Isaac is strengthening

By: Dr. Jeff Masters , 2:58 PM GMT on August 24, 2012

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Tropical Storm Isaac is strengthening. An Air Force Reserve hurricane hunter aircraft measured surface winds of 60 mph on the east side of the center, about 170 miles south of Santo Domingo in the Dominican Republic, at 8:40 am EDT this morning. Winds at the aircraft's flight level of 5,000 feet were hurricane force, 76 mph. The surface pressure remained fairly high, at 1000 mb. Tropical cyclones have a warm core, and the Hurricane Hunters typically find that a storm's lowest pressure is also where the warmest temperature are. However, this morning's flight found that Isaac was still disorganized, with the storm showing almost no evidence of a warm core. Isaac's warmest temperatures were displaced 75 miles to the west of where the lowest pressure was. There were no signs of an eyewall beginning to build. Infrared and visible satellite loops show that Isaac is somewhat asymmetric, with a large band of intense thunderstorms to the east, separated from the core region. This is interfering with both the storm's low-level inflow and upper-level outflow, but the band appears to be dying out. An analysis of upper level winds from the University of Wisconsin CIMSS shows an upper-level outflow channel well-established to the north, and an intermittent outflow channel to the south.


Figure 1. Evening shot of Tropical Storm Isaac taken on August 23, 2012, by the NOAA Hurricane Hunters.

Isaac's rains
Radar imagery from Puerto Rico shows that Isaac is dumping some very heavy rains to the south and east of the center. Ponce, Puerto Rico had a wind gust of 37 mph this morning as a heavy band of rain moved through, and radar-estimated rainfall amounts are in excess of 7 inches for the region just north of Ponce. Power outages to 2,000 homes have been reported in Puerto Rico this morning. NOAA buoy 42085 offshore from Ponce reported a wind gust of 54 mph near 9 am EDT this morning. Rainfall estimates from microwave satellite instruments suggest that Isaac's heaviest rains are to the south of the center, and that the Dominican Republic and Eastern Haiti will escape the worst of Isaac's rains. Haiti's southwest peninsula and Eastern Cuba should suffer the heaviest rains.


Figure 2. Radar-estimated precipitation from the Puerto Rico radar shows the region near Ponce has received up to 7" of rain as of 10 am EDT August 24, 2012.

Latest model runs for Isaac
The latest set of 00Z and 06Z (8 pm and 2 am EDT) model runs have come into better agreement, thanks to the dropsonde mission by the NOAA jet yesterday afternoon and evening. Isaac should move over Haiti's southwest peninsula and then eastern Cuba, then track along the spine of Cuba before popping off into the Florida Straits on Sunday. A trough of low pressure will then pull Isaac to the northwest, and then north, towards the Central Gulf Coast. Landfall locations range from Mississippi (06Z HWRF model run) to the Florida Panhandle south of Tallahassee (06Z GFDL model run.) It is possible that the trough of low pressure pulling Isaac to the north may not be strong enough to pull Isaac all the way to the northeast and out to sea, and the ECMWF model indicates that Isaac could stall out after landfall over the Tennessee Valley for several days.


Figure 3. Predicted 5-day rainfall total ending at 2 am EDT Wednesday August 29, from Tropical Storm Isaac. Graphics were generated from the 6Z (2 am EDT) August 24, 2012 run of the HWRF model. Image credit: Morris Bender, NOAA/Geophysical Fluid Dynamics Laboratory.

