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 reedzone:


Correction: 85W... 80W is the East Coast of FL.
thx
Member Since: July 14, 2009 Posts: 0 Comments: 3003
Quoting Seflhurricane:
my bad yes that is the center


If that's the case, Isaac is a ways from RI. Its really tilted.
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859. CJ5
It is going to be a naked swirl at this rate. He just cannot pull it together. Good for Haiti, though.
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2 things to look for in the 2pm advisory.

* Any watches for South Florida/Keys
* motion and speed ( looks like a wnw to NW motion )
* Wind Speed
Member Since: July 14, 2009 Posts: 0 Comments: 3003
856. Jax82
da mets in Jax seem to think we are going to get 30-65mph winds next tuesday.

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855. VR46L
Quoting sar2401:


I realize that, but it was still good comic relief compared to some of the other doomcasting we've been getting for the past week.


It was the post of the month just my opinion ...
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the COC has moved little north the past 3 hours in fact has been moving almost due west. center is about 16.4/5 N moving wnw will be interesting to see if he slips south of cuba
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Quoting will40:



i agree and think they should get rid of the quote feature. or change it to when it only shows the person being quoted name


I agree with that. Whats the point in IGNORE if every quotes em!
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Quoting gustavcane:
at what time does the the new ECMWF Model comes out? and what website will the newest model appear first?
Try hereLink 12z should begin initialization around quarter to 2PM. I like using the 500mb Heights/SLP (Atlantic View).
Member Since: August 31, 2009 Posts: 0 Comments: 5628
Quoting Seflhurricane:
models has shifted east again looks like REED has been right all along i do not see this storm moving any west than 80W


Correction: 85W... 80W is the East Coast of FL.
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Quoting GetReal:


Hey, it looks like a yin-yang symbol!

One half of the storm doing well, other, not so much.
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Quoting Progster:
Is that the LLC emerging at 17N 71W?
my bad yes that is the center
Member Since: July 14, 2009 Posts: 0 Comments: 3003
We mean no harm, but we need the rains badly in Georgia. Our lakes are well down and need replenishing.
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Time: 17:22:00Z
Coordinates: 18.15N 65.7833W
Acft. Static Air Press: 505.8 mb (~ 14.94 inHg)
Acft. Geopotential Hgt: 5,777 meters (~ 18,953 feet)
Extrap. Sfc. Press: -
D-value: 283 meters (~ 928 feet)
Flt. Lvl. Wind (30s): From 130° at 29 knots (From the SE at ~ 33.3 mph)
Air Temp: -5.0°C* (~ 23.0°F*)
Dew Pt: -5.0°C* (~ 23.0°F*)
Peak (10s) Flt. Lvl. Wind: 30 knots (~ 34.5 mph)
SFMR Peak (10s) Sfc. Wind: -
Member Since: August 18, 2011 Posts: 0 Comments: 994
Quoting wunderkidcayman:
ok I see two places where the LLC could be

#1 near 17.1N 71.0W
#2 near 16.3N 71.0W
back in about 30mins or so to check on how RECON is doing

#1 near 17.1N 71.0W
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Quoting Progster:
Is that the LLC emerging at 17N 71W?
no but its the convection wraping around the center
Member Since: July 14, 2009 Posts: 0 Comments: 3003
Quoting PalmBeachWeatherBoy:
The GFS has been nearly identical for 3 runs now. Impact on upper key west, brush with southwest FL, panhandle hit.


The NHC I hope isn't putting much stock in the Euro as that model has been SH_T lately.

Big trough across the eastern Gulf. The door is open now for Issac to cut up the FL east coast.
Member Since: October 26, 2011 Posts: 0 Comments: 2651
Quoting GTcooliebai:
The core is definitely tightening up on Isaac.
lets see what recon finds but i would not be surprised to see a 65-70mph tropical storm
Member Since: July 14, 2009 Posts: 0 Comments: 3003
Is that the LLC emerging at 17N 71W?
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at what time does the the new ECMWF Model comes out? and what website will the newest model appear first?
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models has shifted east again looks like REED has been right all along i do not see this storm moving any west than 80W
Member Since: July 14, 2009 Posts: 0 Comments: 3003
The core is definitely tightening up on Isaac.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
ok I see two places where the LLC could be

#1 near 17.1N 71.0W
#2 near 16.3N 71.0W
back in about 30mins or so to check on how RECON is doing
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Quoting presslord:
Why on Earth would anyone repost someone using bad language?!?!?!?! It just confounds me...



i agree and think they should get rid of the quote feature. or change it to when it only shows the person being quoted name
Member Since: September 19, 2005 Posts: 2 Comments: 4131
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Quoting StormTracker2K:
Big shift east on the 12Z HWRF.



