Isaac lashing the Lesser Antilles; TD 10 forms

By: Dr. Jeff Masters , 3:06 PM GMT on August 22, 2012

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Tropical Storm Isaac is lashing the entire Lesser Antilles chain of islands with heavy rains this morning, and the winds will be on the increase this afternoon as the storm heads west at 19 mph. Isaac is still a weak and disorganized tropical storm, but that is changing. The Hurricane Hunters are in the storm, and have thus far found no increase in the storm's top surface winds, which remain near 45 mph. In their 7:44 am EDT center fix, an Air Force Reserve aircraft measured a rather unimpressive center pressure of 1007 mb, and top winds at their 1000 foot flight altitude of 56 mph. The plane did not observe an eyewall trying to form, and recent microwave satellite images also show no signs of an eyewall forming. A large area of dry air to the north of the storm, as seen on water vapor satellite loops, continues to interfere with development, and it is unlikely Isaac will become a hurricane today. Satellite loops, however, show that Isaac has developed a Central Dense Overcast (CDO) of high cirrus clouds, the hallmark of a developing storm. These clouds have very cold cloud tops, indicating that the updrafts creating them are quite strong. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow is becoming well-established to the southwest and northwest, but outflow is restricted on the southeast side. A large clump of heavy thunderstorms several hundred miles southeast of the center continues to compete for moisture, and is interfering with the low level inflow and upper level outflow of the storm. The intensification rate of Isaac will increase if the storm is able to integrate this clump of heavy thunderstorms into its circulation, which satellite loops this morning suggest is now beginning to happen. Radar imagery from Barbados and Martinique show plenty of heavy rain showers, mostly on the south side of Isaac where it is moister, but little spiral banding. Hurricane hunter missions are scheduled for Isaac every six hours, and a NOAA hurricane hunter research aircraft will also be in the storm, with missions scheduled every 12 hours. The NOAA jet is first scheduled to fly into the storm on Thursday afternoon, to do a large-scale dropsonde mission to aid model forecasts.


Figure 1. Morning radar image of Isaac taken from the Barbados radar at 10:15 am EDT. Image credit: Barbados Weather Service.

Intensity forecast for Isaac
The latest 8 am EDT run of the SHIPS model shows that wind shear is moderate, 10 - 15 knots, but is expected to relax by this evening to a low 5 - 10 knots. Ocean temperatures have increased to a very warm 29°C, and the storm is now over waters with a high total heat content. The lowering wind shear and warm waters should allow the storm to wall off the dry air that has been interfering with development, and also allow the storm to integrate the thunderstorm clump on its southeast side that has been interfering with low level inflow and upper level outflow. It will take some time for the increase in organization to result in an increase in Isaac's winds, and I still expect top winds of 45 - 60 mph in the Lesser Antilles Islands this evening when the core of the storm moves through. On Thursday, when Isaac will be in the Eastern Caribbean, conditions should be favorable enough to allow steady strengthening to a Category 1 hurricane. The 11 am EDT wind probability forecast from NHC predicted a 47% chance that Isaac will become a hurricane by Friday morning, and a 16% chance it will be a Category 2 or stronger hurricane then. This is a reduction in the odds given in the 5 am advisory.

Impact of Isaac on the Islands
The entire Lesser Antilles Islands chain will have a three-day period of heavy weather Wednesday through Friday. Sustained tropical storm-force winds extend out about 50 miles to the north of the center and 30 miles to the south, so an 80-mile wide swath of the Lesser Antilles will potentially see tropical storm-force winds of 45 - 60 mph this Wednesday evening. Guadaloupe, Antigua and Barbuda, Dominica, Montserrat, and St. Kitts and Nevis at highest risk of these winds.

Winds in St. Croix in the Virgin Islands will likely rise above tropical storm force on Thursday morning, and the south coast of Puerto Rico should see tropical storm-force winds by Thursday afternoon. The San Juan airport may be able to stay open Thursday afternoon and evening, but I think it is more likely they will be forced to shut down.

On Thursday night, heavy rains and tropical storm-force winds should arrive on the southern coast of the Dominican Republic, and all airports in the D.R. will probably be closed on Friday. Rainfall amounts of 8 - 12 inches will likely affect the Dominican Republic Thursday through Saturday, creating dangerous flash floods and mudslides.

