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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There looks like there was a glitch in the UKMET model:

42 hrs.



48 hrs.

Member Since: Posts: Comments:
Center consensus is Mobile Bay, which is almost exactly what I expected all along...

...but wow, look how many west-leaning ensemble members there are now...this is a nightmare for forecasting.




Hopefully in 12 hours when 2 more GFS runs and another Euro run are done, this crap will get sorted out better.


This cycle is a disappointment, because it added more uncertainty rather than decreasing it...

At least the intensity forecast is starting to agree somewhat, but that's a shame too, because they are all pretty strong now.

6 members at or west of the mouth...damn


Yesterday 0,1,2.

This morning 3...

Now 6...
Member Since: January 25, 2012 Posts: 33 Comments: 1520
1769. LargoFl
Quoting coffeecrusader:
The biggest concern for the west coast of Florida (Tampa Bay area south) is that with the shape of the Florida west coast a slight wobble east could mean 50 miles closer to the coast.
yes for sure
Member Since: August 6, 2011 Posts: 4 Comments: 42029
1768. flcanes
Quoting RTSplayer:
Significant re-intensification occurring in the NE quadrant as per radar, which refreshes much faster than most public satellite products.


are you saying isaac just all the sudden isaac just jigged back into the carribean
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1767. Grothar
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it seem like there is a flow riding up the east coast why wont it carry it that way is it too weak
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Quoting Maineweatherguy20023:

Theve been in Cuban Airspace for 20 min now...


I think they have clearance to fly over cuban waters, but no clearance to fly over land.
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1764. angiest
Quoting robj144:


Yes, I agree Levi. That's why I mentioned the team of Ph. D.'s they have there. Honestly though, in the past 10 years, how often have they been way off? I know it happens from time to time, but it's quite rare.


Debby's first 3 or 4 forecasts may well be the absolute worst the NHC has put out in a decade (in terms of where the storm ultimately went, they were actually reasonable). I personally don't remember seeing such a dramatic shift in a forecast as that.
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1763. Levi32
Quoting MississippiWx:


The point is that none of the reliable models ever showed a straight line in that manner. Nearly all of them had Isaac popping out in the location he is in now.


Well the 06z GFS to take an example was pretty much a straight line and was a good 50 miles too far west with the 15hr forecast.

The model tracks are plotted in 12-hour increments. Yes the GFS comes out in 3-hour increments so there can be some little jumps that are not resolved on those maps, but it is very clear how far Isaac is outside yesterday's guidance envelope.

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Quoting Cruiser29:


Gfs has shifted back west


and the Euro, GFDL and HWRF went back east

the models are going to keep switching it appears as the dropsonde data has not given us too much of an agreement beyond 48 hours
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The biggest concern for the west coast of Florida (Tampa Bay area south) is that with the shape of the Florida west coast a slight wobble east could mean 50 miles closer to the coast.
Member Since: August 21, 2006 Posts: 0 Comments: 254
Does anyone else see an eye?
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What atmospheric level is best for modeling atmospheric pressure for tropical cyclones?
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Quoting Cruiser29:
check out the gfs ensemble models concentrated on MS/AL Link


Interesting enough there are maybe one or two ensembles off to the east where the Euro has it moving inland....most of the ensembles are weighted more heavily to the west of the operational...this tells me that I would not be surprised at all to see the 18Z shift even further west maybe coming in line with the CMC
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I have not seen a tropical storm so wide across since Ivan and Katrina. Even at a CAT 1 or 2, if the two halves of this storm re-join, we're looking at an incredible rain event.
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1756. dmh1026
Quoting Hurricanes101:
its pretty clear the models are still going everywhere and cannot come to an agreement. I really think we can drive ourselves crazy hinging on each model run or even figuring out if the center is offshore of Cuba or not

that's why I am focusing on the recon data and the updated watches and warnings for my area

I agree... I don't follow the models past 2-3 days out...
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GFS ensemble now a little west......hmm, and GFDL moved to Moble Bay. Lets see if it holds for a few runs.
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Anyone know the depth of the water off the north Cuban coast where Recon is currently flying? SFMR has issues in shallow water, believe they call it "shoaling". where the chopiness of the water fools the SFMR.
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Quoting louisianaboy444:


Its interesting because one it is consistent on the more western track and two some of the GFS ensembles are starting to come around to the idea....of course the Florida people wont mention that though :) If you are in SE LA or MS I would not take my eyes off of this yet...


No doubt for sure. It's just that folks there have more time to waste hanging out on this blog until they have to hang it up, react and get things done to prepare. Probably wait until Mon afternoon up there. If you're in the FL Keys, you need to be done before dark tonight.
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1751. JasonRE
Quoting MarkTodd2233:


Me to, I started in 2004 when a friend told me about WU, love this season & live tracking, wake up all hours of the night checking, and now I check with my phone,,, do the words ADDICTION mean anything


Haha. Nothing wrong with that. I've been on here since 8 am. Crazy storm this Isaac. We'll see what he does over the next 24 hours. I'm wondering what effect the GOM will have on him.
Member Since: August 6, 2012 Posts: 0 Comments: 122
1750. flcanes
Quoting atmosweather:


He must be onshore right now simply because RECON is flying right around where the center should be and is not getting a center fix yet. There haven't been any dropsondes released either so it is clear Isaac isn't quite clear of the Cuban coast.

he was supposed to leave an hour ago
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Significant re-intensification occurring in the NE quadrant as per radar, which refreshes much faster than most public satellite products.

Member Since: January 25, 2012 Posts: 33 Comments: 1520
1748. Michfan
Quoting GetReal:


It is posted by Colorado St and that is the only info I have besides the link.


