Isaac pounding Haiti and the Dominican Republic with torrential rains

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

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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

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Well what a pathetic storm for us in swfl not even gonna jet wash the house was looking forward to one cat1 winds aswell :( nvm it's only August! Plenty hurricane season left.
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Quoting RevInFL:
Well in light of another shift West at 5pm. I have decided to leave for my planned trip to the panhandle. Glad the Space Coast is once again escaping. Everyone here seems to think a further shift West will happen so I am feeling pretty confident. I am enjoying following all the discussion.


everyone? Not I. I don't see this going to the panhandle even.
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Quoting GetReal:



Euro has now gone way east...


I Officially Hate Models!!!!!!!!!!!
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NEW BLOG
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Yes!  What does the NHC know?
Quoting peterj:
Since Isaac is moving over the water now...do you all feel like it is going to pick up intensity faster than what the NHC is predicting?

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Quoting WindNoise:
Did yall hear TWC talk about the low level center to the southwest of Isaac that it may help steer Isaac more westward. How long will that continue. Hmmm!
Here it is on NOAA official discussion. Also, the low pressure system over Colombia helped keep Isaac farther south away from the trough. Undoubtedly, it was a close dynamic that kept Isaac from eastern Fla. coast path. The ECMWF/EURO was correct on that part of the course and so far. Now the EURO is spinning to the east while the GFS goes west.ISAAC IS CURRENTLY PART OF A
LARGE LOW/MID-LEVEL LOW PRESSURE SYSTEM COVERING THE NORTHWESTERN
CARIBBEAN SEA AND THE ADJACENT LAND AND OCEAN AREAS. THIS LOW
PRESSURE AREA HAS A SECOND VORTICITY CENTER BETWEEN JAMAICA AND THE
CAYMAN ISLANDS...AND FOR THE NEXT 24-36 HR ISAAC AND THIS CENTER
WILL LIKELY ROTATE AROUND EACH OTHER. BASED ON THIS EXPECTATION...
THE FORECAST TRACK SHOWS ISAAC MOVING NORTHWESTWARD FOR 12 HR OR
SO...FOLLOWED BY A TURN TOWARD THE WEST-NORTHWEST. IT IS POSSIBLE
THE STORM COULD MAKE A SHARPER TURN THAN FORECAST...AS THE UKMET IS
SHOWING AN ALMOST DUE WEST MOTION BETWEEN THE FLORIDA KEYS AND CUBA
NEAR THE 36 HR POINT. THE EARLY PART OF THE FORECAST TRACK IS
SHIFTED A LITTLE TO THE LEFT OF THE PREVIOUS FORECAST...BUT STILL
LIES TO THE RIGHT OF THE CONSENSUS MODELS AND THE BULK OF THE
DYNAMICAL MODELS. AFTER 36 HR...ISAAC SHOULD BREAK FREE OF OR
ABSORB THE REST OF THE LOW...WITH THE SUBTROPICAL RIDGE TO THE EAST
BECOMING THE DOMINANT STEERING INFLUENCE. THIS SHOULD ALLOW THE
STORM TO GRADUALLY TURN TOWARD THE NORTHWEST AND NORTH OVER THE
EASTERN AND NORTHERN GULF OF MEXICO...WITH AN EVENTUAL LANDFALL
ALONG THE NORTHERN GULF COAST IN 72-96 HR. WHERE THIS MIGHT OCCUR
IS STILL RATHER UNCERTAIN...WITH THE TRACK GUIDANCE SPREAD BETWEEN
SOUTHEASTERN LOUISIANA AND THE CENTRAL FLORIDA PANHANDLE. THIS PART
OF THE TRACK FORECAST LIES IN THE CENTER OF THE GUIDANCE ENVELOPE
NEAR THE CONSENSUS MODELS AND IS SIMILAR TO THE PREVIOUS FORECAST.







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Quoting Matt1989:
It is going to take atleast 8 hours for the surface center to reorganize.. Chill out guys.. It did just get shredded by mountains..


Ed Rappaport of the NHC just said a possible eyewall could be forming to its NE but that will take 5-12 hours.
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You're kidding!

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Quoting presslord:


rational and sensible post....What the hell is wrong with you?!?!?!?!?!


This person obviously does NOT understand the rules fo this online community...where is the vitriol and un-supported supposition?
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Did yall hear TWC talk about the low level center to the southwest of Isaac that it may help steer Isaac more westward. How long will that continue. Hmmm!
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2457. KEEPEROFTHEGATE (Mod)
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2456. peterj
Since Isaac is moving over the water now...do you all feel like it is going to pick up intensity faster than what the NHC is predicting?
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2455. KEEPEROFTHEGATE (Mod)
T.C.F.W.
09L/TS/I/CX
MARK
21.37N75.83W


