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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1021. Grothar
The HWRF still doesn't want to give up

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1020. LargoFl
Quoting icmoore:


Madeira Beach here and not laughing either.
IC do you think they will evacuate your area?..i see on the storm surge map..all the beach area's flood
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Quoting scott39:
You think models that have gone father W many times and still presently are(GFS)....carry no weight?


The models that are west do carry just as much weight, at least the major ones, its a hard call right now. Even New Orleans can't be ruled out honestly.
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watching closely here in Marco Island.
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the mode runs have been pointing at a strong hurricane the ? is how strong will it get be for land fall


am thinking any where from 100mph too 155 mph if things set up this right
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Quoting Tanner27:
ok im fort lauderdale where is this thing going?




well its jogging north so stay aware
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1015. scott39
Quoting reedzone:
The track makes sense, keeps Isaac east of 85W and doesn't slam the storm into a building ridge to the west. the intensity is overdone though.
You think models that have gone father W many times and still presently are(GFS)....carry no weight?
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Quoting Bluestorm5:
Did you read what Dr. Masters said about Gulf? It's not as favorable as 2005 in term of water hear. He doubts a Category 4 or 5.




do you look at mode runs ???
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1013. icmoore
Quoting oakland:


I'm sitting in Bradenton. Not a laughing matter for me.


Madeira Beach here and not laughing either.
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Quoting drj27:
TWC is a joke i mean im no weatherman but the guy just said the gfs has it going to nola/ms im in ft.walton beach im prepared i hope everyone else is


That is a joke.

There are no systems or meteorological wild cards to sweep Isaac that far west. I'm just a lay person, and I can see that. The thing is, there are times that I don't think these guys get fully immersed in the minutiae of storms like this, and simply act as stenographers for the latest doom and gloom scenarios without doing a bit of actual analysis.
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1011. Chiggy
Can someone post or direct me to the 12Z GFS Ensembles please? Thx
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1010. HrDelta
Quoting GetReal:
66 hours



Never seen a system bomb that clse to the coast in NE GOM>>>


What model is that? Because that is Category 4 strength.
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ok im fort lauderdale where is this thing going?
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Quoting Joanie38:
I am in Central LA, The weatherman here doesn't think we will have anything to worry about with this storm...but me, being a weather fanatic, I am still a little weary..remember Rita? It was suppose to go to the central coast of Texas and it kept creeping more up the coast to LA??...well my gut feeling tells me to just keep watching cause no one REALLY knows where it is going...NOONE...any thoughts???? TIA:)

Joanie :)
Hello Joanie, i use to live in Alexandria, Isaac could go as far west as La. but chances are slim, watch storm and you should be fine. I think this Storm turns North and then Northeast before he gets to La. but that is not 100 percent.
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Quoting TomballTXPride:
At least along the coast east of Apalachicola in the Big Bend Region the population is low. But inland, you start hitting regions with a greater concentration of people per square mile such as Tallahassee, FL...



and some of that area is over 30 inches over the average rainfall for the year, and its only august, could see some 100+ inches for the year in that area come DEC 31
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Quoting Tazmanian:




may be not the water temper in the gulf is hot so if things set up right thing storm could go in too RI
Did you read what Dr. Masters said about Gulf? It's not as favorable as 2005 in term of water hear. He doubts a Category 4 or 5.
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1004. 7544
is issac connecting with the bahama blob?? hmmmmmm
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just repeating incase some missed it

I have found the LLC location 20.3N 76.2W movement W-WNW

surface obs
#1 at 20.8N 76.3W show winds out of ENE
#2 at 19.9N 75.2W has wind out of the WSW

als lastes microwave image does kind agree with this
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Quoting StormTracker2K:
This would be a dangerous storm surge situation for Tampa.




The HWRF is showing no mercy right now for the west coast of Florida lets hope that doesn't play out.
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1001. oakland
Quoting washingtonian115:
Once again the models show that no matter where Isaac goes it will end bad for someone.


Sad to say I agree with you. This will be awful for somewhere.
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1000. GetReal
Quoting Tribucanes:
Could we please cool it with verbage like "what you wish for" That's insulting and insinuating people want death and destruction. People are discussing model track and Jeff has even called for a 40% chance of cat.2 . No one "wishes" for this, we watch in awe and are intrigued by the possibilities.


With all do respect there are those people, mostly individuals that have never experienced one, the clean up, the recovery of corpses, the no electricty, nor food and water, that are pulling for this system to strike the area they reside....

I'm sorry if the truth is uncomfortable for some. Those people know who they are.
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The HWRF looks a little too far north during the first 24 hours since Isaac is already west of its track since the 12z initialization.

However, the interesting part is not the first 24 hours, but the recurvature parallel to the Florida Peninsula, regardless of how far east it is while it is doing so. The HWRF could show the storm 100 miles offshore, but if it shows a track with a different shape than the other models, then that is the most significant difference.
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Quoting Hurricanes305:


Starting to look slightly better now than earlier.



sure is
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Quoting reedzone:
The track makes sense, keeps Isaac east of 85W and doesn't slam the storm into a building ridge to the west. the intensity is overdone though.




may be not the water temper in the gulf is hot so if things set up right thing storm could go in too RI
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Quoting PalmBeachWeather:
There is a logical reason models flip-flop....I'm suprised you don't realize that.


I think the west coast of FL track is good but my question is how is the GFS and Euro seeing a strong ridge out of this.

