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 Grothar:
GFS consensus model

wow almost the whole east coast is going to get a piece of issac
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Quoting floridaboy14:
GFS is further east at the begining but isaac looks to be bending back wnw on the run

but not before moving ashore in s. fl.

vorticity maps on ncep clearly show it.
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GFS consensus model

Member Since: July 17, 2009 Posts: 71 Comments: 27107
The deforested mountains of Haiti are drawing in Isaac.
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Quoting yoboi:
how many peeps from fla in here???


Tyndall Air Force Base, Panama City, FL
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Quoting FLWeatherFreak91:
Starting to look like that worst scenario track for Tampa... just saying. People need to get less complacent around this city...


Better to learn from a TS/cat1 than a Cat3 or higher.
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GFS is further east at the begining but isaac looks to be bending back wnw on the run
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hahaha i so meant to say skirting lol
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253. FOREX
Quoting CaneHunter031472:


In short, if you live in Panama City or east of Panama City FL. You should be starting to get ready for a Mayor impact. Westo of Panama City can fell a bit better about the odds, but still need to keep an eye on it. This is a Panama City event


I do live there, but am hoping the latest GFS is showing a more Easterly track bringing Isaac East of my area.
Member Since: August 17, 2011 Posts: 0 Comments: 2335
Quoting trey33:



Downtown Tampa here

Brandon here. Guess I'm logging out to stack crap in the garage. Anyone want an old bicycle?
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Now that she's a colleague Of Jeff Masters....I better be careful what I say about Stephanie...lest I rack up a penalty....
Member Since: August 13, 2007 Posts: 0 Comments: 10492
12z 60hr shows Isaac hitting Miami then squirting off to the WNW... hmmm..
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SE FL landfall this time.

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Quoting reedzone:
NHC remains west of the model consensus.. They just don't want to let go of their precious EURO model (kidding), which remains west to Alabama.. Not gonna happen. In time, the track will shift to the east. A storm CANNOT bust into the corner of a building ridge. It's impossible. My opinion, Isaac is unfortunately Tampa bound. I see Isaac hugging the west coast of FL, then recurving as soon as it reaches Tampas latitude.


That is a disctinct possibility! Just have to keep our eyes on it . The models are in the "windshield wiper" mode!!!
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Quoting StormTracker2K:
SE FL is going to get hammered.

Right at Miami this run.
ST i have a very bad feeling for us here in central florida with this storm
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Isaac seem like is moving more to the NNW than the WNW for now
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Quoting FOREX:


Yes, Stephanie Abrahms is there and is really pretty in her pink outfit.


Dang, I missed that! only saw the trucks.
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Quoting CaneHunter031472:


In short, if you live in Panama City or east of Panama City FL. You should be starting to get ready for a Mayor impact. Westo of Panama City can fell a bit better about the odds, but still need to keep an eye on it. This is a Panama City event


Last I heard, Isaac was still going rather Westerly. I wonder how big a wrench that might throw into projections if it continues a bit longer like that.
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Quoting reedzone:
NHC remains west of the model consensus.. They just don't want to let go of their precious EURO model (kidding), which remains west to Alabama.. Not gonna happen. In time, the track will shift to the east. A storm CANNOT bust into the corner of a building ridge. It's impossible. My opinion, Isaac is unfortunately Tampa bound. I see Isaac hugging the west coast of FL, then recurving as soon as it reaches Tampas latitude.


Nope. They'll kick on the shields and Isaac will do this :P

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Quoting alvarig1263:


I'm from Naples, FL on the SW coast.


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!
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Same thing happened with Wilma if I remember correctly she was forecasted to come up along the west coast through tampa and ended up cutting right surprised the SE coast.
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Quoting StormTracker2K:
GFS is further north and east again. Miami watchout!!!



While it is further north and east at the start of the run, the trough actually looks weaker later on, so all the further north position will do is give more time over water, and less land interaction.



In this run, Isaac should hit the Keys before moving into the eastern Gulf.
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Quoting ChillinInTheKeys:
Just got back from Key West to my shop on Upper Sugarloaf Key. Must of had some decent wind from that squall line that came thru here earlier. One pine tree and a lot of branches down, doesn't look like much rain though.
While in town I see Miami Channel 10 and TWC have moved in, also lots of billboards coming down.
good to see they down there are preparing
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Quoting 12george1:

If consensus is the central Panhandle of Florida, then why does the forecast track go near the Florida/Alabama border?


