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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GFS has other models to support it... EURO has a few but not a ton (looks like a west outlier at the moment). Doesn't mean they won't shift west again though. Again, look for patterns with these runs.

What is clear is that SW Florida and the Keys should be under the gun on Sunday/Monday from a strengthening storm.
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This is like watching a long tennis rally that starts at the net and meanders back to the base lines!
Member Since: September 21, 2008 Posts: 0 Comments: 266
Quoting Chiggy:
For what it is worth, 160 year history is against Fl landfall, only 3 storms have hit FL from the current position of Issac in August - a staggering statistics!

And with whats going on in the upper atmosphere, a westward track will favor once again.
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1158. BigTuna
Quoting Gino99:
Can someone please explain me the XTRP model? :P I don't even consider it as a model lol.
eXTRAPolated.. if the storm continued on its present course at its present speed without deviation. Not a model.
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1157. WxLogic
Quoting Gino99:
Can someone please explain me the XTRP model? :P I don't even consider it as a model lol.


Is not really a model, but the current path it would take if it were to follow the same direction and speed is currently heading.
Member Since: August 14, 2008 Posts: 4 Comments: 4972
I just tuned in for the day, read the 11am nhc. Looks like it is still a battle between the gfs and euro. If there is a northern component, like they state, and the track is adjusted slightly, this will move it a little east, then a sharper turn nnw later on. But I don't know much about this, but seems like that's what they were forecasting. And it is now it's consistently inching a tad further east, at each update.
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1155. Michfan
Quoting reedzone:
The European Model just..... Doesn't make sense.


Nvm.
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Quoting reedzone:
The European Model just..... Doesn't make sense.


How's that? Looks like a reasonable solution and is still well within consensus.
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Quoting reedzone:
The European Model just..... Doesn't make sense.

Actually it does. Ive explained it many times.
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Has anyone out there in the Caribbean ever found a dropsonde
washed up on the beach?
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Quoting mrjr101:


How about intensity models to the shredder? NHC should really keep intensity forecast to themselves.


Thats the big unknown...the 10,000 ft mtns can tear it down to an open wave or hurl it in some direction;
Also, while the GOM SSTs are above avg, the GOM is rather shallow with floating loop currents which makes proj. intensity harder.
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1150. WxLogic
Quoting coffeecrusader:
12z Euro has shifted even further west than last run and has Isaac making a run at New Orleans. Based on this I don't think NHC will change the track at 5pm infact they may move it a little more west.


96 hours 12z 8/24


96 hours 00z 8/24


Interesting enough there's more TROF(ing) in the 12Z ECMWF than on the 00Z run and you still get a western solution. In the longer term, I have issues agreeing with ECMWF but in the short them is doing OK.

I would have expected ECMWF to stay the same or shifted towards the E under a more pronounced weakness.
Member Since: August 14, 2008 Posts: 4 Comments: 4972
Quoting coffeecrusader:
It's a question of which model do you trust. The NHC puts alot of weight on the Euro and GFS and they are moving further apart again (GFS to the east and Euro to the west). GFS was good with Debbie and bad with Ernesto while Euro was the opposite. Frankly I don't know which one is going to be right with Isaac. However, I live in Sarasota and if the GFS is right I will get alot worse of the storm.
Actually it was the GFS that handled Ernesto well, while most thought that it would strengthen in the Eastern and Central Caribbean it weakened and strengthened again closer to the Yucatan. The Euro had to play catch up and conceded to the GFS.
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1000mb (just in case when it hits an island)




990mb



970mb



Notice, by the time of a few days, the trough will be gone and the gulf ridge will be over florida, so a 970mb or below storm will actually hook west harder than a weaker storm, particularly if it slows down or makes any more significant westward jaunts today, which would bring it more in line with Climatology (Camille, Ivan, Gustav...anything from middle of Cuba westwards).


Other words, if it gets center or left of that next ridge in the 970 layer, then the east casting is done for...


950mb



Same deal, but more pronounced.



Obviously these are gonig to move around a lot, but something interesting on satellite is the pop-up thunderstorms building around the ridge in the 1000mb to 1010mb layer, perhaps showing the intuitive maximum eastward route for the storm....