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2454. GetReal
63
Member Since: July 4, 2005 Posts: 204 Comments: 8871
2453. GetReal
54
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2452. GetReal
45 hrs
Member Since: July 4, 2005 Posts: 204 Comments: 8871
Picture in 2398 shows the eye feature pretty clearly. He's not as ugly looking at that image, he looks quite ready to rap up, land interaction with the convection over Cuba stopping that process at the moment. If Isaac continues to skirt the Cuban coast this could be a continuing issue until he moves northward. What's the deal with all the left over convection over Hispaniola? Is this the largest TS ever in this position? Simply unreal in size. Sucking in this new low will make him bigger or will that process weaken him? I know that's three questions in one post, I'm just intrigued beyond belief at this point.
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2450. Dunkman
18z may be a little slower and slightly weaker than the 12z through 48h but it's pretty much the same.
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2449. MZT
Quoting CaneHunter031472:
So based on the latest discussion by the NHC SE LA and MS are not yet out of the woods

Cone of error is only 2/3 probability after all. It could land in MS and the NHC shrugs and says "we never said we're always right"

1/3 odds of "elsewhere" are not dismissable.
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.
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Quoting RTSplayer:


Either you're a troll, or that guy needs to be fired.


Yea all signs point to a possible hurricane there and nothing less than a strong TS.
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So based on the latest discussion by the NHC SE LA and MS are not yet out of the woods
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Quoting Bielle:
Any word (official or otherwise) about the Bahamas and the other islands north of the COC?


Here on Little Exuma in the central Bahamas we have 35-40 steady, earlier today a band came through with near horizontal rain and 60mph gusts.
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Quoting Bluestorm5:




lol but wait....it hit Mississippi not the Florida panhandle. Thats what I mean. I can't remember any storm hitting the Panhandle after being where Isaac is now.
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36 Hours, GFS 850mb:

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Quoting MAweatherboy1:
Isaac has to be one of the ugliest systems of all time, lol.



Hey now don't boost Jose's self esteem.

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My forecast and intensity for Isaac..

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Quoting Gorty:


Is that an eye?

And it looks like his structure is getting better organized. Am I right?


Isaac doesn't have an eye yet, but the cloud structure appears to gradually become better organized.
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Quoting Grothar:
Moving much faster than earlier.


Ok sir.. for us newbies who have a hard time weeding out the hype and misinformation...I am going to direct my question to you. I live in MS and (barely) survived Katrina. What do these new models mean? I am NOT wish casting but people keep saying MS is out of it. I realize this is guessing at this point but I am a long time lurker and never really posted much until lately but even I know that a storm this size will affect several states when it hits. I also know that the "cone of doom" can shift at the drop of a hat. Why are so many people so sure it won't hit MS?
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Quoting Bluestorm5:




wow and I had even thought of Georges. Good work for doing the research to find the answer.
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dont forget you got a young kid runninh NHC,,,has not been their for 30 years,,he picks a model and sticks with it....hey I remember,,that Andrew was moving into Ft Lauderdale...took them 20 years to figuire out it was cat 5...Gezzzzzzzzzzzzzzzzz
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Quoting HCW:


Ernesto called and said don't forget about me :)


that name is forbidden lol "Ernesto". That was a nightmare for me in predicting.
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Presslord,
Where might I find a link to the present location of the loop eddy in the GOM?
Thanks for your help......
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2431. MZT
Looks like a feed band is developing on the east side, curling around to the south. Isaac is in the water off Cuba and it's "Game On"
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2430. WxLogic
18Z GFS keeping it a bit too weak for quite sometime... that would be a problem in the long run, but we'll see what happens towards the end.
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2429. shawn26
Can someone please post the 18Z GFS run?
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A VERY intense feeder band is consolidating on the S side near the radar station now.

If this keeps up we'll see some symmetry restored soon.




Still lots of convection moving over Hispaniola, so the farther it gets away the better the inflow will be for the storm.

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New Blog
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2426. HrDelta
Quoting Seflhurricane:
8pm


Not counting the fact that we still have 2 hours before 8. It could look better, and be a touch stronger by then.
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18z GFS 24 hours:

Member Since: July 6, 2010 Posts: 113 Comments: 31989
Also from the NWS discussion:

IT SHOULD BE NOTED THAT THE GLOBAL MODELS ARE
FORECASTING A VERY FAVORABLE PATTERN OF UPPER-LEVEL WINDS OVER THE
NORTHERN GULF OF MEXICO BY 72 HR. SHOULD THIS VERIFY AND SHOULD
ISAAC ESTABLISH AN INNER CORE...THE CYCLONE COULD BE SIGNIFICANTLY
STRONGER THAN CURRENTLY FORECAST
.
Member Since: June 16, 2006 Posts: 0 Comments: 1565
2423. LargoFl
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From what I'm seeing now, Isaac is going to shoot the gap between Cuba and Florida. Should keep South FL in the worst part of the storm the entire time. Interested to see how much Isaac is affected by the gyre of low pressure it is embedded in and how much that pushes the center west around the Florida Straits.
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2421. Drakoen
Quoting WxLogic:
B. Center Fix Coordinates: 21°30'N 76°18'W (21.5N 76.3W)


Farther north and west of the NHC coordinates. Not surprising.
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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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