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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1276. guygee
Quoting Levi32:
Interesting that the group of CMC members, of all things, don't strengthen Isaac beyond Cat 2 in general.
Yeah, doesn't the CMC usually have problems with convective feedback and overestimating storm intensities. Colossal Mess Creator.
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1274. BlxMS
Quoting gustaveye:
EURO still not in agreement with GFS....still don't have the tracks in agreement....UGH


Sorry for the amateur question, but been busy and just have a few mins available now...What time was the latest Euro released,,,when is the next and does anybody have a real quick link to the latest Euro...Thanks for the help
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1273. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 41568
My previous post of the center of circulation as suggested by radar was a bit too far southwest. Here is a corrected version, as of 3pm. As you can see the center is just about to reach its closest pass to key west (about 20 miles away). Moving just north of WNW. Winds should shift in next 20 minutes:

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Quoting Hurricanes101:


not saying I dislike it, but everyone is praising the GFS and pretending it has been right all along when in fact for most of the duration of Isaac, it has been wrong; the Euro gets the victory on this one because it was the first to note the westward trend and was the first one to suggest this could be a GOM storm. The Euro was also the furthest west of the major models until yesterday


To me,they both like fish outta water flipflopping around,one will win by default.
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1270. pottery
Quoting cajunkid:


Dude...Microbes have eaten all that oil.

Light crude does not last long outside a container, much less in salt water.

That's why I thought that Dr. Masters statement (as quoted here on the blog) was a little strange......

What oil ???
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The St. Charles Parish Emergency Operations Center is fully activated and taking actions in preparation for Tropical Storm Isaac, which is currently forecast to make landfall as a strong category 2 hurricane near Biloxi, Miss. on Tuesday.

The parish expects to see 105 mph winds with higher gusts and storm surges between 6 to 8 feet beginning Monday night and into Tuesday. The parish is currently under a hurricane watch.

Parish President V.J. St. Pierre Jr. at 10:15 a.m. declared a state of emergency due to Isaac.
Member Since: September 5, 2007 Posts: 0 Comments: 3488
Quoting TallyWeather15:
What is the farthest east you see schools closing along the gulf coast?


i would think as soon as a hurricane warning goes up for your parish that school will be cancelled....some parishes have already cancelled school
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1267. Grothar
This will not take as long to intensify as they currently believe. As it begins to move away from Florida.....



Member Since: July 17, 2009 Posts: 71 Comments: 27068
Quoting NICycloneChaser:


Saw someone make this point earlier, but many storms rapidly intensify out in the Atlantic over much less TCHP than is in the Gulf right now.

And 'below average' doesn't mean 'low'.

Many smaller and more organized storms. Not messes like this.
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Link
Quoting Dakster:


I agree - and I want to know who built the ONE, LONE house that survived... Assuming that house was ORIGINALLY in that spot.


It was indeed in that spot....here's an article about it:

http://articles.cnn.com/2008-09-18/us/ike.last.ho use.standing_1_yellow-house-pam-adams-new-house?_s =PM:US

I know I'm probably being paranoid, but the way the models keep moving west is making me extremely nervous, remembering the early forecast tracks for Ike. So many homes and businesses in my community were decimated by that storm, my parents' home included. I would never wish that on anyone, and I do NOT want to go through that again!!!
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Member Since: September 2, 2011 Posts: 0 Comments: 1149
Quoting Dakster:


It appears that way.. Hopefully they step up to the plate and take care of it. I imagine they will as they do not need yet, another bad PR image in the news.


Dude...Microbes have eaten all that oil.

Light crude does not last long outside a container, much less in salt water.
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1261. TXProf
Just going out on a limb here, but with a name like Isaac it seems almost destined to hit Galveston...
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1260. Axelia
Polk County, FL (central FL) schools closed for Monday, August 27.

As of now, Orange County (also central FL) schools are still open.
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Quoting CosmicEvents:
Hardly, We have deaths and thousands of homes down in Haiti.


I'm taking about right now in the U.S.A.
I assumed everyone would understand that.

