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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by the way they shifted the cone West untill 72 hours then they pulled it East
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Quoting JasonRE:
Is the rainfall HWF's bold black line the path that this thing is taking or are the models showing it more to AL? I'm confused. It looks like it still is moving farther NW than WNW. Should LA stop worrying now that it's Friday?

NO everyone E of Huston TX to Jacksonville FL should keep an eye on it...
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Quoting harleydchick:
HWRF 06Z RUN - possible landfall, MS/LA state line............things that make me go hmmmmmm.


Exactly......the new models show this thing up the gut of AL. How, when the models of HWRF 06Z RUN show the MS/LA possibility.......head spins......falls on floor. Where am I, and what's going on?
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Quoting Tropicsweatherpr:


Why 13? It will interact with plenty od land before it emerges north of Cuba.

Isaa is massive. It's not going to weaken much. When it gets in the Gulf, conditions will be perfect for RI.
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How powerfull that ULL has been in all this processs... It ate Joyce turning into a Ex-tropical system and is responsible of keeping Isaac moving WSW... making model forecast unpredictable...

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I think Dr. Masters update on track in this blog his little old because the 11AM track update is more eastward
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54. 7544
look at the cone maybe no watches at all for east side of fla except key west ?
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Quoting Tropicsweatherpr:


Why 13? It will interact with plenty od land before it emerges north of Cuba.

SSTs in GOM
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HWRF 06Z RUN - possible landfall, MS/LA state line............things that make me go hmmmmmm.
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Quoting weatherman12345:
Ft lauderdale is out of the cone!
wait until the 5am tomorrow, this is just now unfolding with some new info, dont take your eyes off this one
Member Since: August 6, 2011 Posts: 4 Comments: 38483
Is the rainfall HWF's bold black line the path that this thing is taking or are the models showing it more to AL? I'm confused. It looks like it still is moving farther NW than WNW. Should LA stop worrying now that it's Friday?
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Thanks DRM.
Member Since: August 24, 2010 Posts: 0 Comments: 151
Quoting interstatelover7165:
What About the FLANALF? I think Issac is shooting for Charleston Sc.

Sorry but NO
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Quoting TropicalAnalystwx13:
NHC was far too conservative with the intensity forecast.


Why 13? It will interact with plenty of land before it emerges north of Cuba.
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Quoting weatherman12345:
Ft lauderdale is out of the cone!
But that does not mean you will be effected.
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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.


In other words, the feature we identified is not 'an optical illusion' as some here have been claiming. There's "a fairly tight center" and it's now "exposed to view." That's not the same as an eye, of course. But Dr. Masters is looking at the same imagery we are, and he sees what we see.
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24Aug.06amGMT's 16.2n69.4w has been re-evaluated&altered
24Aug12pmGMT's 16.2n69.6w-15.9n70.4w are now the most recent positions
(The previous vectors and straightline projections have been corrected for this mapping)

23Aug.06amGMT: 15.3n63.5w (248.5*WSW@13.5knots) 35knots, 1004millibars, TS
23Aug.12pmGMT: 15.4n64.8w (274.7* West@12.6knots) 35knots, 1003millibars, TS
23Aug.06pmGMT: 15.9n66.4w (288.1*WNW@16.2knots) 35knots, 1004millibars, TS
24Aug.12amGMT: 16.5n68.0w (291.4*WNW@16.5knots) 40knots, 1002millibars, TS
24Aug.06amGMT's numbers are below, before 24Aug.12pmGMT's
Derived from NHC_ATCF data for TropicalStormIsaac for 24August12pmGMT
MinimumPressure remained at 1000millibars
MaxSusWinds increased from 40knots(46mph)74km/h to 45knots(52mph)83km/h
Vector changed from 259.2*West@18mph(29km/h) to 248.9*WSWest@9.5mph(15.3km/h)

IRN-Iriona :: PVA-Providencia :: TTQ-Tortuguero :: CBJ-CaboRojo :: AUA-Aruba

The westernmost dot on the kinked is Isaac's starting position on its 3rd day as a TropicalStorm
The easternmost dot on the longest line is TS.Isaac's most recent position

