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 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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Quoting Hurricanes101:


pretty much

track is headed away from Florida, so no one cares lol
I still feel the models could flip back to the east tonight, especially withe the euro still showing a florida/alabama landfall
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1224. marsHen
the Cedar Keys (NE Gulf, just south of the mouth of Suwannee River) presently has NE winds, 15-18 knts. with gusts over 20 knts.
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I see the NHC Website now has Surge maps displayed right with the forecast track map.
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Quoting Bootsie1:
How come no one ever mentions MS when they talk about this storm or any storm. MS took the brunt of Katrina but seems everyone thanks about LA.

They do mention MS in my experience. Seems Urban populated areas get more attention though just because of the larger population impacted. Had two camps there wiped away by Camille then Katrina. I understand your frustration. All impacted are bonded by these experiences.
Member Since: July 9, 2005 Posts: 2 Comments: 1164
Strengthening Isaac:

Extrap. Sfc. Press: 992.8 mb (~ 29.32 inHg)
Member Since: July 15, 2006 Posts: 17 Comments: 10284
Quoting MississippiWx:


The CMC operational has only shown around a 985mb system.

2 pm
Key West 29.48
Marathon 29.58
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I don't know if it's my eyes playing tricks,but I can see the center of circulation actually moving North west,like in a straight line,this could get the center closer to the west coast of Florida,here in Miami has been a rough day!,a lot of rain and high winds!!,just wondering as Isaac becomes a Hurricane once it cross the Keys and the center will not be terrible far from the coast of we will still feeling bad weather in all south Florida as it moves North.
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1217. Grothar
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One of the issues mentioned by Dr. Masters on TWC is the very large circulation associated with Isaac.
This very large circulation will take more time to intensify (because its so large).
It should be more of a slow and steady intensification.

A smaller system (Charley like)can ramp up much faster.

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Quoting yonzabam:



TCHP is below average throughout the GOM, except for a small patch in the middle which Isaac wont be going over.


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'.
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1214. Dakster
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.


I agree - and I want to know who built the ONE, LONE house that survived... Assuming that house was ORIGINALLY in that spot.
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Quoting LargoFl:
GFS at 18 hours..its useless posting anything today here huh


pretty much

track is headed away from Florida, so no one cares lol
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Isaac's pressure is dropping and he's getting a CDO together, so its only a matter of time before winds start going up. My prediction is that he'll be declared a hurricane at 2AM tonight or 5AM tomorrow morning.
Member Since: February 11, 2012 Posts: 83 Comments: 7839
Quoting weatherh98:


I hope that means no school。

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.
Member Since: July 6, 2010 Posts: 113 Comments: 32286
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.


Amen!! Amen!!! and Amen!!!!
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1209. Hhunter
Quoting weatherh98:


I hope that means no school。


gotta say the weather channel maybe about to put on it's best hurricane coverage show with Northcross, NHC director on alot, DR. M , etc.. Pretty impressive so far..(glory days coverage)
Member Since: August 19, 2007 Posts: 0 Comments: 2973
Texas may come into play with Isaac.
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Most preparation is complete for me. Now, just waiting to see when (or if) the casinos closes. I work at the Hard Rock, so things could get really crazy here shortly.
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1205. LargoFl
GFS at 18 hours..its useless posting anything today here huh
Member Since: August 6, 2011 Posts: 4 Comments: 39309
Quoting ClevelandBob:
Proud of TWC's coverage, they sure are pulling in all the resources they can get.
I don't think Isaac deserved all of the coverage it got, at least till today. They should have waited until Isaac started to get better organized.
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Jefferson Parish Councilman Chris Roberts reported, "Surf already way up on Grand Isle beach, with it is tar balls.'
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Looks like Key West is going to get the (EDIT: northeastern) eastern side of the developing eye as the bulk passes to the southwest of the island.
Member Since: June 18, 2010 Posts: 3 Comments: 946
1201. ITCZ
Latest NHC track has a Cat. 2 #Isaac making landfall near Gulfport, MS Wednesday
Member Since: September 5, 2008 Posts: 0 Comments: 47
Quoting tennisgirl08:
You know what...I like the NHC track. Models are just a guide. NHC uses the guide, but adds a human element.

MS landfall likely, in my opinion.


For the Mississippi Gulf Coast actually a Biloxi - Ocean Springs landfall wouldn't be as bad as a Bay StLouis landfall because of the surge the way Mississipi coast is shaped tend to trap the water and a Hurricane pushing it from the west side would make it all worse giving the water no way to escape back IE Katrina.
Member Since: August 1, 2007 Posts: 0 Comments: 193
Quoting ClevelandBob:
Proud of TWC's coverage, they sure are pulling in all the resources they can get.


That Heather girl has got to go though, she is just getting under my skin for some reason.
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1198. Dakster
SO, this could be Isaac the Oil 'Cane?

