall Wednesday night." -KFDM 6


Thank you for posting.
Member Since: October 14, 2008 Posts: 0 Comments: 179
Quoting sar2401:


I think you're absolutely wrong. There isn't even a hurricane to warn about yet. The NHC doesn't jump the gun like that.

If they upgrade it than warnings could very likely ensue
Member Since: August 1, 2012 Posts: 0 Comments: 269
1714. angelafritz (Admin)
Quoting duajones78413:
What are the chances Isaac goes far enough west to effect Corpus Christi? That is where I live.


So far Isaac is low risk for you. Even the models that are pushing Isaac west aren't reaching Corpus Christi.
1712. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 39702
Basically, if you live along the Central Gulf Coast right now, especially along the coasts of Louisiana, Mississippi, and Alabama, you need to prepare for a major hurricane or evacuate. Isaac has the potential to produce significant damage to this area with a combination of very strong winds, driving rains, and massive storm surge. If it were me, I'd find someplace to go farther north. Go east, northeast...not north or northwest because this storm is expected to ride up the states. Make sure you have everything essential.
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Quoting capeflorida:
Can someone please twit Joe Bastardi and tell him that Florida is still here and did just fine.! To think that people here pay to listen to that garbage!!!


using the words 'twit' and 'Bastardi' in the same sentence is redundant
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Quoting Clearwater1:
When the levees where re-built, after Katrina, what were they built to withstand? Also, did they install a better pumping system, to withstand a Katrina strength storm? Anyone know? I hope they planned for a worse case situation. . . and hope they don't have to hope it works.

The hurricane protection levee and drainage system has been significantly improved in the New Orleans area since Katrina, however, the work is still ongoing. Levees have been raised and fortified and pumping stations have been protected against backwash in the event they fail. Concrete safehouses have been contructed at the pumping stations that are completely self-contained and require no outside power, tap water or sewage removal. They are at an elevation of 30 ft above mean sea level.
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Quoting newportrinative:


It is?


In this case it was..but you had to catch the post
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My goodness I would not want to be in Brawley, CA. The little quakes keep coming. That would freak me out.
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1706. Michfan
Good afternoon everyone. Looks like Issac is still having issues getting his act together and the models are trending westward again. What a mess this is.
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Quoting Bluestorm5:br
/> How big?


click and view the yellow bar near the top
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1704. sar2401
Quoting weatherganny:

Why???She is being truthful.You can NEVER go wrong being prepared.


All in bold for the furthest western point of any model? Preparedness is good, alarmism, not so much.
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1703. JGreco
I feel sorry for New Orleans and S.Mississippi that they're gonna get hit again by a major. We in the western florida panhandle look to dodge another one....hopefully the forcefield over my region can be extended to NO....You really don't need another one there...
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Hopefully once this passes through West Palm, the worse will be over for southeast Fla.

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Quoting Bobsled27:


Actually, the Superdome was never intended as a shelter. In fact, people were told NOT to go there because there were no resources and it wasn't safe. But they came anyway, and you saw the results.


People were in fact directed to the Dome, RTA buses were delivering them to the dome, and people were told to bring their own supplies.

If you want to re-fight the myth battles, I suggest we find someplace off the board.
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Quoting Stormchaser121:

Exactly as i said days ago

What does that mean?
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Can someone please twit Joe Bastardi and tell him that Florida is still here and did just fine.! To think that people here pay to listen to that garbage!!!
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1698. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 39702
Other than a bit of dry air initially, I'm not sure if Isaac could ask for better upper level conditions. He has an upper low backing away to his SW, another upper low to the SE of him, and an upper low along the Eastern Seaboard giving him excellent poleward outflow. Combine that with upper level ridging in the Gulf and the stage is set.

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1696. bappit
Funktop shows a slight decrease in convection near the end of this loop, but compared to the beginning of the loop convection is on an upward path.

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Quoting Stormchaser121:

No its on the KFDM site


Ok. Thanks.
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Quoting yoboi:
what are peoples thought with the 5?? west, east or same cone???


The NHC will probably split the GFS and EURO down the middle.
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Quoting angelafritz:


We're expected a hurricane warning for the northern Gulf in the 5pm. A hurricane warning is issued 36 hours in advance of the anticipated onset of tropical-storm-force winds.


Thanks Angela nice call
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1692. sar2401
Quoting angelafritz:


We're expected a hurricane warning for the northern Gulf in the 5pm. A hurricane warning is issued 36 hours in advance of the anticipated onset of tropical-storm-force winds.