Right back over my house again.

Make up your mind, Isaac. Pensacola, or not?
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Quoting Progster:


I rather like to visit chocolate-munching pike-bearing banker country. However..Levi seems to like the CMC 12Z. the Euro will be interesting.


I resemble that remark. Don't make me send the visigoths over to pay you a visit.
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CMC is spitting out 10" to 14" of rain across eastern FL on the 12Z CMC. YIKES!!
Member Since: October 26, 2011 Posts: 0 Comments: 2651
The GFS has been nearly identical for 3 runs now. Impact on upper key west, brush with southwest FL, panhandle hit.
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Quoting ProgressivePulse:
Interested to see what recon finds, one could say the center is just off the coast of DR ATM.
The NHC cone has been pretty good, they said it would make landfall on the border of DR/Haiti where that piece of land juts out at 2PM today, it must be getting close now. Although it might have taken a jog west to even out the wnw heading.



Member Since: August 31, 2009 Posts: 0 Comments: 5628
Quoting VAbeachhurricanes:


rhetorical ;)


Phew :>)
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Quoting HurricaneDean07:

12z GFS continues 5 days out from where he posted that...
Brings "Kirk" out to sea as it clips PR. and DR. Makes it a Hurricane.

12z GFS also shows Leslie, and Michael by September 9th.


Yeah they were rolling them out one after another. Where is El Nino? I know, I know he'll show up later.
Member Since: August 24, 2010 Posts: 0 Comments: 60
The flow in from the mountains of the DR are doing a number on Isaac right now. Poor guy just can't catch a break and get organized.
Member Since: February 6, 2010 Posts: 0 Comments: 214
The blog is really gonna freak if/when it bounces off Haiti.

I've seen a few storms do it.
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12Z CMC takes Issac up the east coast of FL which as well has shifted to the east now. So far 3 global models have shifted to the east. I think the biggest reason is the weakening of Joyce toa Remant Low.
Member Since: October 26, 2011 Posts: 0 Comments: 2651



COC at 17 N - 71 W
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Quoting Seflhurricane:
it looks like it but its not its the convection wraping around the center but the center to me looks like its 100 miles to the south of cabo rojo


That's what I figured, one of the minion circulations. Doesn't appear to be coming back around however, lol.
Member Since: August 19, 2005 Posts: 5 Comments: 5209
821. wpb
time over islands is the key. when does it emerge off the north cuban coast.?what do the models say about forward spped going forward from cuba rainfall totals
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Convection weaker, but more organized than ever.
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Quoting mojofearless:


Is that a rhetorical question, or do you honestly want my opinions about the causes of Haiti's failed state? Because I don't think there's any one person to blame - from where I stand, it sure looks like a long term, ongoing series of systemic failures. But if you need me to spend my afternoon analyzing it, I can try ;)


rhetorical ;)
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Quoting Seflhurricane:
thats not hi9s forecast thats bogus and someone copied his logo/name


I realize that, but it was still good comic relief compared to some of the other doomcasting we've been getting for the past week.
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Quoting scott39:
If Isaac tracks up the spine of Fl, I will eat 2 vultures, 8 crows and a chocalate covered canary for dessert.


Great post
Member Since: August 13, 2007 Posts: 10 Comments: 4150
Quoting ProgressivePulse:
Interested to see what recon finds, one could say the center is just off the coast of DR ATM.
it looks like it but its not its the convection wraping around the center but the center to me looks like its 100 miles to the south of cabo rojo
Member Since: July 14, 2009 Posts: 0 Comments: 3003
Quoting Bluestorm5:
Note to myself: DO NOT EVER POST A SINGLE SENTENCE OF POLITICS AGAIN...

Anyway, Recon is in the air.


All of the political histrionics on this blog ruin it for people like me who enjoy the weather commentary and are trying to learn more about meteorology. Twitter is a much better forum to voice your political and sociological opinions. Now if you'll excuse me I'm going to trip out on Largo's pictorial musings......
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813. eddye
ppl come on tropics chat lets talk about issac
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the NW jog cantinues looks like Isaac may make landfall just west of port au prince , haiti if trends continue its approx 100 miles south of cabo rojo dominican republic
Member Since: July 14, 2009 Posts: 0 Comments: 3003
Hippo-thetically, if it's shifting to the east, would Mobile AL be affected?
Member Since: August 18, 2011 Posts: 0 Comments: 994

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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.