The left side of these storms is their lower levels, the east side is the cloud tops being blown off by shear. So this is showing a rare "hard" boundary for steering.
Member Since: January 25, 2012 Posts: 33 Comments: 1520
1146. Chiggy
For what it is worth, 160 year history is against Fl landfall, only 3 storms have hit FL from the current position of Issac in August - a staggering statistics!
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Quoting CarolinaHurricanes87:
Is central Cuba as damaging to storms (mountainous) as eastern Cuba and Haiti? Or more similar to Florida (flatter) in that storms can cross without losing as much intensity?
Central and West are flat
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1144. Gino99
Can someone please explain me the XTRP model? :P I don't even consider it as a model lol.
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Quoting GTcooliebai:
You can't look past the fact that it didn't handle Debby well and missed seeing clearly a trough that was sweeping down and picking her up to the NE. The GFS had a better handle with the strength and timing of the trough, as well as the placement of the ridge.


The Euro is consistantly thinking that Issac as was the same with Debby would slip under the TX ridge while the trough bypasses to the north. could be another epic fail for the NHC if they continue to put all it's faith in the Euro.

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Quoting AtHomeInTX:
What was that saying about models can and do change as they always do...

I have a question though. Is the data from yesterday's GIV
in these models too. Just seems they've both changed since 00z last night.



No, but there is another GIV flight today.
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Quoting sunlinepr:
Looks like the mountains of Hispaniola are already affecting Isaac... and there is dry air into it...



nope its dmin thats having an affect on convection.
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G-IV flight up sensing the ridge, maybe the trough is overdone by the GFS, it has a history of doing that, but time will tell...So far im leaning to the GFS still
EURO is WAAAAY out there
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I am thinking 5pm will show another shift east on the 3-5 day track, with the Euro going from TX/LA border yesterday to MS today. Trend is East. Panama City...
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1138. 7544
Quoting wpb:
watvhes are due 48 hour in advance. models show very close to se coast of fla upper keys in 48 hours.
????think they will post at 6pm
??tropical storm watch or hurricane watch??
anyone have a good answer


nhc will prob wait but its only 2 days a way they might wait to see how he is around the trip to cuba , or maybe by 11pm tonight or maybe not at all lol
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The European Model just..... Doesn't make sense.
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1136. angiest
hurricanealley.net (not official)

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During discussions, due to the fact that model science is more accurate forecasting path than intensity, we concentrate on storm paths forecasts....

Isaac is an example of that... 60mph... (and that is good)

The question is How strong will Isaac be when it reaches Conus?.... (either FL or any area in GOM)

What will determine loss of life is the intensity rather than path... Its not the same thing to have a 60mph over my area than a 110 mph hurricane...

Maybe in the future models will be able to improve Intensity forecasting...
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Time: 18:22:00Z
Coordinates: 17.1N 70.4167W
Acft. Static Air Press: 842.8 mb (~ 24.89 inHg)
Acft. Geopotential Hgt: 1,509 meters (~ 4,951 feet)
Extrap. Sfc. Press: -
D-value: -
Flt. Lvl. Wind (30s): From 155° at 61 knots (From the SSE at ~ 70.1 mph)
Air Temp: 13.9°C* (~ 57.0°F*)
Dew Pt: -*
Peak (10s) Flt. Lvl. Wind: 64 knots (~ 73.6 mph)
SFMR Peak (10s) Sfc. Wind: 47 knots (~ 54.0 mph)
SFMR Rain Rate: 6 mm/hr (~ 0.24 in/hr)
(*) Denotes suspect data
Member Since: August 18, 2011 Posts: 0 Comments: 994
Well, we're now seeing some intermittent sun, but winds have picked up considerably due to the pressure gradient induced by the approach of Isaac.

What are the odds of the heavy rainfall / thunderstorm activity switching from the south side of the circulation to the north side as Isaac crosses Cuba? I thought I saw something similar earlier this week as it crossed into the CAR...

On the subject of US international aid, I applaud the efforts, both by government and private entities, to assist other nations in their time of crisis, especially poor nations with limited resources, like Haiti. Nevertheless, I am also reminded that giving money is rarely the solution to the problems that make the effects of such crises linger. And recent hurricane strikes in the US show that Americans have their own vulnerablities to concern themselves with. So maybe a little less "rock-throwing" on all our parts might be wise...

Member Since: October 25, 2005 Posts: 19 Comments: 22150
Getting flight level winds exceeding hurricane force.
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What was that saying about models can and do change as they always do...

I have a question though. Is the data from yesterday's GIV
in these models too. Just seems they've both changed since 00z last night.

Member Since: August 24, 2010 Posts: 0 Comments: 161
Quoting Unfriendly:


So... landfall will be in the everglades somewhere?