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The formation of tropical cyclones is the topic of extensive ongoing research and is still not fully understood. While six factors appear to be generally necessary, tropical cyclones may occasionally form without meeting all of the following conditions. In most situations, water temperatures of at least 26.5 °C (79.7 °F) are needed down to a depth of at least 50 metres (160 ft); waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms. Another factor is rapid cooling with height, which allows the release of the heat of condensation that powers a tropical cyclone.High humidity is needed, especially in the lower-to-mid troposphere; when there is a great deal of moisture in the atmosphere, conditions are more favorable for disturbances to develop. Low amounts of wind shear are needed, as high shear is disruptive to the storm's circulation. Tropical cyclones generally need to form more than 555 kilometres (345 mi) or 5 degrees of latitude away from the equator, allowing the Coriolis effect to deflect winds blowing towards the low pressure center and creating a circulation. Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place.

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Quoting Bluestorm5:
HWRF tends to overdo it, right? 945 mb is unrealistic... then again, Florida's tallest mountain is 300 feet near Alabama. Flat state.

No mountain more like highest ground/plateu
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Quoting RTSplayer:
The CoC is not where you guys think it is.

It is directly due N of the radar station, and moving WNW.

The center is negative space on radar, not the rain bands; unless it's collapsed, which it has not collapsed.



1

You are right on!!!

That radar is not "primitive" as Levi states. It's an ARSR 4, which is one of the most modern high altitude/long range radars out there. It was the last modern upgrade to the FAA CONUS. It's primary purpose (ie..it's calibration) is for the purpose of Air Traffic Control, not weather.

It does process weather, but in a different format than dopplar. Weather is a lower priority.

The reason the precip appears limited is because it actually is. The vast majority of it is located E/SE and beyond the 200 mile range of the radar. This is obvious on sattelite imagery.

Isaac is disorganised and the radar clearly shows that. Hence the COC being difficult to discern with the vast majority of tools at our disposal. HH's will tell us where the center is.
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Quoting PalmBeachWeather:
There is a logical reason models flip-flop....I'm suprised you don't realize that.
This is true, weather patterns, in particular upper level steering changes daily even in hours, minutes, seconds. It's a progressive thing, it doesn't wait for you.
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The track makes sense, keeps Isaac east of 85W and doesn't slam the storm into a building ridge to the west. the intensity is overdone though.
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Looks like a relation between isaac and the FL mess... Isaccum cleaner has been activated
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Quoting icmoore:


If they are wishing for it they must not have their homes and lives on the line. I would suggest they go watch a disaster movie and get it out of their system.


I'm sitting in Bradenton. Not a laughing matter for me.
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Quoting MiamiHurricanes09:
So IKE has major hurricane Issac visiting him in 3 days on the HWRF:





looks like i may be right if that comes ture a strong cat 3 or 4 land fall but am not sure if it will go that far E am thinkig more W
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HWRF tends to overdo it, right? 945 mb is unrealistic... then again, Florida's tallest mountain is 300 feet near Alabama. Flat state.
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Quoting floridaboy14:
WOAH. HWRF out to lunch. major hitting tampa. too far east and too strong


Strong = more poleward, so not out too lunch if it had the intensity right. Intensity forecast are not very good. I think a cat3 is pushing it a bit quick for 60 hours.
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Quoting MiamiHurricanes09:
It has it near 22.5N in about 12 hours, and it initialized the circulation at the latitude where I believe it is right now. We'll see what it does; the mean motion is NW, if it has more N-ward wobbles than W-ward wobbles, a track that poleward could verify, especially since I believe it's moving NNW-ward right now based on latest satellite imagery.


Starting to look slightly better now than earlier.
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Quoting Grothar:
This will keep the blog going next week. This is the one to really watch. I saw it first.



No, I did! lol
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This would be a dangerous storm surge situation for Tampa.

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Quoting StormTracker2K:
Geesh. Can we get a consensus on the models already instead of all this flip flopping.

There is a logical reason models flip-flop....I'm suprised you don't realize that.
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09L
TROPICAL STORM 09L 12:00UTC 25August2012
UW-CIMSS Experimental Vertical Shear and TC Intensity Trend Estimates

Current Conditions (from TPC) :
Latitude : 19:45:45 N
Longitude : 73:53:31 W
Intensity (MSLP) : 994.0 hPa

Max Pot Int (MPI,from Emanuel) : 914.0 hPa
MPI differential (MSLP-MPI) : 80.0 hPa

CIMSS Vertical Shear Magnitude : 7.4 m/s
Direction : 236.9 deg

Outlook for TC Intensification Based on Current
Env. Shear Values and MPI Differential
Forecast Interval : 6hr 12hr 18hr 24hr
F F F F

Legend : VF-Very Favorable F-Favorable N-Neutral
U-Unfavorable VU-Very Unfavorable

-- Mean Intensity Trend (negative indicates TC deepening) --
6hr 12hr 18hr 24hr
VF <-3.0mb/ 6hr <-6.0mb/12hr <-9.0mb/18hr <-12.0mb/24hr
F -3.0 - -1.0 -6.0 - -2.0 -9.0 - -3.0 -12.0 - -4.0
N -1.0 - +1.0 -2.0 - +2.0 -3.0 - +3.0 -4.0 - +4.0
U +1.0 - +3.0 +2.0 - +6.0 +3.0 - +9.0 +4.0 -+12.0
VU >+3.0 >+6.0 >+9.0 >+12.0
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Quoting StormTracker2K:
Geesh. Can we get a consensus on the models already instead of all this flip flopping.

Oh no then it would be too easy, Isaac likes to play games, and keeps us guessing.
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So IKE has major hurricane Issac visiting him in 3 days on the HWRF:

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Quoting WxGeekVA:


True. Possible, but highly unlikely.


How about no possible chance at all.
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Quoting reedzone:


I agree, the intensity is probably overdone, but the track is good.
You think the track is good? I always thought the HWRF did better with intensity and bad with track.
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Once again the models show that no matter where Isaac goes it will end bad for someone.
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HWRF visually shows my forecast but about 50 miles to the west and ending in KTLH.
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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