Isaac is potentially a very dangerous storm for Haiti, where 400,000 people still live outside underneath tarps in the wake of the 2010 earthquake. Heavy rains from Isaac will begin on Friday morning in Haiti, and last through Sunday. Rainfall amounts of 8 - 12 inches are possible, which will be capable of causing extreme flooding on the vegetation-denuded slopes of Haiti. It will be a major challenge to keep those Haitians living outside safe, if rainfall amounts of 5 - 10 inches occur.

Impact on Florida, Cuba, Jamaica, and the Bahamas
Florida, Cuba, Jamaica, and the Bahamas are all at high risk of receiving hurricane conditions from Isaac. The latest set of 06Z (2 am EDT) model runs for Isaac are fairly unified for the coming three days, showing a westward track to a point on the south coast of Hispaniola. All of the models then predict a more west-northwest track across the island and into eastern Cuba, as Isaac responds to a trough of low pressure over the Southeast U.S. Most of the models then predict a path for Isaac along the spine of Cuba, then into the Florida Straits off the coast of Miami by five days from now. A notable exception is our best-performing model, the ECMWF, which keeps Isaac just south of Cuba, and takes the storm more to the west between Jamaica and Cuba on Saturday, then into the Yucatan Channel between Mexico and Cuba by Monday. However, this model is keeping Isaac weaker than the other models, and thus predicts the storm will have a weaker response to the trough of low pressure over the Southeast U.S. If the official NHC intensity forecast is right and Isaac becomes a hurricane on Thursday, the more southerly track of the ECMWF is not going to verify, and Isaac will spend considerable time over Cuba on Saturday and Sunday. Where Isaac pops off the coast of Cuba will be critical in determining its future path and intensity. Some models predict a more easterly exit point, allowing Isaac to move up the east coast of Florida, and potentially make landfall in the Southeast U.S. The latest 06Z GFS model run predicts a more westerly track, which would potentially allow Isaac to move up the west coast of Florida towards Tampa. Keep in mind that the average error in a 5-day forecast is 260 miles. The two most recent runs of the GFS model, at 00Z and 06Z (8 pm and 2 am EDT), gave positions for Isaac that were about 250 miles apart--the earlier run putting the center near West Palm Beach, and the more recent run giving a location between Key West and Havana, Cuba. While passage over the high mountains of Hispaniola and then Cuba will substantially disrupt Isaac and probably reduce it below hurricane strength, the storm is quite large, and should be able to re-intensify once it emerges over the Florida Straits. Waters will be very warm, near 30°C, wind shear is predicted to be light, and forecasts of the upper-level winds show the possibility of an upper-level outflow pattern very favorable for intensification. If Isaac spends a day over water, that should be enough time for it to intensify into a Category 1 hurricane, and if the storm takes a longer 2-day track over water up either the east or west coast of Florida, a Category 2 or stronger storm is possible.

Isaac is a threat to affect Tampa during the Republican National Convention, August 27 - 30, and the official NHC forecast now has Tampa in the 5-day cone of uncertainly. The latest 11 am EDT wind probability forecast from NHC gives Tampa a 9% chance of receiving tropical storm-force winds for the 24-hour period ending on the morning of first day of the convention (Monday). I blogged about the climatological chances of a hurricane causing an evacuation of Tampa during the convention in a post last week, putting the odds at 0.2%. I put the odds of an evacuation occurring during the convention in the current situation at 3%.


Figure 2. Morning satellite image of Tropical Depression Ten.

Tropical Depression Ten forms in the Eastern Atlantic
The large tropical wave in the Central Atlantic, midway between Africa and the Lesser Antilles Islands, has become Tropical Depression Ten. The depression has an impressive amount of spin and heavy thunderstorm activity, as seen on visible satellite loops, and should be Tropical Storm Joyce by Thursday. None of the models show that TD 10 will be a threat to the Lesser Antilles Islands.

Which model should you trust?
Wunderground provides a web page with computer model forecasts for many of the best-performing models used to predict hurricane tracks. So which is the best? Well, the best forecasts are made by combining the forecasts from three or more models into a "consensus" forecast. Over the past decade, NHC has greatly improved their forecasts by relying on consensus forecast models made using various combinations of the GFS, GFDL, NOGAPS, UKMET, HWRF, and ECMWF models. If you average together the track forecasts from these models, the NHC official forecast will rarely depart much from it, and the NHC forecast has been hard to beat over the past few years. The single best-performing model over the past two years has been the ECMWF (European Center model). The GFS model has been a close second. You can view 7-day ECMWF and 16-day GFS forecasts on our wundermap with the model layer turned on. Ten-day ECMWF forecasts are available from the ECMWF web site. The European Center does not permit public display of tropical storm positions from their hurricane tracking module of their model, so we are unable to put ECMWF forecasts on our computer model forecast page that plots positions from the other major models. As seen in Figure 3, the HWRF and GFDL were well behind the ECMWF and GFS in forecast accuracy in 2011, but were still respectable. The BAMM model did very well at 4 and 5-day forecasts. The UKMET, NOGAPS, and CMC models did quite poorly compared to the ECMWF, GFS, GFDL, and HWRF. For those interested in learning more about the models, NOAA has a great training video (updated for 2011.)


Figure 3. Skill of computer model forecasts of Atlantic named storms 2011, compared to a "no skill" model called "CLIPER5" that uses just climatology and persistence to make a hurricane track forecast (persistence=a storm will tend to keep going in the direction it's current going.) OFCL=Official NHC forecast; GFS=Global Forecast System model; GFDL=Geophysical Fluid Dynamic Laboratory model; HWRF=Hurricane Weather Research Forecasting model; NOGAPS=Navy Operational Global Prediction System model; UKMET=United Kingdom Met Office model; ECMWF=European Center for Medium Range Weather Forecasting model; TVCA=one of the consensus models that lends together several of the above models; CMC=Canadian Meteorological Center (GEM) model; BAMM=Beta Advection Model (Medium depth.) Image credit: National Hurricane Center 2011 verification report.

I'll have a new post this afternoon.

Jeff Masters

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Quoting BahaHurican:
Wonder if mosquitos care whether the blood they suck or the person who dies from West Nile virus is Democrat or Republican...
There should be some things that all Americans have: health care, retirement security for workers, and public systems [like weather forecasting] that save lives. [...]
Thank you Baha for saying what needs to be said, but in a better world would simply exist without mention: Justice for All.
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Quoting Grothar:


I doubt that model though. I think a little trip through the Windward Passage is more likely then a brush on the East Coast of Cuba. However, if it stay more west longer, it would most likely emerge in Central Cuba then Northward.


You really like that Windward Passage, don't you!
Member Since: June 29, 2009 Posts: 13 Comments: 5315
Quoting WPBHurricane05:
GFDL with a south/westerly shift.


Spine cracker there eh?
Member Since: July 3, 2005 Posts: 428 Comments: 129903
Quoting jeffs713:

Interesting... tries to run the length of Cuba.


I doubt that model though. I think a little trip through the Windward Passage is more likely then a brush on the East Coast of Cuba. However, if it stay more west longer, it would most likely emerge in Central Cuba then Northward.
Member Since: July 17, 2009 Posts: 71 Comments: 27210
Quoting Grothar:


baddd gfs baddd :(
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<
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GFDL with a south/westerly shift.
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NWS out of Miami has now just updated their forecasts with possible tropical storm conditions for Sunday night into Monday with possible hurricane conditions forecasted for the upper keys on Monday. Not surprised with the wording.
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Hi Everyone,

Watching Isaac like the rest of you from SE Florida.

Yesterday we were in center of cone today the cone moved to west but we are still in east side.

Of course that cone can move every update for next 5 days.


And the Mountains could rip the storm apart like they did 2006 Ernesto.


I will be lurking as much as possible.

Thanks everyone for the updates and folks from New Orleans to North & South Carolina need to be watching this storm.

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Quoting MississippiWx:


It's still the same concept. Georges would not be an analog, however. I haven't seen any similarity in the long range between 1998 and now.
So basically a pattern we haven't seen in awhile. Interesting.
Member Since: August 1, 2011 Posts: 28 Comments: 8075
20mph moving storm due west....gonna cover some Longitude, and if speed doesn't slow down.....track shift is in order.
Member Since: July 14, 2008 Posts: 2 Comments: 9686
Quoting SSideBrac:


Have been trying - unsuccessfully - for a while now to get into NEMA Bahamas Website - I presume this is the site for official Bahamas Govt Advisories???
Have you got any other appropriate links?
Trying them right now... That's weird... the website is just not there...

I'll get back to you on this...

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Phew, was wondering. All good, hope your having a good one.
Member Since: April 18, 2012 Posts: 0 Comments: 2437
Quoting SouthTXWX:
looks like Recon just flew through the center, about 50 miles due east of Guadeloupe.



The radar concurs with that CoC

Member Since: July 3, 2005 Posts: 428 Comments: 129903
RECON says center is at 16.0N
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Recon indicates that Isaac is a little better organized than when they last investigated earlier today, there is now a pronounced windshift, which confirms a closed circulation.
Member Since: July 8, 2005 Posts: 259 Comments: 24579
Quoting Bluestorm5:
It's little different because Georges was a major hurricane while Isaac is a tropical storm. Sorry if I didn't make it clear.


It's still the same concept. Georges would not be an analog, however. I haven't seen any similarity in the long range between 1998 and now.
Member Since: July 15, 2006 Posts: 17 Comments: 10284
Quoting Bluestorm5:
It's little different because Georges was a major hurricane while Isaac is a tropical storm. Sorry if I didn't make it clear.


Barely a tropical storm. Having a rough go of it is Isaac.
Member Since: August 17, 2005 Posts: 26 Comments: 16874
Winds will likely be strongest in the northeastern part of the storm, just like yesterday.
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recons center is 16.1 N
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Quoting Grothar:
This is the ECMWF. Wonder what the new run will look like.



WOW HOLY COW BATMAN. I need a FRESCA!!!!
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recon say 16N 60.3W it looks like
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Rainrates are low over the CoC, that means the center is somewhat exposed still.
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Quoting MississippiWx:


Well, I'm sure there have been more, but off the top of my head Georges in 1998 is one of them. I remember it because I was impacted by it.

It's little different because Georges was a major hurricane while Isaac is a tropical storm. Sorry if I didn't make it clear.
Member Since: August 1, 2011 Posts: 28 Comments: 8075
Quoting bamagirl1964:
Hello everyone! I desperately need your help! I truly admire the wealth of knowledge you all have and need your input. My husband and I are 48 years old and are taking our first cruise this weekend! We are leaving out of Miami on Friday, going to Nassau, and returning to Miami on Monday. It is getting down to the wire and I don't know what to do. I have been glued to the computer watching the models and I see they are tracking west, which would be good for our cruise. I know that nothing is definite, but do you think this trend will continue? I have the vacation protection package and could reschedule for next week, but I may run into the same scenario as this week! I am very grateful for any insight you can give me and I do truly admire your input! Thanks! Marisa
best thing to do is cancel because very bad weather will affect the whole area from the bahamas to the eastern gulf of mexico
Member Since: July 14, 2009 Posts: 0 Comments: 3029
Quoting bamagirl1964:
Hello everyone! I desperately need your help! I truly admire the wealth of knowledge you all have and need your input. My husband and I are 48 years old and are taking our first cruise this weekend! We are leaving out of Miami on Friday, going to Nassau, and returning to Miami on Monday. It is getting down to the wire and I don't know what to do. I have been glued to the computer watching the models and I see they are tracking west, which would be good for our cruise. I know that nothing is definite, but do you think this trend will continue? I have the vacation protection package and could reschedule for next week, but I may run into the same scenario as this week! I am very grateful for any insight you can give me and I do truly admire your input! Thanks! Marisa

If you have the vacation protection plan, you should be OK. That said, leaving on Friday may be a bit choppy.

Which cruise line are you sailing on?
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NOAA G-4 Gulfstream



20090825N1 Aircraft 49RF
Genesis/SALEX Mission around Trop. Dist. 92L (pre-Danny)2009


(Manuscript received 17 March 2000, in Final form 6 March 2001)


ABSTRACT

The values of surface winds simulated by the University of Washington (UW) two-layer similarity planetary boundary layer (PBL) model are compared with National Oceanic and Atmospheric Administration Hurricane Research Division global positioning system dropsonde observations and the surface wind analyses of a numerical weather prediction model.

These three wind products compare fairly well at moderate wind speeds, away from the center of the storms where the coarse resolution of the numerical model is not a major factor. In the very high wind regime, the UW PBL model winds match the dropsonde observations fairly well, which is consistent
with the unique characteristic of the PBL model being able to account for the nonlinear effects of organized
large eddies.

These eddies transport momentum and heat Boxxes more efficiently than the smaller-scale, local turbulence can, leading to simulations of higher winds with mesoscale variability.
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Quoting StormHype:


You are one of the better informed ones here. How sophisticated are any of these models when it comes to dealing with rugged land masses like DR and eastern Cuba?


The hurricane models are more adept to it than the global models but both account for boundary layer friction.
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Coordinates: 16.0667N 60.3667W
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looks like Recon just flew through the center, about 50 miles due east of Guadeloupe.
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Indeed we have a center with the strongest winds most likely to be found to the E and NE... typical.
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Quoting CybrTeddy:
Pressure down to 1004, winds are very light though.
Isn't the pressure 1005 mb since the pressure is at 1004.7 mb or do they lower it down to 1004 mb despite rounding?
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Quoting RitaEvac:



#776
:) Thanks I needed that :)
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Hello everyone! I desperately need your help! I truly admire the wealth of knowledge you all have and need your input. My husband and I are 48 years old and are taking our first cruise this weekend! We are leaving out of Miami on Friday, going to Nassau, and returning to Miami on Monday. It is getting down to the wire and I don't know what to do. I have been glued to the computer watching the models and I see they are tracking west, which would be good for our cruise. I know that nothing is definite, but do you think this trend will continue? I have the vacation protection package and could reschedule for next week, but I may run into the same scenario as this week! I am very grateful for any insight you can give me and I do truly admire your input! Thanks! Marisa
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12ZGFDL and OOZUKMET show very little land interaction for Isaac.
Member Since: August 19, 2005 Posts: 5 Comments: 5458
801. GetReal
6:09 PM GMT on August 22, 2012
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800. NICycloneChaser
6:09 PM GMT on August 22, 2012
We have our centre, folks.

16.07N, 60.37W.

Extrap. pressure 1005.2mb.
Member Since: August 10, 2010 Posts: 2 Comments: 1971
799. Bluestorm5
6:09 PM GMT on August 22, 2012
Quoting NICycloneChaser:


Nice to have someone polite and wanting to learn and have reasonable discussions on the blog.
I do really want to be a good forecaster but I got a long way before I am anywhere near Levi or couple of others. They've tracked hurricanes for years. I've tracked closely for 2 years now, but I've watch hurricanes on Weather Channel since 2003.
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797. CybrTeddy
6:09 PM GMT on August 22, 2012
Pressure down to 1004, winds are very light though.
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796. MississippiWx
6:08 PM GMT on August 22, 2012
Quoting Bluestorm5:
So what it look like to me is that this is a whole different storms than what we've seen in the past. I don't think I've ever seen storm going over Haiti and then going west of Florida.


Well, I'm sure there have been more, but off the top of my head Georges in 1998 is one of them. I remember it because I was impacted by it.

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795. StormJunkie
6:08 PM GMT on August 22, 2012
Center at 16.06 per recon.
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794. masonsnana
6:08 PM GMT on August 22, 2012
Quoting RitaEvac:
You forgot Moses Presslord....
lol!!
Member Since: February 14, 2004 Posts: 2 Comments: 664
793. NICycloneChaser
6:07 PM GMT on August 22, 2012
Quoting Bluestorm5:
Just learning the lessons, that's all :) I never feel the need for attention or taking my frustration out on others. Also it's reasonable that surface winds is lower than flight winds by some.


Nice to have someone polite and wanting to learn and have reasonable discussions on the blog.
Member Since: August 10, 2010 Posts: 2 Comments: 1971

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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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