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 wunderkidcayman:
ok guys what I think is that we have weak elongated LLC or multiple LLC and a kinda weak MLC

the multi or elongated LLC are over cuba and the MLC is on the Northern SE tip coast I still think that the main weak LLC is on the southern half of the SE tip of cuba wind obs from cuba does show this and radar show N half of the tip what ever is there moving W and South half hardly show much but the few speck that are there are moving E so circulation is weak but its over cuba


I have to agree with you here, WKC. Although, I don't think its moving W and South, but W/NW.

The circulation is so broad it is really hard to tell. I think he might still be over Cuba though but on the Northern part of Cuba.
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Quoting GetReal:
54 hours


Into Tampa like many on here are looking for... Be careful for what you wish for...
Would put the convention in danger from storm surge and flooding.
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896:

Look at that...the NAM picks up on that LLC I pointed out near Caymans, doesn't do too much with it, but at least it noticed...
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Quoting tennisgirl08:


No, Taz! You are hyping.



always expect the unexpected

isaac is not a joke
and will be far from
a laughing matter
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Quoting CybrTeddy:


That observation was taken while recon was ascending in altitude.


Very good point. I did not notice that at the time.
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Quoting Tazmanian:




this storm can be come a cat 5 look how big the W PAC storms and they can be come strong cat 4 and 5 storms no matter how you look at small storms and big storms can be come vary powerfull


True. Possible, but highly unlikely.
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Quoting tennisgirl08:


No, Taz! You are hyping.

Agreed but he did say "could"...
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Quoting TampaCat5:
After Debby, I don't put much stock in the NHC's Winds graphic. They showed much of the bay and surrounding area with a 100% chance of TS force winds, but there weren't any.


No, not quite. In fact, we had tropical storm force winds ongoing in Pinellas for hours well before a tropical storm watch or warning was issued. Sustained tropcial storm force winds with gusts 50 to even 60 mph occurred here in Pinellas for a long time.


They were actually bit behind and the west coast of Florida received a much greater impact on both wind rain and surge than anticipated. Besides the NHC wind graphic actually states that wind speeds of that strength are possible within the wind field shading but it doesn't mean all areas will see them. That's the nature of the physics of wind especially as it interacts with land, some areas will have surprisingly weak winds while other areas will have very strong winds.


As you get closer to the eye-wall or center of circulation, wind speeds gradually grow more organized and uniform. However even with the eye-wall of a hurricane winds even then are not uniform. a category 4 hurricane listed at 140 mph might give a period of 155 mph gusts or more to one given area in a deeper convective band while another location in the eye wall may be seeing 130 mph gusts in weaker convection. That's just how it is with low pressure systems, they aren't that general.

Because of this the NHC creates a general wind graphic so they aren't so confusing, and for the fact that properly representing all wind speeds in sall parts of the storm is pretty much near impossible.
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Quoting GetReal:


Link

Gracias.
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Quoting Tazmanian:
i think we could be looking at a strong cat 4 or low cat 5 some where in the gulf then some sight weaking be for land fall too a strong cat 3 with cat 4 or 5 waves

The heat content in the eastern gulf will not support that strong of a storm. Cat 2 at most!
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ok wait hold up new sat images came out plus sfc obs

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
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Quoting scott39:
What do you think about the most recent GFS putting Isaac in the Mobile Bay again?


Mississippi Alabama line

But that's splitting hairs
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54 hours


Into Tampa like many on here are looking for... Be careful for what you wish for...
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Quoting Maineweatherguy20023:

that is a hype but you never know MAYBE CAT 4 but its too big and disorganized to be a CAT 5




this storm can be come a cat 5 look how big the W PAC storms and they can be come strong cat 4 and 5 storms no matter how you look at small storms and big storms can be come vary powerfull
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For reason's like Isaac I'm glad I decided to stay here in D.C and not move to Florida.
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Quoting chrisdscane:



you still seeing the northward jog or did it stop??


You won't know for 2-3 hours at least.
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12z HWRF falls right in line with Levi's idealism. A leftward track scraping along the western Florida coast.
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Quoting Grothar:


That is why I put jog in quotes. Pay attention 98. :)


Yes I know. I was stating the obvious :)

G=6.67 x e-11 m2/kg2
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....SEE..issac has a 1 in 3 chance to be OUTSIDE the cone
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Quoting cchsweatherman:


They are actually quite far from the center. With the center at 20.1 N and 74.6 W as most recently updated by the NHC and that observation at 17.7 N and 64.8 W, that places that 999.5 mb observation 605 miles away from the center of the storm.


That observation was taken while recon was ascending in altitude.
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Quoting TampaCat5:

Thank you. Do you have a link for that?


Link
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Quoting LargoFl:
...look at all those warnings
yeah..I hope someone tells Issac he's not allowed to cross from Volusia to Flagler..lol..warnings stop at Flagler beach..at least for today..lol of course I'm 2 miles north of Flagler beach..and 10 miles inland...
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Quoting Tazmanian:
not sure is the recon is at the center yet but they have found

999.5 mb
(~ 29.52 inHg


They are actually quite far from the center. With the center at 20.1 N and 74.6 W as most recently updated by the NHC and that observation at 17.7 N and 64.8 W, that places that 999.5 mb observation 605 miles away from the center of the storm.
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Quoting Tazmanian:
i think we could be looking at a strong cat 4 or low cat 5 some where in the gulf then some sight weaking be for land fall too a strong cat 3 with cat 4 or 5 waves

that is a hype but you never know MAYBE CAT 4 but its too big and disorganized to be a CAT 5
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Quoting Levi32:
I have seen the GFDL and HWRF exhibit both poleward and westward biases in huge ways. I don't see them as particularly susceptible to one or the other. The UKMET is the same way. Some models are "balanced" in their errors.
What do you think about the most recent GFS putting Isaac in the Mobile Bay again?
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Quoting GetReal:



HWRF

Thank you. Do you have a link for that?
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Quoting Levi32:
I have seen the GFDL and HWRF exhibit both poleward and westward biases in huge ways. I don't see them as particularly susceptible to one or the other. The UKMET is the same way. Some models are "balanced" in their errors.
so what are the best models to use right now? GFS and Euro? CMC shifted way west towards lousiana in its 12z run
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Quoting Levi32:
I have seen the GFDL and HWRF exhibit both poleward and westward biases in huge ways. I don't see them as particularly susceptible to one or the other. The UKMET is the same way. Some models are "balanced" in their errors.



