Isaac pounding Haiti and the Dominican Republic with torrential rains

By: Dr. Jeff Masters , 2:24 PM GMT on August 25, 2012

Tropical Storm Isaac is pounding Haiti and the Dominican Republic with torrential rains that are causing extremely dangerous flooding and landslides. Isaac's center passed over Haiti's southwest peninsula early this morning, tracking about 50 miles west of the capital of Port-au-Prince. As the center pulled away to the northwest, Isaac's heaviest thunderstorms moved ashore over Hispaniola near sunrise, and are now dumping heavy rains with rainfall rates approaching one inch per hour, according to recent microwave satellite estimates. Barahona on the south coast of the Dominican Republic had received 5.14" of rain as of 8 am EDT this morning, and it is probable that some mountainous areas in Haiti and the Dominican Republic have already received up to 10" of rain from Isaac. These rains will continue though much of the day, and have the potential to cause high loss of life in Hispaniola.


Figure 1. A river north of Port-au-Prince, Haiti in flood due to rains from Isaac. Image from Amélie Baron via Twitter.

Latest observations
Isaac built a partial eyewall last night as the storm approached Haiti, but passage over the rough mountains of Haiti has destroyed the inner core, and the surface center of the storm is now fully exposed to view on satellite images. Radar out of Guantanamo Bay, Cuba shows no sign of an organized center, but does reveal some very intense thunderstorms affecting Eastern Cuba, Western Haiti, and nearby islands. Latest data from the Air Force Hurricane Hunters confirm that Isaac has weakened; during their penetration to obtain their 7:08 am EDT center fix, the aircraft reported top surface winds of 55 mph with their SFMR instrument, top flight-level winds at 5,000 feet of 68 mph, and a pressure rise of 3 mb, to 998 mb. Infrared and visible satellite loops show that Isaac remains a large and well-organized storm, though it lacks an inner core. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow is still good to the north, but is lacking elsewhere. An impressive large multi-day satellite animation of Isaac is available from the Navy Research Lab.


Figure 2. Rainfall rates estimated by the NOAA F-17 polar orbiting satellite at 6:21 am EDT August 25, 2012. Rainfall rates of 1 inch per hour (orange colors) were occurring in a large area to the south of Hispaniola, and these heavy rains have now moved onshore. Image credit: Navy Research Laboratory.

Latest model runs for Isaac
The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs are similar in spread to the previous set of runs. Our two best models, the GFS and ECMWF, are virtually on top of each other, with a landfall location in the Florida Panhandle between Fort Walton Beach and Panama City. It is likely that the trough of low pressure pulling Isaac to the north will not be strong enough to pull Isaac all the way to the northeast and out to sea, and Isaac has the potential to drop torrential rains capable of causing serious flooding over the Southeast U.S. The latest 8-day precipitation forecast from the GFS model (Figure 3) calls for 10 - 15 inches of rain over portions of Georgia and South Carolina from Isaac. The ECMWF model, however, predicts that a ridge of high pressure will build in and force Isaac to the west after landfall, resulting in a slow motion across the Tennessee Valley into Arkansas by Friday. Arkansas is experiencing its worst drought in over 50 years, so the rains would be welcome there.


Figure 3. Predicted precipitation for the 8-day period from 2 am Saturday August 25 to 2 am Sunday September 2, from the 2 am EDT August 25 run of the GFS model. This model is predicting a wide swath of 5 - 10 inches of rain (orange colors) will affect portions of Cuba, Florida, the Bahamas, and the Southeast U.S. Image credit: NOAA/NCEP.

