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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Quoting GTcooliebai:
102 hrs. landfall somewhere along the Northeastern Gulf Coast between Gulfport and Pensacola it looks like:


over Mobile,AL
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Quoting AllyBama:
Keeper - thanks for posting the model run!
iam not enjoying what i see ally iam very concearned at the moment
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Quoting KEEPEROFTHEGATE:

okay, ivan landfall it is
( and near the 8th anniversary to)
(pouts)
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566. wpb
Quoting Levi32:
Good morning.

Blog update:

Tropical Tidbit for Saturday, August 25th, with Video
12z has your impossible track again central gom. and a path more west over lower keys
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Quoting hurricanehunter27:
I am not sure a 980MB pressure indicates a major hurricane. More like in the 960-970MB range.


That might be true but one thing is for sure. I think that a major hurricane is now more likely with the time over water. NOAA and forecasters now warning the storm will become a hurricane 24 hours before previously thought. That is 24 hours more over warm water as a hurricane which to me adds to the CAT 2 already forecasted at landfall. That being one level higher at CAT 3. The worse thing that could happen would be rapid intensification beyond that. I seen they are forecasting a CAT 1 hurricane in my area. That is seriously not good news.
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Quoting flcanes:

that spells tampa under water
yes im afraid so,and most of the excape routes like causeways etc..this could..be a disaster if it comes true..please die off issac
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102 hrs. landfall somewhere along the Northeastern Gulf Coast between Gulfport and Pensacola it looks like:

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Quoting Dunkman:


GFS resolution is way too high to give a truly accurate pressure representation that far out. Suffice to say something in the 970s and deepening on the GFS is a major. You will see the Euro go deeper on tropical since it has a much higher resolution than the GFS.

true, you get a cookie
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Keeper - thanks for posting the model run!
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559. CJ5
It looks like the Sierra Maestra Range is putting a hurting on Isaac.

Full disclosure: I had to google the mountain range.

As weak as he is, the shear may be having some effect as well. Maps show around 20 and increasing to 30 to the NW.
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Quoting TropicalAnalystwx13:
I'm usually not bold when it comes to USA landfalls, but people living in the Panhandle of Florida need to prepare for a Category 3 major hurricane. Residents living on both sides need to prepare for slightly less intense winds.
Huge anticyclone very warm sst and a moist environment.All things needed to create a beast...
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Quoting LargoFl:
man its going to be pushing all that water up and eastward..oh boy

that spells tampa under water
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Quoting hurricanehunter27:
I am not sure a 980MB pressure indicates a major hurricane. More like in the 960-970MB range.


GFS resolution is way too high to give a truly accurate pressure representation that far out. Suffice to say something in the 970s and deepening on the GFS is a major. You will see the Euro go deeper on tropical since it has a much higher resolution than the GFS.
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Morning.....I am missing Patrap's live radar feeds right about now.....Pat come back! Come back Pat! (Pat/Shane)
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Quoting louisianaweatherguy:
the new GFS looks like a Hurricane Georges path... coming at New Orleans then turning at the last second.... we still had power outages from Georges though... :/

again, it'll be further east, i know it
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Quoting GTcooliebai:
78 hrs.

man its going to be pushing all that water up and eastward..oh boy
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550. 900MB
Quoting weatherh98:
Alassippi


Wow! 918MB, that is low!!!!!!

Andrew was 922Mb
Camille 909
Katrina 904
Member Since: December 31, 1969 Posts: Comments:
I'm usually not bold when it comes to USA landfalls, but people living in the Panhandle of Florida need to prepare for a Category 3 major hurricane. Residents living on both sides need to prepare for slightly less intense winds.
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If that were to pan out instant retirement off the back and that will be the 3rd year in a row the "I" storm has joined the list.
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Quoting Levi32:
Good morning.