Hello Levi... Yeah the windshield wiper model runs continue with some....
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.WATCHES/WARNINGS...
A HURRICANE WARNING CONTINUES FOR THE FOLLOWING LOCATIONS...
COASTAL COLLIER...MAINLAND MONROE AND FAR SOUTH MIAMI DADE.

FOR MARINE INTERESTS...A HURRICANE WARNING CONTINUES FOR
THE LOCAL GULF WATERS.

A TROPICAL STORM WARNING AND A HURRICANE WATCH CONTINUES FOR THE
FOLLOWING LOCATIONS...INLAND MIAMI DADE...METRO MIAMI DADE AND
COASTAL MIAMI DADE.

FOR MARINE INTERESTS...A TROPICAL STORM WARNING AND A HURRICANE
WATCH CONTINUES FOR THE ATLANTIC WATERS SOUTH OF GOLDEN BEACH AND
BISCAYNE BAY.

A TROPICAL STORM WARNING CONTINUES FOR THE FOLLOWING LOCATIONS...
GLADES...HENDRY...INLAND PALM BEACH...METRO PALM BEACH...INLAND
COLLIER...INLAND BROWARD...METRO BROWARD...COASTAL PALM BEACH AND
COASTAL BROWARD.
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That's about 75 to 80 miles due N from the Guantanamo radar, and still just about half over land, found by extrapolating the circle around some...
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Somebody posted the 2 AM models and it's on the southern end of those along the gfdl
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Quoting weatherh98:


It's pretty much wnw again


That is why I put jog in quotes. Pay attention 98. :)
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Quoting wunderkidcayman:
ok guys what I think is that we have weak elongated LLC or multiple LLC and a kinda weak MLC

the multi or elongated LLC are over cuba and the MLC is on the Northern SE tip coast I still think that the main weak LLC is on the southern half of the SE tip of cuba wind obs from cuba does show this and radar show N half of the tip what ever is there moving W and South half hardly show much but the few speck that are there are moving E so circulation is weak but its over cuba
http://www.ssd.noaa.gov/PS/TROP/floaters/09L/flash -swir-short.html


im not so sure look at this loop
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Quoting PalmBeachWeather:
wx13.......I agree.. You might want to but an intensity over Key West....But very good

I had 12 hours increments at first, but when I added the intensity text boxes, the image as a whole was really cluttered so I resorted back to 24 hour increments. For anybody that is interested, I think it will pass through the Keys as a 75-80 mph hurricane.
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Quoting GetReal:
42 hours

Isn't the HWRF the one to look at since it has the HH data or something like that? I remember somebody posting that yesterday..
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NAM MODEL
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Quoting scott39:
How can anybody discount Isaac going W of the panhandle? The GFS currently and the Euro for many runs before last night has also. To say it will not go father W on the Gulf Coast is ill advised.


I think the furthest West he will go is Mobile.

But my guess right now would be Destin/FWB/Panama City
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Quoting depalma13:


The Northern Gulf will not sustain that strength unless it makes a strong cat 3 well before landfall. The storm will be a cat one borderline cat two at landfall.


haha you are mistaken...water temps in the mid 80's low shear, no dry air...theres no reason it cant sustain at cat 3 or even higher. after camile did make landfall basically as north as you can go in the gulf
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Quoting weatherh98:


It's pretty much wnw again


Yes, jog only so far. If you look at vis sat and enable the TC forecast points, its still right on the track line.
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Quoting TampaCat5:
GetReal which model is that?



HWRF
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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.

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Tampa next. Concerning run.

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(click to enlarge)


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Quoting Tazmanian:





you dont no that

So you expect a cat 5 in the GOM? I see a mid grade CAT 3.
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I have seen the GFDL and HWRF exhibit both poleward and westward biases in huge ways. I don't see them as particularly susceptible to one or the other. The UKMET is the same way. Some models are "balanced" in their errors.
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Quoting wolftribe2009:


Yes I agree that it would be good for the storm to weaken. This isn't a game or something to laugh about. This is a serious storm with potential to threaten plenty of lives and property. This is the worst thing to being a weather forecaster. We here at the blog can be wrong and no harm done except one's ego but the forecasters at NOAA and other forecast places get crap from the public (including this blog) but in the end they are the ones that have to make the life and death decision. That isn't something I would look forward to. I agree that sometimes I am critical of NOAA but at least I don't have to tell people when to move and then take the heat when I don't tell others to move and they lose their lives or property in the storm.


Well said.......
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Quoting WXGulfBreeze:
One thing I haven't seen anyone discuss - when Isaac arrives, we're going to be close to a full moon. If he comes in around Mobile / Pensacola, the full moon will definitely increase storm surge potential in both Mobile and Pensacola Bays (as well as all bays / sounds from Mobile east to probably Panama City Beach).
Fl. is the main topic of disscusion right now. By Monday we could be talking about that, and yes it would not be good.
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48 hours


Going right?
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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