Intensity forecast for Isaac
Isaac survived passage over Hispaniola relatively intact. It's large size aided this, and this will also help it survive passage over Cuba today and Sunday. By the time Isaac pops off the coast of Cuba into the Florida Straits on Sunday, it will likely be a 50 mph tropical storm with a large, intact circulation. Isaac will be over very warm waters of 31°C (88°F) in the Florida Straits, wind shear will be light to moderate, and the upper-level wind pattern will feature an upper-level anticyclone over the storm, aiding its upper-level outflow. As I discussed in my previous post, Crossing Hispaniola and Cuba: a history, there have been five storms since 1900 with an intensity similar to Isaac, which crossed over both Haiti and Cuba, then emerged into the Florida Straits. These five storms strengthened by 5 - 20 mph in their first 24 hours after coming off the coast of Cuba. Given the relatively intact structure of Isaac so far, and the favorable conditions for intensification, I expect Isaac will intensify by 15 - 20 mph in 24 hours once the center moves off of the north coast of Cuba. If Isaac spends a full two days over water after passing the Florida Keys, it is possible that it will have enough time to develop a full eyewall and undergo rapid intensification into a Category 2 or 3 hurricane. The latest 06Z (2 am EDT) run of the GFDL model is calling for Isaac to intensify to Category 2 strength, then weaken to Category 1 at landfall in Mississippi on Tuesday. The 06Z HWRF run is calling for landfall in the Florida Panhandle near Fort Walton Beach as a borderline Category 2 or 3 hurricane. The 5 am EDT NHC wind probability forecast gave Isaac a 19% chance of becoming a Category 2 or stronger hurricane in the Gulf. I expect these odds are too low, and that Isaac has a 40% chance of becoming a Category 2 or stronger hurricane in the Northern Gulf of Mexico. I doubt the storm has much of a chance of hitting Category 4 or 5 status, though. While the surface waters in the Gulf of Mexico are very warm, near 30 - 31°C, the total heat content of these waters is unusually low for this time of year. We got lucky with the Gulf of Mexico Loop Current this summer, as it did not shed a big warm eddy during the height of hurricane season, like happened in 2005 (I discuss this in my Gulf of Mexico Loop Current Tutorial.) Without the type of super-high heat energy we had in 2005 in the Gulf of Mexico, Category 4 and 5 hurricanes in the Gulf of Mexico in 2012 will have difficulty forming.

Invest 97L off the coast of Africa
A tropical wave (Invest 97L) is located about 350 miles west-southwest of the Cape Verde Islands off the coast of Africa. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 30% chance of developing by Monday morning. The 8 am EDT SHIPS model forecast predicts moderate shear for the next 5 days over 97L, so some development is possible if 97L can fend off the dry air to its north. None of the reliable models foresee that 97L will be a threat to the Lesser Antilles Islands. However, both the GFS and ECMWF models predict that a tropical wave that has not yet emerged from the coast of Africa may develop next week, and potentially take a more westward track towards the Lesser Antilles.

The Weather Channel's hurricane expert, Brian Norcross, is now writing a blog on wunderground.com. For those of you unfamiliar with his background, here's an excerpt from his first post, from last night:

"This evening 20 years ago the sun set on the horrendous first day after Hurricane Andrew. I was in downtown Miami at the studios of the NBC station. We knew that there was "total" destruction in South Dade County, but even that didn't describe it. Here's to the people that went through it... and held their families together in a situation that most people can't imagine."

Angela Fritz will have a new post here by 6 pm EDT. For the next few days, I plan to do the morning blog post, and Angela will be doing the late afternoon post.

Jeff Masters


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

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Is it looking like Tallahassee is going to get much wind and rain from Isaac? We've had about a foot of rain this month so it wont take much to cause flooding
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90 hrs.

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514. 900MB
Quoting weatherh98:


Moving slow


I can't read the pressure on that, but looks real low! Cat 3/4 by that run???

2 days ago I predicted S Fla, yesterday NOLA, guess looks like somewhere in between, or both.

A couple of things concern me here. The path seems like it is as good as it gets considering the area. The Center could be off the Cuban coast all the way, rather than over. This could make for a slightly stronger (65mph/70mph) entrance to the Gulf. Second, looks like plenty of time to experience RI over the Gulf. NHC now has peak intensity to 100mph storm. I think there is an above average chance that we get to a Cat 3 storm. That would be a disaster!
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Quoting barotropic:


I think isaac has slowed also. Its going to be a long day. I think any delay in movement further reduces the already remote chance of any S Fla direct hit.


I agree.
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Quoting MiamiHurricanes09:
9mb stronger than the previous run. Major hurricane.