NASTY -- and you thought just water was bad....
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Based on objective analysis of average model track error, the UKMET (EGRR) is 3rd at 72 hours and first at 120 hrs. GFS (AVNO) not doing so well


Model Name 0hr 24hr 48hr 72hr 96hr 120hr
OFCL 1.1 61.3 66.4 56.6 84.8 208.7
TVCN 0 63.6 68.1 67.7 92.1 136.8
EGRR 26.7 64.5 93.8 75.3 59.5 67.6
HWRF 7.2 80.5 77.4 79.3 87.8 148.3
GFDL 19.8 59.6 90.7 114.8 161.1 187.6
AVNO 27.5 77.1 120.5 150.9 201.1 265.7
CMC 39.6 67.9 150 161.4 302.7 453.2



Link
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1196. Levi32
12z JMA is with the Euro on landfall:

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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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Quoting GOLSUTIGERS:
I am here in Baton Rouge, watching not looking forward to no power for 5 days.


13 days with Gustav
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1193. JasonRE
Quoting cajunkid:


When was this model run? WOW....that slightly East of Lafayette but might feel a little bit of that if it follows this track.
Member Since: August 6, 2012 Posts: 0 Comments: 122
Quoting TomballTXPride:
18Z SHIPS out. Virtually the same very unsettling data--very ripe upper-level dynamics coupled with high TCHP. It's only detriment to becoming a monster could be the lower RH values in the 500-700 MB level. Other than that, conditions are very conducive for RI along with strengthening Isaac into a MAJOR hurricane before making landfall somewhere along the Gulf coast.

The storm surge could be devastating because of Isaac's enormous size and projected wind-field.

TIME (HR) 0 6 12 18 24 36 48 60 72

SHEAR (KT) 15 13 7 6 6 5 1 13 15
700-500 MB RH 69 65 59 60 63 58 62 54 56



TCHP is below average throughout the GOM, except for a small patch in the middle which Isaac wont be going over.
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1191. yoboi
Quoting jazzygal:
Mandatory Evacuations for all residents of Grand Isle, La. Starting tomorrow morning at 9AM. Per WWL. So it begins.


and good be shifting west at 5pm...
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JUst went outside for a little while ... still sprying [u guys call it drizzle, I guess?] but one can see the sun behind the thin high level clouds... first time in 2 days...

Member Since: October 25, 2005 Posts: 19 Comments: 22322
Quoting franklincounty:
never seen so much wishcasting in my life
Yes, but some folks are just anticipating the worst scenario and hoping for the best, too. I am assuming it's coming up Mobile Bay as a category 4 and stalling with the eye wall on my house (and reading every report trying to prove myself wrong). ;-)
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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


I understand you like it because it shows your eastward track but I will put a stop to this now....What happened with Debby huh lol? GFS Florida Euro Texas...what happened at the last minute....Euro came around to the GFS idea at the last minute right? Okay! Euro can't be discounted but as of now it is an outlier
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1187. ITCZ
Breaking: Public schools in St. Charles Parish have been canceled for Monday, Tuesday and Wednesday. State of emergency declared
Member Since: September 5, 2008 Posts: 0 Comments: 47
1186. pottery
Quoting Hhunter:
He stated a big storm like Isaac can send currents all the way to the ocean floor, this the ploughing affect of the ocean floor and possibly pulling up some oil...

It was never made clear where that oil settled.
As far as I know, fortunately, most of it is on the sea-floor, some 2 MILES below the sea-surface.

If the oil was deposited 100 feet below sea level, I would think a large storm could possibly stir it up.

2 miles down? A mile down? Half-mile down?? I doubt it very much.

Would really love Dr. Masters to make this clear to us all.
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Oil washing up is the least of our worries right now.

It has all been broken down by now.

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Looking at the Key West Radar it appears as though the NW portion of the "eye" wall is about to push through key west
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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.
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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.
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This is nuts.

SHEAR ADJ (KT) -3 4 0 -1 2 -1 6 -2 -1 -2 0 0 6

ADJ. POT. INT. 157 155 159 160 161 152 144 140

Prob of RI for 25 kt RI threshold= 36% is 2.8 times the sample mean(12.8%)
Prob of RI for 30 kt RI threshold= 18% is 2.1 times the sample mean( 8.4%)
Prob of RI for 35 kt RI threshold= 11% is 2.2 times the sample mean( 5.0%)
Prob of RI for 40 kt RI threshold= 7% is 2.2 times the sample mean( 3.4%)


SHIPS
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1179. LargoFl
GFS at 15 HOURS..is THIS when we get Our taste of this storm?...look how the yellow reaches into central florida..................
Member Since: August 6, 2011 Posts: 4 Comments: 39309
Quoting MAweatherboy1:
Rick Knabb just said warnings will be going up for the Gulf coast soon.
yes he did at 5pm today to help with any evacuation orders
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1177. yoboi
stores in la starting to see a big surge....get ya stuff now before the crowds.....
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Quoting Levi32:
Interesting that the group of CMC members, of all things, don't strengthen Isaac beyond Cat 2 in general.

god i hate the cmc model lol almost as much as the nogaps. first it was going west of bermuda than east coast of florida and now mississippi. so what are your thoughts on isaac right now levi?
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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.