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 angelafritz:


You make it sound like this storm has been forecast to get strong over and over and it never plays out. Which is not the case.

I think everyone in this blog needs to take a deep breath. Or two.
LOL Yes this blog is full of released, pent up energy. It's a bit infortunate, really. i posted a piture of my house under my photos if anyone wants to take a gander... if it's approved
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Quoting Grothar:

houston landfall?
o_o
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All this talk of Texas landfall is fairly ridiculous we are less then three days away from landfall. Typically NHChas very good track of less then 72 hours which is within the 100 mile region. I would guess this to go from new Orleans to Alabama or some where in the middle. Frankly I will be surprised if Galveston even sees a small sprinkle from this
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Quoting flcanes:

my comment was about the northern gulf coast, though


People here are so quick to blast a post. The previous poster posted an opinion of the storm for where HE/SHE LIVES. They did NOT say anything about where it may go and what it would be like when it gets there.
Why couldn't you have seen that without having to post a comment about the people in the gulf and what they think?
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Raining hard here, still vertical, in pretty far South Dade (near the Cutler Ridge Mall) but hasn't yet reached the "who turned the fire hose on my windshield" stage. As for the school closings, IMHO, the school officials made the prudent, safe call for thousands of kids and we're now getting lucky. (Besides, we've all survived the first week of school, we deserve a day off. :) )
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Quoting angelafritz:


You make it sound like this storm has been forecast to get strong over and over and it never plays out. Which is not the case.

I think everyone in this blog needs to take a deep breath. Or two.


It's had it's fair share of issues, but definitely getting healthier by the minute. The moisture shield on radar has been increasing, and now that it's cleared the northern part of the island we're starting to see banding on the southern part of the circulation. I think it's been steadily restructuring the past 12 hours, should begin strengthening soon, core is looking much healthier...
Member Since: December 18, 2006 Posts: 7 Comments: 2685
Quoting angelafritz:


You make it sound like this storm has been forecast to get strong over and over and it never plays out. Which is not the case.

I think everyone in this blog needs to take a deep breath. Or two.





heh heh heh good post
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Quoting angelafritz:


You make it sound like this storm has been forecast to get strong over and over and it never plays out. Which is not the case.

I think everyone in this blog needs to take a deep breath. Or two.


This storm was forecasted to be a Hurricane prior to reaching Haiti. Then it was forecast to be a Hurricane before reaching Key West.

So the intensity forecast has been greater than reality (so far).

That can quickly change but Isaac has been weaker than forecasted (so far).
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Quoting angelafritz:


You make it sound like this storm has been forecast to get strong over and over and it never plays out. Which is not the case.

I think everyone in this blog needs to take a deep breath. Or two.

You're asking too much of the blog to stay calm when a storm is threatening the USA, lol.
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isaac is getting better and better organized
Quoting washingaway:
Low Cloud Product
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Quoting angelafritz:


You make it sound like this storm has been forecast to get strong over and over and it never plays out. Which is not the case.

I think everyone in this blog needs to take a deep breath. Or two.


After the last couple of days of attempting to forecast Isaac's track, I certainly am.
Member Since: November 24, 2005 Posts: 628 Comments: 26465
Quoting angelafritz:


You make it sound like this storm has been forecast to get strong over and over and it never plays out. Which is not the case.

I think everyone in this blog needs to take a deep breath. Or two.
Agree.
Member Since: August 1, 2011 Posts: 23 Comments: 7508
Quoting Levi32:
This storm is a huge win for the ECMWF, redemption for its performance with Debby, as it led the models into the Gulf of Mexico from the very beginning.

It is so ridiculous to have two storms like this in one year.


Hi Levi,

South Floridian here. Just wanted to say thanks for all you do! I follow your video blogs and they have been very useful in helping me understand more about weather patterns, etc.

Getting some bands here in North Palm Beach. Nothing major yet, but keeping an eye on the radar.

Thanks again for keeping me updated!

[Edit] Can someone tell me how to add a user as a favorite? Is this possible? I clicked 'Add to favorites' next to the users name, but it's not showing up when I select 'favorites' at the top of the page. TIA.
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Closest buoy to Isaac
Conditions at SMKF1 as of
(12:00 pm EDT)
1600 GMT on 08/26/2012:
Unit of Measure: Time Zone:

Click on the graph icon in the table below to see a time series plot of the last five days of that observation.
5-day plot - Wind Direction Wind Direction (WDIR): ENE ( 60 deg true )
5-day plot - Wind Speed Wind Speed (WSPD): 36 kts
5-day plot - Wind Gust Wind Gust (GST): 40 kts
5-day plot - Atmospheric Pressure Atmospheric Pressure (PRES): 29.54 in
5-day plot - Pressure Tendency Pressure Tendency (PTDY): -0.09 in ( Falling )
5-day plot - Air Temperature Air Temperature (ATMP): 78.1 °F
5-day plot - Dew Point Dew Point (DEWP): 77.7 °F
5-day plot - Tide (above MLLW) Tide (above MLLW) (TIDE): 1.08 ft
5-day plot - Heat Index Heat Index (HEAT): 80.8 °F
5-day plot - Wind Speed at 10 Meters Wind Speed at 10 meters (WSPD10M): 31 kts
5-day plot - Wind Speed at 10 Meters Wind Speed at 20 meters (WSPD20M): 33 kts
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Sand Key Light weather station is finally back up and running. Awesome!