The longest line is a straightline projection through TS.Isaac's 2 most recent positions to its closest approach (within 18miles or 29kilometres) to a coastline
23Aug.12pmGMT: TS.Isaac had been headed for landfall ~17.7miles(28.6kilometres)East of Iriona (right,IRNdumbbell)
23Aug.6pmGMT: TS.Isaac had been headed for passage ~2.2miles(3.6kilometres)SSWest of AltoVeloIsland (left,nearCBJdumbbell)
24Aug.12amGMT: TS.Isaac had been headed for passage over southern LagoDeOviedo (right,nearCBJdumbbell)
24Aug.6amGMT: TS.Isaac had been headed for passage 13.2miles(21.3kilometres)NNWest of Providencia
24Aug.12pmGMT: TS.Isaac was heading for passage 6.3miles(10.2kilometres)SSEast of Tortuguero in ~4days1hour from now (when this was posted)

Copy&paste irn-15.954n84.916w, cbj, 17.439n71.647w-17.706n71.363w, aua-12.626n70.055w, ptp, dom, 15.8n62.2w- 15.3n63.5w- 15.4n64.8w- 15.9n66.4w, 15.9n66.4w-16.5n68.0w, 16.5n68.0w-16.2n69.6w, 16.5n68.0w-13.583n81.421w, pva, 13.397n81.372w-13.583n81.421w, 16.2n69.6w-15.9n70.4w, ttq, 16.2n69.6w-10.484n83.475w into the GreatCircleMapper for a larger map and other information
The previous mapping for comparison
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Quoting JBirdFireMedic:
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.

WOW

CAT 3 easy with GOM waters
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Quoting AllStar17:
80 mph hurricane at landfall. That may need to be upped.

...by a lot.
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Quoting SFLWeatherman:
No Tropical Storm Watch up????

i didn't see anything...
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Quoting Maineweatherguy20023:

Where u get that??
ok tampa is now in the cone, lets see the cone at 8pm tonight and better yet 5am tomorrow morning
Member Since: August 6, 2011 Posts: 4 Comments: 38483
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.

WOW
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80 mph hurricane at landfall. That may need to be upped.
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Whoa, cat 3 landfall
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...winds up...still strengthening...track shifted back East...looking more like AL/Fl will be in Isaacs's sights. Not breathing a sigh of relief here in LA yet, but... Hope everyone is getting their plans in order.
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Quoting oracle28:
Anyone know what time the XTRP model will update?
What About the FLANALF? I think Issac is shooting for Charleston Sc.
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Quoting weatherman12345:
Ft lauderdale is out of the cone!

Dosent mean u wolnt have impacts
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No Tropical Storm Watch up????
Quoting Maineweatherguy20023:

Where u get that??
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The dropsondes have spoken. Get ready Northern Gulf Coast...
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Quoting VR46L:
Looks like he is finally getting his game together ... a little

Rainbow Gif Isaac


Definitely. Looks good
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right guys I'll wait for HH RECON to fly in at 1pm and get the center fix from that because NHC has a history with this storm putting the LLCOC N of where it really is via recon fix so I'll wait if recon finds it where NHC put it then fine but I think they may again find it S of the NHC plot
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Quoting Bluestorm5:
As I expected, a minor shift to east.


shift east at the end but thru 72 hrs it shifted slightly west of S. FL.
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Quoting superpete:
20 years since Andrew..amazing how time flies.
i was amazed to hear on the radio this morning he destroyed 68,000 homes..geez
Member Since: August 6, 2011 Posts: 4 Comments: 38483
Quoting SFLWeatherman:
lol on this it going NW!!

Just a jog for now...
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Quoting SFLWeatherman:
WHat why??

Where u get that??
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Looks like he is finally getting his game together ... a little

Rainbow Gif Isaac

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He is starting to shimmy...
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As I expected, a minor shift to east.

Member Since: August 1, 2011 Posts: 28 Comments: 8009
lol on this it going NW!!
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Quoting cajunkid:
No eye yet


Soon very soon HH SAID EYEWALL WAS FORMING
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WHat why??
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Thanks Dr. Masters.

Nobody's watching her anymore, but poor lil Joyce is refiring some convection. I'm pullin' for her. ;)
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Quoting TropicalAnalystwx13:
NHC was far too conservative with the intensity forecast.

Definitely
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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.
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Hurricane by landfall in Haiti??
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NHC was far too conservative with the intensity forecast.
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Quoting cajunkid:
No eye yet


False Panic Alarm.
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gamma :)

Thanks Dr M.

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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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