I don't know.
Member Since: July 17, 2009 Posts: 71 Comments: 27107
234. 7544
1
2
3stronger this run further east like 80 miles here we go
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233. FOREX
Quoting ChillinInTheKeys:
Just got back from Key West to my shop on Upper Sugarloaf Key. Must of had some decent wind from that squall line that came thru here earlier. One pine tree and a lot of branches down, doesn't look like much rain though.
While in town I see Miami Channel 10 and TWC have moved in, also lots of billboards coming down.


Yes, Stephanie Abrahms is there and is really pretty in her pink outfit.
Member Since: August 17, 2011 Posts: 0 Comments: 2335
12z GFS has shifted even more to the east... according to this run, it's a Miami storm... hmmm... interesting... Does anybody have a good site to look at the EURO as it comes out? thanx
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High cirrus clouds now showing up. Outflow???

Member Since: July 17, 2009 Posts: 71 Comments: 27107
You can see where center is clear now on this image.Link
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Quoting Catgypsy:
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.

Thanks Doc.


It all depends on the conditions aloft. If it does not have high pressure over it, it will not stregthen a whole lot regardless of how warm the waters of the GOM may be. UL conditions are more important in development of a tropical system.
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Quoting WalkingInTheSun:


How does that look then if the storm goes RI & much larger than expected?
Also, what about the possibility of the ridge shrinking or not being as strong as expected?
...or all of the above?
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SE FL is going to get hammered.

Right at Miami this run.
Member Since: October 26, 2011 Posts: 0 Comments: 2651
Quoting Grothar:

If consensus is the central Panhandle of Florida, then why does the forecast track go near the Florida/Alabama border?
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A lot of DOOM sayers lately hmm?

Anyway, I would love to see this in VIS.
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222. FOREX
Quoting bucyouup68:


Sitting Patiently in St Petersburg, On the water.


I live in Panama City Beach.
Member Since: August 17, 2011 Posts: 0 Comments: 2335
Quoting reedzone:
NHC remains west of the model consensus.. They just don't want to let go of their precious EURO model (kidding), which remains west to Alabama.. Not gonna happen. In time, the track will shift to the east. A storm CANNOT bust into the corner of a building ridge. It's impossible. My opinion, Isaac is unfortunately Tampa bound. I see Isaac hugging the west coast of FL, then recurving as soon as it reaches Tampas latitude.


How does that look then if the storm goes RI & much larger than expected?
Also, what about the possibility of the ridge shrinking or not being as strong as expected?
...or all of the above?
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I believe Levi said it best that storms tend to be 'sucked' into the mountains of haiti/DR. And by that jolt to the NW, i'd say thats exactly whats happening, thanks Levi.
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Just got back from Key West to my shop on Upper Sugarloaf Key. Must of had some decent wind from that squall line that came thru here earlier. One pine tree and a lot of branches down, doesn't look like much rain though.
While in town I see Miami Channel 10 and TWC have moved in, also lots of billboards coming down.

Think I saw the Village People at the Home Depot.
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Member Since: October 26, 2011 Posts: 0 Comments: 2651
Quoting bucyouup68:


Sitting Patiently in St Petersburg, On the ewater.



Downtown Tampa here
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216. 7544
im getting ready to scream whos with me
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Quoting Elena85Vet:
What's all that scurrying about noise coming from the dirrection of Panama City, FL?


In short, if you live in Panama City or east of Panama City FL. You should be starting to get ready for a Mayor impact. Westo of Panama City can fell a bit better about the odds, but still need to keep an eye on it. This is a Panama City event
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Further E than what I expected @42HR:



Looks like the incoming TROF is a bit more stronger on this one:

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GFS is further north and east again. Miami watchout!!!

Member Since: October 26, 2011 Posts: 0 Comments: 2651
Quoting 7544:
if the gfs shows miami landfall this run im going to scream lol looks to be going more north this run so far


Florida is gonna get hammered according to the 12Z GFS. This slight intensification to 60 MPH is really affecting the models.
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Quoting reedzone:
NHC remains west of the model consensus.. They just don't want to let go of their precious EURO model (kidding), which remains west to Alabama.. Not gonna happen. In time, the track will shift to the east. A storm CANNOT bust into the corner of a building ridge. It's impossible. My opinion, Isaac is unfortunately Tampa bound. I see Isaac hugging the west coast of FL, then recurving as soon as it reaches Tampas latitude.


Ya know what? If I had to bet, I'd bet you're pretty close to right...
Member Since: August 13, 2007 Posts: 0 Comments: 10492

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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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