I think it will be 20-40 miles west of Miami
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Quoting Dunkman:


Yes we share all of our data with the Euro. I mean you can write off the best model we have for forecasting tropical cyclone track but that seems unwise.
You can't look past the fact that it didn't handle Debby well and missed seeing clearly a trough that was sweeping down and picking her up to the NE. The GFS had a better handle with the strength and timing of the trough, as well as the placement of the ridge.
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It's a question of which model do you trust. The NHC puts alot of weight on the Euro and GFS and they are moving further apart again (GFS to the east and Euro to the west). GFS was good with Debbie and bad with Ernesto while Euro was the opposite. Frankly I don't know which one is going to be right with Isaac. However, I live in Sarasota and if the GFS is right I will get alot worse of the storm.
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1127. WxLogic
So far on the Late 12Z Dynamical:

Member Since: August 14, 2008 Posts: 4 Comments: 4972
1126. shawn26
Levi, what are your thoughts on the latest Euro?
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Quoting southernema:


so is this model just way off...or does it know something the others dont..just wondering


I think the Euro has always been farther W than the rest.
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Quoting alvarig1263:
I see intensifying!

Time: 18:21:30Z
Coordinates: 17.1N 70.3833W
Acft. Static Air Press: 842.2 mb (~ 24.87 inHg)
Acft. Geopotential Hgt: 1,515 meters (~ 4,970 feet)
Extrap. Sfc. Press: 1004.0 mb (~ 29.65 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 156° at 67 knots (From the SSE at ~ 77.0 mph)
Air Temp: 15.0°C* (~ 59.0°F*)
Dew Pt: 15.0°C* (~ 59.0°F*)
Peak (10s) Flt. Lvl. Wind: 70 knots (~ 80.5 mph)
SFMR Peak (10s) Sfc. Wind: 48 knots (~ 55.2 mph)

SFMR Rain Rate: 3 mm/hr (~ 0.12 in/hr)


thats not too high
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Quoting TampaFLUSA:
People are posting last run of Euro.



It is the current 12Z run. Go to Allen Huffman's page.
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If I were a bettin' man....and, actually, I am....I'd bet the NHC emerges from all this with their credibility well intact...
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1121. pottery
Quoting Bielle:


You have my sympathies.


heheheheh
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The cone may stay the same or shift ever so slightly to the right for the first 48 hrs.
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Quoting coffeecrusader:
12z Euro has shifted even further west than last run and has Isaac making a run at New Orleans. Based on this I don't think NHC will change the track at 5pm infact they may move it a little more west.


96 hours 12z 8/24


96 hours 00z 8/24


so is this model just way off...or does it know something the others dont..just wondering
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1118. pottery
Isaac is DMINishing right now.

Still a Massive system.

Horrors and Dread down the line....
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Flight level winds are up to 70kts. Probably 60-65 on the surface.
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People are posting last run of Euro.
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Here are the links to the 12z run of Euro model. Further shift to the west. After clicking on link go down to 12z and choose 500mb Heights/SLP (Atlantic View)

Link
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I see intensifying!

Time: 18:21:30Z
Coordinates: 17.1N 70.3833W
Acft. Static Air Press: 842.2 mb (~ 24.87 inHg)
Acft. Geopotential Hgt: 1,515 meters (~ 4,970 feet)
Extrap. Sfc. Press: 1004.0 mb (~ 29.65 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 156° at 67 knots (From the SSE at ~ 77.0 mph)
Air Temp: 15.0°C* (~ 59.0°F*)
Dew Pt: 15.0°C* (~ 59.0°F*)
Peak (10s) Flt. Lvl. Wind: 70 knots (~ 80.5 mph)
SFMR Peak (10s) Sfc. Wind: 48 knots (~ 55.2 mph)

SFMR Rain Rate: 3 mm/hr (~ 0.12 in/hr)
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Quoting GTcooliebai:
And how do we know if this model isn't out to lunch? Does anyone know if they got the Gulf Stream Data into this model?


The odd thing is the Euro is doing the same thing that it did with Debby which many on here fell for. Looks as if the NHC hasn't learned their lesson yet. FL get ready please as all other models show atleast minimal hurricane sliding up the west coast of FL.

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1112. Bielle
Quoting pottery:

I don't get it.....

:):))


You have my sympathies.
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1111. GetReal
Quoting RTSplayer:


How do you get that so quickly?

The site Wunderground links to is usually another hour behind people posting individual frames, and it doesn't have as much detail...

Can you provide a link to the hosting site please?



Wrong run sorry for the mix up...
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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.