The weather has been much worse in Miami than Key West this afternoon. Yet Key west was right in the path of the center.
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WPB!! Isaac power outages: 1,190 people without power in Palm Beach County, FPL says



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What is the farthest east you see schools closing along the gulf coast?
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2mb/h fall, not to shabby, especailly as Cuba trails off to the WSW, and TCHP goes up.
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1255. HCW
By 11pm the NHC track will be like the TVCN track

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1254. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 176 Comments: 55503
Quoting Hurricanes101:


looks to be right on track to me


Yeah, for some reason the 5 day track on google earth didn't update to the new position, but the 3 day did. So yes, it's right on track.
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1252. Levi32
Core winds are not impressive yet, but pressures are falling.

More recon plots here

Member Since: November 24, 2005 Posts: 635 Comments: 26695
Quoting nofailsafe:
From Mission 21:

Time: 18:46:30Z
Coordinates: 24.0167N 81.7167W
Acft. Static Air Press: 843.0 mb (~ 24.89 inHg)
Acft. Geopotential Hgt: 1,440 meters (~ 4,724 feet)
Extrap. Sfc. Press: 992.8 mb (~ 29.32 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 282° at 2 knots (From the WNW at ~ 2.3 mph)
Air Temp: 20.3°C (~ 68.5°F)
Dew Pt: 18.0°C (~ 64.4°F)
Peak (10s) Flt. Lvl. Wind: 7 knots (~ 8.0 mph)
SFMR Peak (10s) Sfc. Wind: 24 knots (~ 27.6 mph)
SFMR Rain Rate: 2 mm/hr (~ 0.08 in/hr)
(*) Denotes suspect data

This might be the center right now from the latest pass. Regardless, it's on the south side of the 2/3rds error forecast track.


looks to be right on track to me
Member Since: March 10, 2010 Posts: 1 Comments: 7874
Quoting TropicalAnalystwx13:

I don't see why they would cancel school along the Gulf Coast tomorrow. They didn't the day before Irene here last year...which was exactly a year ago actually. You won't have school Tuesday, Wednesday, and maybe even Thursday though. I can guarantee that.


Once evac orders go up, schools have to close. They have to allow the students and families to prepare. Here, mandatory evacs are already beginning for surrounding areas. As a school employee, I wouldn't be shocked if schools start to close tomorrow. If not, there will be mass chaos on the interstate monday-tuesday leading to gridlock.
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1249. Dakster
Quoting gulfcoastmom1969:


well BP my have some oil beaches to come back and finish cleaning... the tar mats ( sunk oil) are just off shore...what a mess we will have


It appears that way.. Hopefully they step up to the plate and take care of it. I imagine they will as they do not need yet, another bad PR image in the news.
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Dr. Bill Read, former NHC director, on Isaac: "So far the track and intensity of Isaac has fit rather nicely to what the GFS (and ECMWF) had been predicting. Now, however, the bifurcation of ensembles in the GFS runs that started showing up yesterday suggests that the steering pattern near the northern Gulf coast is very sensitive to minor changes in the data the initializes the model. This sensitivity probably explains the role reversal of the ECMWF shifting to the right most solution since yesterday, but I don’t have data to support the speculation. This model mess creates rather low confidence in any single solution which of course is unfortunate given the long lead time for decision makers in the region. And then there is the intensity forecast…

Back to your question. Not a zero percent chance on Texas but certainly still an outlier. The TS wind probability off the NHC (not GFS) forecast give a 15% chance for Beaumont and 8% chance of tropical storm force winds."
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EURO still not in agreement with GFS....still don't have the tracks in agreement....UGH
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1246. DVG
The purple blob that is Isaac has tripled in size on the current water vapor loop.
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1245. dmh1026
Quoting coondini:

She is nice looking though.