Intensity forecast for Isaac
Isaac has not intensified as much as predicted, and I think that the storm's very large size is partially responsible for that. It's tough to spin up as much atmosphere as Isaac is attempting to do very quickly. Conditions remain favorable for intensification today, with wind shear low, 5 - 10 knots, ocean temperatures warm, 29°C, and dry air mostly mixed out of the storm's core. The large band of intense thunderstorms to the east, separated from the core region, appears to be dying out now, which will help the storm grow more organized. The storm's structure has improved considerably between 9 am - 10 am EDT, with a fairly tight center forming, exposed to view, on the north edge of Isaac's heaviest thunderstorms. A curved band of heavy thunderstorms is now trying to wrap around this center to the northeast, and this band will bring very heavy rains to Haiti and the Dominican Republic this afternoon. I expect that the Hurricane Hunters will observe a partial eyewall in their vortex reports between 2 - 4 pm EDT this afternoon. The storm's large size and disorganized structure suggests that Isaac will be able to intensify only slowly today, and will have top winds of 70 - 75 mph before encountering Southwest Haiti and Eastern Cuba tonight and Saturday. Isaac will likely be a 50 - 60 mph tropical storm on Saturday and Sunday as it moves over Cuba. Once Isaac pops off the coast of Cuba into the Florida Straits, it will be over very warm waters of 31 - 32°C (88 - 90°F), wind shear will be light to moderate. The upper-level wind pattern favorable may be quite favorable for intensification, with low wind shear due to an upper-level anticyclone over the storm--though the models disagree on whether or not this anticyclone will set up directly over Isaac or not. It will probably take at least 24 hours with the storm's center over water for it to become a hurricane. It is possible that Isaac could be approaching Category 3 strength by the time it makes landfall on Tuesday on the Gulf Coast, as suggested by the latest 06Z run of the HWRF model.

Impact on Tampa, Florida
The Republican National Convention begins on Monday in Tampa, Florida. The latest 11 am EDT wind probability forecast from NHC gives Tampa a 17% chance of receiving tropical storm-force winds and a 1% chance of receiving hurricane-force winds on Monday. Tampa is in the NHC cone of uncertainty, though near the edge of it. At a minimum, Tampa will receive very heavy rains and wind gusts in excess of 40 mph. Isaac is going to be hard-pressed to bring hurricane-force winds to the city, though, since any path that takes it close to Tampa would keep the storm too close to land for significant intensification to occur. I put the odds of a mass evacuation being ordered for Tampa during the convention at 1%. I have detailed information on Tampa's storm surge vulnerability in a post from last week.

Invest 97L off the coast of Africa
A tropical wave that moved off the coast of Africa on Thursday has been designated Invest 97L by NHC this morning. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear of 10 - 20 knots. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 30% chance of developing by Sunday morning. The 8 am EDT SHIPS model forecast predicts that 97L will track west-northwest over the next few days, and encounter a region of high wind shear associated with an upper-level low on Monday and Tuesday. This low may be capable of tearing the storm apart, as happened to Tropical Storm Joyce. None of the models currently foresee that 97L will affect the Lesser Antilles Islands, but 97L may pass near Bermuda 7 - 8 days from now.



20th anniversary of Hurricane Andrew
Today, August 24, is the 20th anniversary of Hurricane Andrew, which hit South Florida as a Category 5 hurricane with 175 mph winds--one of only three Category 5 hurricanes ever to hit the U.S. With Isaac churning through the Caribbean this week, I didn't have time to prepare a special post on Andrew, but our Hurricane Andrew archive page has links to satellite and radar images, newspaper headlines, and 49 YouTube videos. Here's an additional link for an Andrew damage video shot by wunderblogger/storm chaser Mike Theiss, when he was 14 years old.

Jeff Masters

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Quoting SFLWeatherman:
New GFS is on S FL now??
yes
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Been watching for awhile and been on this blog many times and man you guys (except a select few) are on the ball and smart as hell!! I am a met in the Navy stationed at NAS JAX however I live near Starke, FL and I have been watching this and watching the models bounce all over as well.
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Quoting yoboi:
how many peeps from fla in here???