Link
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Quoting 145:
PalmBeachWeather, no one asked you for your 'Northern Gulf' opinion, I just simply made my comment about what I think about the storm in ft. Lauderdale.

my comment was about the northern gulf coast, though
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Low Cloud Product
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Quoting Levi32:
This storm is a huge win for the ECMWF, redemption for its performance with Debby, as it led the models into the Gulf of Mexico from the very beginning.

It is so ridiculous to have two storms like this in one year.


Not yet. It has been saying west while everyone else said east. Now everyone is saying west and it is saying east. It could end up being the wrost peformer on Isaac
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Quoting Articuno:


It can't be weakening...
If it is, then it's like ernesto, good satellite presentation but bad winds.

Maybe I should call this Trollsaac?

It's not weakening. They've not sampled the northern side yet. As it stands, winds remain 65 mph.
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Quoting TropicalAnalystwx13:

Latest compter model runs have shifted significantly west in the past six hours, and the threat of a strike on New Orleans by Katrina as a major hurricane has grown. The official NHC forecast is now 170 miles west of where it was at 11am, and still is to the east of the consensus model guidance. It would be no surprise if later advisories shift the forecast track even further west and put Katrina over New Orleans. Until Katrina makes its northward turn, I would cast a very doubtful eye on the model predictions of Katrina's track. So much for the model prediction being high confidence, as I was surmising at 8am this morning! Recurvature is a difficult situation to forecast correctly.

The pressure of Katrina has continued to slowly drop, to 965mb. Dry air on the northwest side of the hurricane has interfered with the strengthening process, and may continue to do so over the next day. I still expect Katrina to attain Category 3 status Saturday, but Category 4 is looking less likely due to the dry air to the north. As one can see from the latest long range radar out of Key West, the northwest side of the eyewall is fragmented.

Some fairly prodigious rain amounts fell in the Miami area today. Homestead south of Miami measured 13.2 inches, and isolated amounts of 15 - 20 inches were observed between Homestead and Miami. The 7.55 inches at Key West was its 10th heaviest rainy day in history.

Jeff Masters


Gives me chills to read that....I hope and pray that this is not another "K"...
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Quoting nofailsafe:


Well this is a fantastic way to start the school year here at Rice off. Welcome freshmen! Here's a hurricane!


I know. my daughter starts her final semester at Loyola, New Orleans tomorrow. The kids just moved in Thursday and Friday. same thing for them.
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Quoting ProgressivePulse:
Those that are interested in the status of Palm Beach County Schools.

Any changes in school operations for Monday, August 27 will be announced tomorrow, Sunday, following the county’s 2 p.m. Emergency Operations Center briefing.


This is what Broward should have done!
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Quoting nofailsafe:


Well this is a fantastic way to start the school year here at Rice off. Welcome freshmen! Here's a hurricane!


We usually had to pull ours out of school in September here. This would be an early one. My youngest is out now though.
Member Since: August 24, 2010 Posts: 0 Comments: 459
Quoting SykKid:


Winds are NOT as strong so stop it


You are clueless. Anyone can see that this storm is pulling together.
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People here are just going to have to accept that Isaac was a dud for Miami and South Florida. If all of you would pay attention to what the storm had being doing and is doing, not what you thought it should be doing, you would have come to that conclusion a long time ago!
He has been a large TS in terms of size but nothing more than that!
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250. angelafritz (Admin)
Quoting SykKid:


why? isaac is struggling greatly right now everytime he's forcasted to get stronger he never does.


You make it sound like this storm has been forecast to get strong over and over and it never plays out. Which is not the case.

I think everyone in this blog needs to take a deep breath. Or two.
Quoting DataNerd:
Link


Eye is elongated and winds have fallen some.