She's not hard to look at....
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Quoting TallyWeather15:
I still feel the models could flip back to the east tonight, especially withe the euro still showing a florida/alabama landfall


I would not be surprised if it did. There is a big possibility that it will do, just keep an eye on the other models. Any tendency to the left would mean that the Euro is proving to be the right one.
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Quoting Sfloridacat5:
What's funny (or interesting) is the worst weather associated with Isaac have been on the S.E. coast of Florida.

Hardly, We have deaths and thousands of homes down in Haiti.
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.
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Quoting NICycloneChaser:
I think that just about everybody here would be happy to agree that Isaac will be at least Cat 2 at landfall.

I've posted this before (before Irene last year), but Cat 2 Ike did this:



This doesn't need to be a major to wreak havoc. Everyone in it's path needs to remember this.


That image will stick with me for the rest of my life -- I saw that house and the devastation about 10 days after Ike. Completely stuns me to this day.
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Just went to get extra dog food and cigarettes (all our other supplies just stay in place or get rotated annually), and wow - NOLA is shifting into high gear. Gas stations have lines, and everybody is stocking up.
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Quoting E46Pilot:


That Heather girl has got to go though, she is just getting under my skin for some reason.


The word "vapid" comes to mind.
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What's funny (or interesting) is the worst weather associated with Isaac have been on the S.E. coast of Florida.

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From Mission 21:

Time: 18:46:30Z
Coordinates: 24.0167N 81.7167W
Acft. Static Air Press: 843.0 mb (~ 24.89 inHg)
Acft. Geopotential Hgt: 1,440 meters (~ 4,724 feet)
Extrap. Sfc. Press: 992.8 mb (~ 29.32 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 282° at 2 knots (From the WNW at ~ 2.3 mph)
Air Temp: 20.3°C (~ 68.5°F)
Dew Pt: 18.0°C (~ 64.4°F)
Peak (10s) Flt. Lvl. Wind: 7 knots (~ 8.0 mph)
SFMR Peak (10s) Sfc. Wind: 24 knots (~ 27.6 mph)
SFMR Rain Rate: 2 mm/hr (~ 0.08 in/hr)
(*) Denotes suspect data

This might be the center right now from the latest pass. Regardless, it's on the south side of the 2/3rds error forecast track.
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1236. Gearsts
Quoting MississippiWx:
Strengthening Isaac:

Extrap. Sfc. Press: 992.8 mb (~ 29.32 inHg)
995 and now 992?
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Quoting cajunkid:

Oil washing up is the least of our worries right now.

It has all been broken down by now.

I agree. Just never thought about it. Will it be emulsified and become airborn(only half kidding)?
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1234. dmh1026
Squalls underway in Estero, FL right now..
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Tropical Cyclone Heat Potential is not below average in the Gulf this year. It is average to above average.

Member Since: July 6, 2010 Posts: 113 Comments: 32687
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Quoting presslord:
Stormjunkie enroute to the Gulf coast for Portlight can be seen here Link it's pretty boring right now...but he's kinda fragile...and his ego would eat up the attention if y'all wanna check it out....
You know I'm gonna quote u... lol
Member Since: October 25, 2005 Posts: 19 Comments: 22682
1230. barbamz
Earlier this link to some guys cruising the streets of Key West and livestreaming was sent. They are still online.

http://www.livenewsvideonetwork.com/chris-collura -live-video/
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1229. SykKid
Quoting matara28:
At landfall Isaac will be a monster cat.3 hurricane.GOM water is boilling.

Link

Combine this with the weakening of the wind shear...People must prepare for the worst.


dry air exist though
Member Since: August 17, 2009 Posts: 0 Comments: 451
Quoting Dakster:
SO, this could be Isaac the Oil 'Cane?

NASTY -- and you thought just water was bad....


well BP my have some oil beaches to come back and finish cleaning... the tar mats ( sunk oil) are just off shore...what a mess we will have
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Has nobody posted the 992.8 pressure?


EDIT: Seconds to slow, LOL...
Member Since: December 18, 2006 Posts: 7 Comments: 2687
Quoting E46Pilot:


That Heather girl has got to go though, she is just getting under my skin for some reason.

She is nice looking though.
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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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