Port St. Lucie here
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Ft. Lauderdale here still gonna watch til its north of us
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Quoting SWLACajun:
Dirty side is NE quadrant correct?
Correct, because of the forward speed of the storm pushes the heavy thunderstorms out to the right.
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No worries folks! The Uber-Rich down here in Naples will be activating their weather shield so issac (whatevr is left of him) will bounce rite off & up the coast! Someone was asleep at the controls when Wilma was approaching otherwise...
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16.1N 70.7W if that area is the LLCOC it is moving or wobbled now W it looks like
Member Since: June 13, 2009 Posts: 2 Comments: 12716
302. 7544
Quoting LargoFl:
that guy in ft lauderdale had better stay tuned, he is not out of the woods yet


yeap the gfs has been on south fla all week you are nuts to let your guard down at this stage of the game
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I wonder if this model shift has anything to do with all of the data they were waiting on from the Gulf stream aircraft?
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Quoting SWLACajun:
Dirty side is NE quadrant correct?


Correct... you don't want to be on the NE/E/SE periphery of a tropical system.
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Quoting WPBHurricane05:
Right over St. Pete at 78 hrs.
yes plywood goes up early this weekend, watching the timing and speed of the storm, man i have alot of things outside to bring in..whew
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Quoting Torgen:

Brandon here. Guess I'm logging out to stack crap in the garage. Anyone want an old bicycle?



Brooksville here
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Quoting NICycloneChaser:


That's exactly what the 12Z GFS is showing. Has Isaac off the north coast of Cuba by 36 hours.



How long ago did the latest GFS come out.

Also in another couple of hours heavy thunderstorms will likely completely surround the center as they are now 3/4 wrapped around now.
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Biggest difference here is no joyce, no joyce = farther east
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New GFS is on S FL now??
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Tormenta tropical Isaac. Una foto de como esta inundada la panamericana a la altura de San Antonio




Tropical Storm Isaac is having an impact on two more cruise ships sailing out of Florida.

Royal Caribbean says the 2,350-passenger Majesty of the Seas and 2,390-passenger Monarch of the Seas will not call this weekend at CocoCay, the line's private island in the Bahamas. The ships are setting sail today from Florida's Port Canaveral and Port of Miami, respectively.

The announcement brings to five the number of vessels affected by Isaac. Industry giant Carnival's 2,974-passenger Carnival Valor skipped a call Thursday in Curacao to stay ahead of the storm. The ship instead is today visiting Nassau in the Bahamas. Another Carnival ship, the 2,978-passenger Carnival Liberty, also is visiting Nassau today after skipping a call Thursday in Grand Turk.

Also on an altered course this week due to Isaac is Royal Caribbean's 5,400-passenger Allure of the Seas, which departed from Fort Lauderdale on Sunday. The world's largest cruise ship has sailed for the Western Caribbean instead of heading to the Eastern Caribbean as planned. It's visiting Falmouth, Jamaica and Cozumel, Mexico instead of St. Thomas and St. Maarten.

As of 11 a.m. ET, Isaac was 165 miles south-southwest of Santo Domingo, Dominican Republic and moving to the west-northwest at 14 miles per hour. The storm has maximum sustained winds of 60 miles per hour.
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996 mb. strong TS/ Cat. 1 Hurricane over Tampa Bay:

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i think the 12z GFS is too far east initially. has it over s florida then into the gulf.
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Quoting airforceone:


Tyndall Air Force Base, Panama City, FL


My uncle worked there in communications I believe until he retired a few months ago. Beautiful area!
Member Since: September 2, 2006 Posts: 110 Comments: 6878
Quoting chevycanes:
850 mb vort at 63 hrs

well we are..going to get whacked,anyone not prepared as of yet..better get to the stores pronto..when this hits the local news whenever..shelves will be empty
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Levi, if you're aboard this morning, could you address the steering pattern setting up for the CV storms? From the looks of Joyce and the doc's discussion about the new invest it appears the situation is not favorable for development of CONUS affecting CV storms in the near future. I'm guessing we're safe from CV storms for now.