Blog update:

Tropical Tidbit for Saturday, August 25th, with Video

I agree with your path, I am just a little stronger intensity wise.
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Quoting barotropic:


Alot of beer pong!!!

what
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the new GFS looks like a Hurricane Georges path... coming at New Orleans then turning at the last second.... we still had power outages from Georges though... :/
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Quoting MiamiHurricanes09:
Not only does Raleigh not display the full resolution of the GFS, but the actual model isn't configured to display the the MSLP due to it's resolution; so in actuality, that 978mb pressure is a very intense hurricane.
Ok did not know that.
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Quoting AllyBama:


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


That just brings chills up the spine...
and Ivan was worse.....
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Quoting MiamiHurricanes09:
Not only does Raleigh not display the full resolution of the GFS, but the actual model isn't configured to display the the MSLP due to it's resolution; so in actuality, that 978mb pressure is a very intense hurricane.




right 978mb on the mode runs could mean like strong cat 4 or low cat 5 in real life i think
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Quoting Bluestorm5:
100 mph in Gulf according to NHC... fun!

Sarcasm Flag: ON

My brothers would turn the sarcasm flag off.
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If Isaac stays on, "initial motion is a somewhat uncertain 315/15", It will arrive over Miami at 6PM Sun. Translates to 17 MPH

Miami is 530 miles 315 degrees from the 11 AM EDT Position.

Handy distance calculator

http://www.movable-type.co.uk/scripts/latlong.htm l
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Quoting TropicalAnalystwx13:

Because the high is building in, which will gradually curve it west-northwest.


The models keep it at the same strength
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Quoting TallyWeather15:
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


Alot of beer pong!!!
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Quoting hurricanehunter27:
I am not sure a 980MB pressure indicates a major hurricane. More like in the 960-970MB range.
Not only does Raleighwx not display the full resolution of the GFS, but the actual model isn't configured to display the the exact MSLP due to it's resolution; so in actuality, that 978mb pressure is a very intense hurricane.
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Tropical storm watch now for Pinellas county southward. I have a feeling this may be more east than they have it now in the cone. I hope the RNC is taking this seriously as of right now still no update. Tampa will be getting some serious weather from Isaac regardless if it goes to the west panhandle area. People coming from all over the country for this and most have not experienced a hurricane before. As of right now no word on if they have done anything. I know going over the bridges during Tropical Storm Debby was not fun. Can't imagine them going over the bridges in hurricane winds in a bus. Also the bridges close at 40 mph sustained (at least the Skyway does.)
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Quoting TomballTXPride:

GFS has Mobile's number. However it's flip-floped like windshield wipers for the past 4 days. Take it for what it's worth.


I know! I should just back away from watching and reading for at least 30 minutes..who knows what will transpire during that time - lol
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my research shows that Punta Gorda was in the cone, but people focused too much on the line. Actually caused them to make changes in the way they presented the line.
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529. LRC
Just a note: NSIDC has released there raw iced extent numbers for today coming in at 4,087,031 km2 which beats the 2007 mark of4,140,000 km2 but not only that 3 weeks early. These are the raw numbers which are not put into the normal official totals for another 5 days or so (they use a 5 day run average). The impact of this on the NH winter this year could have major consequences for everyone. And if the weather in the Arctic stays too warm and violent all winter next summer could be a game changer as for as weather models are concerned, because no one knows what that could do to things.
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Quoting MiamiHurricanes09:
9mb stronger than the previous run. Major hurricane.



Yikes. Models have moved a little further westward again.

This windshield wiper effect is in full force now. I think we will know for sure by Monday when he bounces off of Cuba.
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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.


Me too ! It's like ok we good ... Oh maybe not.... Then we good again.... It is driving me crazy. With me being a weather nerd as some may call it everyone is asking me what will he do.... I just don't know....is all I can say lol
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Quoting MiamiHurricanes09:
9mb stronger than the previous run. Major hurricane.





could SE FL still see a direct hit?
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Quoting weatherh98:
Alassippi
Looks like almost where the "K" storm of 05 was headed.
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Quoting tennisgirl08:


True. But the worst of Isaac will be on his Eastern side. I think LA will be ok.


I agree.
Dont think anyone in LA needs to be freaking out, but remain watchful at least.
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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