I am not sure a 980MB pressure indicates a major hurricane. More like in the 960-970MB range.
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Good morning.

Blog update:

Tropical Tidbit for Saturday, August 25th, with Video
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Alassippi
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12z GFS is like a whole category stronger (at least) than it has been...also slower and west of recent runs.
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Quoting KEEPEROFTHEGATE:

baddddddd
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Quoting wpb:
national hurricane service

rick scott

what about who
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Here come the freakout mode...
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Quoting Bamatracker:
Food for thought. Hurricane Fredrick was a tropical storm and a mess when it emerged from Cuba. The rest is history.

Link


Bama, did you have to bring up Frederic!..lol
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Quoting Chicklit:


Isaac's saying, 'gimme some a dat!'


Isaac would like a piece of that trough to its north. not funny


If it starts grabbing a little of that trough, that could be a lot of rain.
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Quoting wpb:
gov scott on tv soon. has no clue

disgrace
yes and i hear he's allowing the car ins companies to raise their rates again..a big disgrace he is
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Quoting JBirdFireMedic:


I disagree and dont count LA out until Isaac is in the straights and models plot off that position and intensity. Remember only a slight deviation results is large changes to the cone. This is a huge storm and even if landfall of the center is in the big bend, the rain bands and winds will be thrown over 200 miles away.
IMO


True. But the worst of Isaac will be on his Eastern side. I think LA will be ok.
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Quoting weatherh98:
Where's the gulf coast?

that will be catastrophic
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494. wpb
national hurricane service

rick scott
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Quoting wpb:
gov scott on tv soon. has no clue

disgrace

Skeletor will be making his statement frpom Castle Grayskull... all hail Skeletor!

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Quoting weatherh98:
Where's the gulf coast?


Looks like Ivan
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78 hrs.

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According to the GFS Isaac likes his new environment in the Gulf...
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982 is a Category 2 pressure right?
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Quoting AllyBama:
Please do NOT use the "W" word - however,the "E" word is fine..lol

what
(oh, i get it
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Where's the gulf coast?
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Quoting flcanes:

it's a joke

Private?
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9mb stronger than the previous run. Major hurricane.

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Quoting barotropic:


I think isaac has slowed also. Its going to be a long day. I think any delay in movement further reduces the already remote chance of any S Fla direct hit.

remember the 18z gas last night........................
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Please do NOT use the "W" word - however,the "E" word is fine..lol
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Quoting RTSplayer:
1500z

1000mb



990mb



970mb



950mb


hmm at the pressuer level Isaac is at would agree with W-WNW
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479. wpb
gov scott on tv soon. has no clue

disgrace
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Quoting JasonRE:


Yeah, guess I'm just tired of this storm showing models that flip flop from night to day. Last night I started thinking about sand bags bc we flooded after 3 hours of rain 2 weeks ago, and this morning I'm thinking not so much.


Same here..I'm tired of monitoring ol' Isaac
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Food for thought. Hurricane Fredrick was a tropical storm and a mess when it emerged from Cuba. The rest is history.

Link
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Quoting weatherh98:


Moving slow

Thank you!
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Quoting TropicalAnalystwx13:
Audio podcasts now up courtesy of our new director, ck Knabb.
thanks I'm listening
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GFS now shows a major!
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Quoting RTSplayer:
Ahem


how appropriate
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Quoting stormpetrol:
To my eye the center of Isaac looks to half on shore , half offshore on the SE coast of around 19.9/20N 74.9/75W , barely moving at all.


I think isaac has slowed also. Its going to be a long day. I think any delay in movement further reduces the already remote chance of any S Fla direct hit.
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Quoting Clearwater1:


Quoting Jedkins01:



Punta Gorda wasn't in the cone as Charley approached.

*****
SE FL wasn't in the cone as Wilma Approached either.
*****

Exactly. Nice we have long range radar, out of Tampa and Key west to watch it's progress off shore. See if it is indeed heading nw as it gets closer.
I believe S. FL was in the five day cone. They were pretty accurate on that one.
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Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather