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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2071. angiest
Quoting weatherh98:


They can't find the center?


Very close to the coast. It will be hard to fly through the center without flying over land, and they don't fly recon over land.
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I'll be watching late tonight and tomorrow to see if COC
can tighten up more and core develop..It had barely got going before It crossed Haiti .
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2069. WxLogic
For now 18Z NAM @36HR showing weaker ridging in the N Gulf states compared to earlier runs. In this 18Z NAM received 5 more DROP & 30 RAOBF Data than the standard.

Will be interesting to see if the passing TROF over the Great Lakes leaves a weakness open long enough.
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he could be a Hurricane at 11, its only 14 mph.
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2067. wpb
what needs to be written the land mass is getting closer to sea level he slowly moving away from the 12k mountain into land mass less than 250 ft.
Member Since: May 28, 2011 Posts: 0 Comments: 573
2066. Michfan
Quoting weatherh98:


They can't find the center?


They can't get close enough to the Cuban coast to get to it. 997 is the lowest they have found so far.
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Quoting Tazmanian:




am looking at what the recon is fineing not where the nhc put the center at 2pm

I know, I was showing the previous position to get a idea where it moved to.
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Quoting CaneHunter031472:
ANT alert! My backyard is flooded in Ants!


thats not a good thing. lol the ants might know better than your average weatherman. NOAA had a blog this past week of how studies show animals can sense when a hurricane is coming.
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Quoting WeatherNerdPR:
Judging from recon, Isaac should be at 60mph, 998mb at 5PM. Should be a hurricane by tomorrow night.



they found a 997mb not too long a go
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Quoting cajunkid:
COC opening up now. Not as tight as earlier.


In the time recon has been out there the pressure has come down from 1000mb to 997.7mb.
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Quoting UKHWatcher:


Can you stop policing the blog please...

cr*p spelt as it was, is not a swear word.

and sykkid did NOT say the storm was dead.

Isaac will hit cane status by 1pm UTC Sunday i think

Oh yes he did...and im not policing either!!Said lots of times this morning and yesterday too.Even now he says the coc is weak lol! Its not!! So just go on with it and leave it alone.
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Member Since: August 18, 2008 Posts: 0 Comments: 628
Judging from recon, Isaac should be at 60mph, 998mb at 5PM. Should be a hurricane by tomorrow night.
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Quoting Levi32:


They are trying hard. 998mb is the new extrapolated pressure minimum.



They can't find the center?
Member Since: June 17, 2011 Posts: 11 Comments: 6539
Quoting TampaFLUSA:
2:00 PM EDT Sat Aug 25 Location: 20.8°N 75.3°W


sorry correction 21n 76W
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2056. Levi32
Quoting Hurricanes101:


yea true Levi, am I wrong to say that in the 12Z runs, the GFDL, HWRF, Euro and Ukmet all shifted east?


UKMET is about the same as the 00z.
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Not directly related to Isaac.

The Cayman low is down to 1004mb along a stationary front.

A nearby buoy has a reading of "29.73in and falling," which is 1006.5mb.


I wonder what this is going to do for steering if it keeps deepening?

Wind barb overlay still suggests a partially formed circulation at the lower levels.
Member Since: January 25, 2012 Posts: 33 Comments: 1520
Quoting Realtormama:
Can someone explain why it is supposed to be Cat 2 then fall off to Cat 1 at landfall somewhere in the northern gulf. What makes it fall off?

Land interaction
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2053. Michfan
Time: 20:06:00Z
Coordinates: 21.3667N 75.7333W
Acft. Static Air Press: 842.5 mb (~ 24.88 inHg)
Acft. Geopotential Hgt: 1,483 meters (~ 4,865 feet)
Extrap. Sfc. Press: 997.7 mb (~ 29.46 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 130° at 18 knots (From the SE at ~ 20.7 mph)
Air Temp: 19.6°C (~ 67.3°F)
Dew Pt: 15.5°C (~ 59.9°F)
Peak (10s) Flt. Lvl. Wind: 19 knots (~ 21.8 mph)
SFMR Peak (10s) Sfc. Wind: 26 knots (~ 29.9 mph)
SFMR Rain Rate: 9 mm/hr (~ 0.35 in/hr)
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Issac is still a 60mph tropical cyclone.

As I stated in my blog, I'm expecting Hurricane Issac by sunrise.


201200 2134N 07531W 8432 01488 //// +152 //// 139053 056 051 015 01
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Quoting TampaFLUSA:
2:00 PM EDT Sat Aug 25 Location: 20.8°N 75.3°W




am looking at what the recon is fineing not where the nhc put the center at 2pm
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Time: 20:13:00Z
Coordinates: 21.5667N 75.4667W
Acft. Static Air Press: 842.1 mb (~ 24.87 inHg)
Acft. Geopotential Hgt: 1,494 meters (~ 4,902 feet)
Extrap. Sfc. Press: -
D-value: -
Flt. Lvl. Wind (30s): From 147° at 50 knots (From the SSE at ~ 57.5 mph)
Air Temp: 15.0°C* (~ 59.0°F*)
Dew Pt: -*
Peak (10s) Flt. Lvl. Wind: 52 knots (~ 59.8 mph)
SFMR Peak (10s) Sfc. Wind: 51 knots* (~ 58.6 mph*)
SFMR Rain Rate: 14 mm/hr* (~ 0.55 in/hr*)
(*) Denotes suspect data
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its offshore heading about 310-315 itll be a close call for SEFL wouldnt be surprised to see hur-warnings up later, watch and see hope for the best
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it almost looks like Isaac is now connected with all of the rain over Florida as its rotating the same way lol

bye bye blue skies, see you on Tuesday here in Tampa
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LAND.
Quoting Realtormama:
Can someone explain why it is supposed to be Cat 2 then fall off to Cat 1 at landfall somewhere in the northern gulf. What makes it fall off?

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Quoting Levi32:
Those newer thunderstorms are putting up 50kt+ flight-level winds, but the system is obviously weaker than before it came ashore Haiti.



Oh yea, there's no question about that. Though it's not dying as some have been claiming all day.
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Quoting Tazmanian:




yup take in at 21N 75W
2:00 PM EDT Sat Aug 25 Location: 20.8°N 75.3°W
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Quoting CybrTeddy:
This satellite image that Drak posted is very, very helpful on determining where the center is going as it automatically tracks the center to keep it exactly in frame. Heading WNW at this time, away from Cuba. On track per NHC.
Link


Yep def best for COC Locating
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Quoting Tribucanes:


MOST??? of the people on the blog are wishcasting. I've seen posts like this over and over. I know this post got lots of pluses, ya'll must be reading a different blog than I; because I don't see that at all. People are excited by the models which bring the storm closest to them. Guess why? Because it well may. Saying they WANT to be hit by a monster Isaac is a very big leap. I'm sure someone will post something in an hour stating the same ridiculous thing because it'll get lots of pluses. I'm sure SOMEONE or two may WANT to get hit directly by a major. Rest assured MOST do not in any way.


yea I like hurricanes but I am not crazy enough to want twisters raining down on my town. Hurricanes like isaac are dangerous storms. Those on the coast don't want to stick around to see how high the waves get.
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Time: 20:12:00Z
Coordinates: 21.5667N 75.5167W
Acft. Static Air Press: 843.2 mb (~ 24.90 inHg)
Acft. Geopotential Hgt: 1,488 meters (~ 4,882 feet)
Extrap. Sfc. Press: -
D-value: -
Flt. Lvl. Wind (30s): From 139° at 53 knots (From the SE at ~ 60.9 mph)
Air Temp: 15.2°C* (~ 59.4°F*)
Dew Pt: -*
Peak (10s) Flt. Lvl. Wind: 56 knots (~ 64.4 mph)
SFMR Peak (10s) Sfc. Wind: 51 knots (~ 58.6 mph)
SFMR Rain Rate: 15 mm/hr (~ 0.59 in/hr)
(*) Denotes suspect d
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Quoting Civicane49:
Hey guys. I'm back.

I'll probably come to this site almost everyday to see what's new.

I see that Isaac is currently affecting the northern Caribbean Islands, and is expected to become a hurricane and threaten the southeastern United States.


The Isaccum cleaner sucking up the FL Mess?
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Quoting chrisdscane:


21n 75w u agree?
Yeah right around there.
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2037. angiest
Current steering layer (based on the best information we have for pressure)

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ANT alert! My backyard is flooded in Ants!
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2035. Levi32
Those newer thunderstorms are putting up 50kt flight-level winds, but the system is obviously somewhat weaker than before it came ashore Haiti. It looks ready to restrengthen though.

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2034. Drakoen
We are already seeing signs that the system wants to get going. Pressures decreasing and solid convection developing in the eastern half.
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Quoting cajunkid:
COC opening up now. Not as tight as earlier.

Not what recon is seeing!
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Flight level winds are 64mph, SFMR 58mph. Isaac will probably stay at 60mph at 5pm in my opinion.
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Quoting Hurricanes101:
recon found winds of 55mph, I think they will keep the intensity at 60mph



and found 997.7mb


997.7 mb
(~ 29.46 inHg)
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Quoting Levi32:


BAM suite is initialized off of the previous GFS run anyway, so where the GFS goes, they generally go.


yea true Levi, am I wrong to say that in the 12Z runs, the GFDL, HWRF, Euro and Ukmet all shifted east?
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Quoting stormhank:
I see some models jumped west....dunno how far west isaac would track in gulf...im thinkin somewherre from mobile to apalachicola myself


Until the models jump back to the East. I've watched some local meteorologist look pretty silly following each model run like it was the absolute truth. Then they have to back track and change their forecast 6 hours later.
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Quoting wn1995:


That is a good view... looks like it is finally JUST offshore.


Yep.
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Quoting CybrTeddy:
This satellite image that Drak posted is very, very helpful on determining where the center is going as it automatically tracks the center to keep it exactly in frame. Heading WNW at this time, away from Cuba. On track per NHC.
Link
for sure its offshore lets see if the convection covers the center but looking at recon info thus far the pressure has remained steady
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2026. 900MB
Quoting chrisdscane:


21n 75w u agree?


I'm down w that!
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I think the new NHC track will go over Pensacola... the EURO will actually be the eastern outlier for this one - MAINLY BECAUSE it shows Isaac going up into the Bahamas - and it's def not doing that... I would suspect the next EURO will jump westward as well... That High Pressure is a lot stronger than we all thought apparently...
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Quoting SykKid:
97L going fishing
Too early
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Quoting cajunkid:
COC opening up now. Not as tight as earlier.


You don't get sub 1000mb pressures with a CoC that is opening up as the recon is finding.
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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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