Time: 16:08:00Z
Coordinates: 23.3167N 79.1167W
Acft. Static Air Press: 842.5 mb (~ 24.88 inHg)
Acft. Geopotential Hgt: 1,541 meters (~ 5,056 feet)
Extrap. Sfc. Press: 1005.0 mb (~ 29.68 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 166° at 40 knots (From the SSE at ~ 46.0 mph)
Air Temp: 17.9°C (~ 64.2°F)
Dew Pt: 16.2°C (~ 61.2°F)
Peak (10s) Flt. Lvl. Wind: 41 knots (~ 47.1 mph)
SFMR Peak (10s) Sfc. Wind: 42 knots (~ 48.3 mph)
SFMR Rain Rate: 1 mm/hr (~ 0.04 in/hr)


It can't be weakening...
If it is, then it's like ernesto, good satellite presentation but bad winds.

Maybe I should call this Trollsaac?
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Quoting TomballTXPride:
Good long range radar link.

Link

thx
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246. 145
Quoting PalmBeachWeather:
Do you think the people in the northern gulf think it is a bust 145?
PalmBeachWeather, no one asked you for your 'Northern Gulf' opinion, I just simply made my comment about what I think about the storm in ft. Lauderdale.
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the south side is trying to create a barrier from the dry air. its a pretty ugly day here in miami. as levi noted, if he develops an inner core and mixes out the dry air, he has plenty of time over the gulf to get much stronger than the NHC and especially if he hits a warm eddy. the ECMWF has won the battle in terms of having isaac go into the gulf all along but the war of the eventual landfall is still a mess. people along the gulf coast should prepare for at least a cat2 to a 4.
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Quoting Grothar:


Yes.

idk
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Hi all! Long time lurker, occasional commenter here...

Wondering if anyone has any webcam links for the Keys? Also, is there a link to watch The Weather Channel online? I've tried the site, but it doesn't seem to have anything live. Thanks in advance, and stay safe!
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Quoting Hurricanes101:


looks like he is moving NW to me

and expect a possible shift northward through the keys if he continues to wrap up

agreed
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Quoting raggpr:
is anyone having problems with http://raleighwx.americanwx.com/ page?


Yes.
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Quoting Chiggy:


Where are you located (roughly) in TX?

I am in Sugar Land but work at NASA-JSC - Will have to watch this carefully as even though we may likely to be on the West (weak) side of Issac (IF GFS pans out) NASA will probably begin to shutdown from Tues/Wednesday....!?

GFS keeps moving that little bit West over the past three run. Consistency is there..


Yeah it does keep moving west. I'm on the TX/LA border in Orange County. I don't know whether to hope for a model consensus now or not.
Member Since: August 24, 2010 Posts: 0 Comments: 459
Quoting flcanes:
Quoting AtHomeInTX:


OK Everyone. It's called the Missouri Dome this year.
?
Because Missouri was at the heart of the drought and the heat high most of the summer.
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State of Emergency for Plaquemines P. per WWL
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Quoting AtHomeInTX:


Thanks I didn't know how west it got. sigh...guess EURO'S next.


Well this is a fantastic way to start the school year here at Rice off. Welcome freshmen! Here's a hurricane!
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Quoting MiamiHurricanes09:
Plenty of tropical storm-force gusts on a consistent basis. Rain is very light though.

quite the contrary here
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2pm to 7pm worst weather for dade
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Quoting Clearwater1:
Nice long range radar out of Key west, clear shot at direction and it's nearly closed eye will pass directly over several of the keys. It's a large area.


looks like he is moving NW to me

and expect a possible shift northward through the keys if he continues to wrap up
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Isaac is definitely moving more to the NW than it was earlier this morning. This may be temporary, but just interesting to note:

Member Since: December 18, 2006 Posts: 7 Comments: 2685
Quoting leftlink:


The radar now has enough frames offshore to show that the path is northwest, and will hit or go just barely west of Key West (4pm). This is confirmed by a much faster pressure drop at Key West buoy. Last reading was 11:42am and the pressure was 1002Mb. Updates every 6 minutes:

http://www.ndbc.noaa.gov/station_page.php?station =kywf1


So we can see from these pressure readings whether the storm is able to take advantage of its movement to open water.


Yeah, looks like KW may take the RFQ of a minimal cat1. Closest RQF hit for KW in quite some time. Also a deepening storm will put winds down to surface.
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Quoting goldenpixie1:


You can understand what Max Mayfield is saying? Wow! He just mumbles along. For me, it's like listening to that teacher from the Peanuts cartoons.


LOL! I do have to turn the volume up signifantly to hear him and then down again after he's done!
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Nice long range radar out of Key west, clear shot at direction and it's nearly closed eye will pass directly over several of the keys. It's a large area.
Member Since: August 26, 2006 Posts: 0 Comments: 1547
226. wpb
Quoting gulfshoresAL:
Excellent explanation...appreciate all you do and totally respect your input.
Look forward to your next tidbit. Thanks...have a great day.
he is very very good plus he effort is aaa
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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.