Angela, you I'll believe. Where exactly on the northern Gulf do you expect the warnings to be issued? ARES ham radio operators in Alabama have already been told to stand down, a bit premature IMHO.
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Quoting yoboi:
if you look at the texas high it is starting to retreat west.....

Exactly as i said days ago
Member Since: September 2, 2011 Posts: 0 Comments: 1148
1690. yoboi
Quoting Clearwater1:
When the levees where re-built, after Katrina, what were they built to withstand? Also, did they install a better pumping system, to withstand a Katrina strength storm? Anyone know? I hope they planned for a worse case situation. . . and hope they don't have to hope it works.


ya have to look past storm surge in nola the pumps can't keep up with a surge and 20 inches of rain....
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Quoting sar2401:


Good heavens. VA, you need to take a nap or get a good stiff drink. :)

Why???She is being truthful.You can NEVER go wrong being prepared.
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Quoting hurricanehanna:

oh Lili....that was not a fun one. 5 days no power, in the heat


In the heat? Lili hit around 7 October and the weather was nice and cool. I also lost electric. That the weather was cool was the saving grace.
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Been getting hammered in Lake Worth (W. Palm Bch) for the past 20 min. Heavy rain, probably 20-30 mph winds with some higher gusts, and thunder.

Now speaking of getting hammered.....
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1686. ITCZ
@wflatampa: BREAKING: Pasco County government offices and schools closed Monday.
Member Since: September 5, 2008 Posts: 0 Comments: 47
What are the chances Isaac goes far enough west to effect Corpus Christi? That is where I live.
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Quoting MAweatherboy1:
Several decent earthquakes in California.


Holy cow! Just had a look, LOTS of medium ones in an hour :O If I were there I'd be about freaking out right now. I know smaller ones are supposed to be good and take off pressure....but don't see it quite like that. Reminds of pre-volcanic explosions TBH. Earthquakes down where they are are extra creepy with how far below sea level the salton Sea area is.

Anyway, been out all day, Ernesto...I mean Isaac looks way better than when I left. When I left earlier a bunch of people said they could see an eye...all I could see was a very disorganised looking TS! But getting sorted it looks now.
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1683. pottery
Quoting Bobsled27:


Actually, the Superdome was never intended as a shelter. In fact, people were told NOT to go there because there were no resources and it wasn't safe. But they came anyway, and you saw the results.

Point taken.
But in the absence of an alternative, what ?

My point was really, that the people had no clear directions and reacted in panic.
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Live Isaac chaser video driving around Key West:
Live mobile feed - Key West FL




Member Since: May 31, 2007 Posts: 0 Comments: 1204
1681. peterj
Quoting Bobsled27:


Actually, the Superdome was never intended as a shelter. In fact, people were told NOT to go there because there were no resources and it wasn't safe. But they came anyway, and you saw the results.


The Superdome had been used as a shelter before. It was was for Hurricane Georges in 1998 and Ivan in 2004. The issue with Katrina was that the city of New Orleans failed to plan.
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Quoting presslord:
let's not be cute here folks....'moron' is code for an ugly racial epithet....


It is?
Member Since: May 20, 2009 Posts: 0 Comments: 205
1679. sar2401
Quoting AtHomeInTX:


Crap! Sorry I'm running out of nicer words.
Quoting WxGeekVA:
Basically, if you are in New Orleans, or you have family in New Orleans, you might want to leave or tell them to leave now before feces hit the rotating oscillator. This could potentially be a KATRINA SIZE disaster, and people need to be aware of this now and leave while they still have a chance.

Please follow instructions of local officials though, as I am not an official source of imformation. It is just my opinion that this could be very bad, and that I want to try to help people avoid loss of life or property.


Good heavens. VA, you need to take a nap or get a good stiff drink. :)
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1678. yoboi
if you look at the texas high it is starting to retreat west.....
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Quoting breeezee:

did you see what those morons did to the superdome during katrina

There is a plan in place to evac those who do not have transportation out of New Orleans. New Orleans no longer will shelter people in place so the Superdome and the convention center will not be used. No more voluntary evacuation orders either. All evac orders will be mandatory. However, if you decide to stay you must stay inside the confines of your house or apt until the curfew is lifted. Those who stay are warned that no emergency services will be available to them after the curfew takes effect. They will be completely on their own.
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Quoting AtHomeInTX:


Was this on Facebook?

No its on the KFDM site
Member Since: September 2, 2011 Posts: 0 Comments: 1148

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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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