And since El Nino isn't performing as predicted, are we now under a greater risk of getting slammed by a backyard storm?
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Dirty side is NE quadrant correct?
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Right over St. Pete at 78 hrs.
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Until Recon takes off, the National Data Buoy Center has a ship taking observations 12 Nautical Miles away from Tropical Storm's Issac Current Position.

Link
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Quoting CypressJim08:


Me too. I am definately watching with interest. It looks like we will be on the "dirty" side of storm. A s it stands it looks like we will be getting copious amounts of rain, and possibly tropical storm winds. Stay safe!


According to the latest 12Z GFS model run at 66 hours we're going to get pounded by the center. At least by a Cat 1. or maybe even Cat. 2 hurricane. The GFS shows Isaac going over a lot more water, rather than going over Cuba.

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Quoting ncstorm:
from the previous blog

Isaac Track Window
Aug 24, 2012; 7:18 AM ET

Commentary

1. Isaac appears to be going through an intensification stage this morning based on the satellite and radar data. When you see the outflow showing the gravity waves on the satellite image, that usually means good venting aloft and intensification of the storm. Just an observation I have seen over the years.

2. Below is a satellite image showing the water vapor channel. Notice the trough over Colorado. I believe that trough is the key to the track of Isaac as it comes off Cuba. The other track map shows the two possible tracks Isaac can take. If the trough is weaker and goes north, Isaac will end up in the eastern Gulf of Mexico. If the trough is stronger and goes east, it will bring Isaac up the East Coast of Florida which I have been in the camp for happening all week. I know none of the models show that possibility, but I am going to hold on to the track until I see otherwise. Yes, I know I am going out on a limb and against all models, but it just seems to me that a track into the Gulf seems odd given the trough in the Rockies and the trough associated with Joyce. I believe that Isaac will try to squeeze between the two up east of Florida.

3. Once Isaac does clear Cuba, the storm will be in position to go through another rapid intensification period. It may go to a Category 3 hurricane before making landfall. Isaac should not be taken lightly because in my mind, the storm has a lot of potential to do a considerable amount of damage and flooding once it impacts the U.S.


You will though get much gratitude and recognition from Levi32 if it turns out that you are right. :-)
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277. dezli
Quoting trey33:



Downtown Tampa here


Melbourne Florida, but son in college in Tampa!
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Are you guys kidding me? seriously?

I leave for awhile with the track in the GOM and now some models are coming back east????

is that for real????

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This run is all Florida all the time. What a miserable scenario with the E eyewall just raking the whole W coast.
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Last comment before I depart for real:

I just want to remind about the "sweet spot" in between Cuba, the BAhamas, and the Keys, where several Twaves blew up suffeciently earlier this season to get AOIs and coloured circles from the NHC, despite having little more than the typical wind shift associated w/ Twaves. If Isaac gets off Cuba east enough in that area, with time to move through most of that warm eddy, I can suggest we might see more rapid rebuilding than expected.

BBL
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I gotta catch up. Question. For the latest GFS thats currently running did it initialize the COC correctly?
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init 12z gfs

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Look at GFS, see it hitting miami, come here to see blog explode :p
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Quoting 7544:
SORRY BUT OUCHHHHHHHHHHHHHHH MIAMI IT IS !!!!!!
that guy in ft lauderdale had better stay tuned, he is not out of the woods yet
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Quoting airforceone:


Tyndall Air Force Base, Panama City, FL


Boca Raton, FL
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850 mb vort at 63 hrs

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Right over key Largo then to Naples...
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latest few visible frames are hinting at yet ANOTHER ISAAC Western jog....
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Directly over Tampa by Monday.
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264. HCW
Expect another 20 to 30 mile shift in the NHC track at 5pm to the east
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Please help me sort this out - what's the dirty side of the storm? And in the case of Isaac and it's current track - what cities would be on the dirty side? We've been arguing this out at work and can't come to a consensus.Thanks for your help!
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Quoting Grothar:
GFS consensus model

wow almost the whole east coast is going to get a piece of issac
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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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