Yeah i recall there being a custom product by them where you can input how many frames you wanted width height etc. Just can't seem to find it anymore.
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Quoting violet312s:
Looks like recon not allowed to enter Cuban airspace. :(

Theve been in Cuban Airspace for 20 min now...
Member Since: August 1, 2012 Posts: 0 Comments: 269
Quoting chrisdscane:


look at the sat hes offshore sorry


We need the center fix coordinates from the HH. Then I will believe you...lol.
Member Since: July 17, 2008 Posts: 0 Comments: 1242
1745. robj144
Quoting Levi32:


The foundation of science is doubt.

Isaac is completely outside the 24-hour supercomputer guidance envelope from yesterday (see below). There is more than enough room to question computer forecasts, always. That's why the NHC exists, and other human forecasters have jobs.



Yes, I agree Levi. That's why I mentioned the team of Ph. D.'s they have there. Honestly though, in the past 10 years, how often have they been way off? I know it happens from time to time, but it's quite rare.
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So 999, still a little higher, which makes sense, but not bad considering what Isaac has just traversed the past 24 hours.
Member Since: September 5, 2008 Posts: 0 Comments: 125
Quoting louisianaboy444:


Its interesting because one it is consistent on the more western track and two some of the GFS ensembles are starting to come around to the idea....of course the Florida people wont mention that though :) If you are in SE LA or MS I would not take my eyes off of this yet...


just like you whined when the tracks shifted east last night?

fact is the models are in less agreement now then they were yesterday, lets see how this progresses, but a stronger Isaac will be further east
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1742. LargoFl
Quoting LargoFl:
....anyone Know what this model is? WRF
Member Since: August 6, 2011 Posts: 4 Comments: 42029
1741. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 42029
Quoting TropicalAnalystwx13:
Off-topic, but..

According to NBC News, the first man ever to set foot on the moon, has died at the age of 82.

:'(
Member Since: August 1, 2012 Posts: 0 Comments: 269
Recon can't make a center fix. Isaac is either on the coast or too close for the HH to fly through.
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Quoting tennisgirl08:


I respectfully disagree. He is already on the southern edge of the NHC cone right now.
Just onshore in Cuba.


He must be onshore right now simply because RECON is flying right around where the center should be and is not getting a center fix yet. There haven't been any dropsondes released either so it is clear Isaac isn't quite clear of the Cuban coast.
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Looks like recon not allowed to enter Cuban airspace. :(
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In looking at visible satellite imagery this afternoon, it will be interesting to see whether or not the circulation center of Isaac currently moving parallel to the Cuban coastline can capture the convection ongoing to the west between Cuba, Bahamas, and Florida. If so, then this could help the storm recover quicker and strengthen at a faster clip.
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Quoting tennisgirl08:


I respectfully disagree. He is already on the southern edge of the NHC cone right now.
Just onshore in Cuba.


To be even more fair, the width of the cone at that spot is like 5 feet
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Quoting GetReal:
CMC 12z 96 hours


I'm not buying this model... But being fair to the CMC fans...


Its interesting because one it is consistent on the more western track and two some of the GFS ensembles are starting to come around to the idea....of course the Florida people wont mention that though :) If you are in SE LA or MS I would not take my eyes off of this yet...
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Quoting Levi32:


I don't know what you mean by "interpolated," but those plot points are at the coordinates of the low pressure center depicted by each model.

The interpolation that occurs on those plots is simply during the early model cycles when the late models have not yet updated. During that time, the previous model run is interpolated given the new position of the cyclone. That's why you see "AVNI" for "AVN interpolated from the previous run" during the first couple of hours after a model cycle. The real GFS 12z has not come out yet on that map.


The point is that none of the reliable models ever showed a straight line in that manner. Nearly all of them had Isaac popping out in the location he is in now.
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1731. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 42029
Quoting tennisgirl08:


I respectfully disagree. He is already on the southern edge of the NHC cone right now.
Just onshore in Cuba.


look at the sat hes offshore sorry
Member Since: July 21, 2010 Posts: 0 Comments: 1181
999.1mb.

191500 2119N 07527W 8438 01473 9991 +170 +170 119037 039 045 008 01
Member Since: September 2, 2009 Posts: 130 Comments: 21194
1728. GetReal
Quoting Michfan:


Can you pm me the link to get those loops. I'm having trouble finding it in my favorites. Used to have a perfect Rapid Scan site but lost it all in my format.


It is posted by Colorado St and that is the only info I have besides the link.
Member Since: July 4, 2005 Posts: 204 Comments: 8898
Quoting CaneHunter031472:
My wife just got done telling me that I'm down playing this a bit. We are in Gautier MS which is 30 miles from Mobile AL. Should we be worried about Isaac. I mean run after run the most reliable models take this system east of Pensacola. The official NHC forecast follow this logic as well and the close we get to next week IMO the more sure I feel that we will not feel other than heavy rains with Isaac over here. My prayers and thoughts to the ppl pf Haiti and I will keep Florida in my prayers as well.
I'm from Gautier too!!
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Quoting RTSplayer:


That's because it moved west back over land.

It was definitely over water a few frames ago.

visible floater



Will probably wobble on and off the coast till it gets around where Cuba topography curves away to the west away from the average track forecast.


He also said it was move WNW at 17mph... pretty fast. There's the sling shot effect Levi called.
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1725. hamla
untill issac gets out in the gom past the fl keys the models then will have a real good idea of a track so late sun.night imo
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Gretna, LA here..... keeping an eye on it as well!
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Quoting Levi32:
Recon gliding along the coast of Cuba now.



Can they fly over Cuba inland?
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its pretty clear the models are still going everywhere and cannot come to an agreement. I really think we can drive ourselves crazy hinging on each model run or even figuring out if the center is offshore of Cuba or not

that's why I am focusing on the recon data and the updated watches and warnings for my area
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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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