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:
Geez, gams, I never thought u'd turn down a chance to rag on press... lol



I am trying my best to be a nice person; especially since I may be here on this blog a lot more often the next few days!!!!

but it was in my "second nature" to give a smart ass reply! LOL

but I took the high road! I will let Pottery do it.
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1092. LargoFl
Quoting Grothar:
Full GFS ensemble not out yet, Here is the first.

..this is the track I..believe in and am preparing for, tampa bay IS going to feel something from this storm,could only be tropical storm force winds and rain, but..we will..be prepared if it gets worse than that huh
Member Since: August 6, 2011 Posts: 4 Comments: 33491
A center reforming to the south could be bad news for the US as that would keep it more off of Hispaniola on a more southerly track.
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Quoting MississippiWx:


Yeah but looking at the wind barbs on their path, it would make one assume that the center truly is reforming to the south. We'll see.


Seeing that also.
Member Since: July 8, 2005 Posts: 259 Comments: 23017
Quoting GeorgiaStormz:
End of Euro, Isaac dissapating in Arkansas, while Joyce is stuck under the ridge....is that really going to recurve with a 1020mb isobar above it?




Wow, there's a new train of thought. Hate to use the name, but that could be a potential Andrew-like track.
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Quoting BeanTech:


your.misuse.of.periods.is.very.annoying....


give him a break, his spacebar is broken..
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Not many tropical storm winds seemed to have been found by the recon...Possible downgrade?
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1086. Grothar
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It seems like alot of the same thigs were being said about Ernesto at this stage of the game. Is it possible that similar conditions are going to result in similar outcomes? For example, he is moving too fast to really strengthen? I am sure there are hundred of thousands of people who would love for that scenario to play out. Also it appears he is actually speeding up from last night, does that not lower the odds of being in the right place to feel the trough?
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Quoting CybrTeddy:
Recon heading back to the old circulation now.


Yeah but looking at the wind barbs on their path, it would make one assume that the center truly is reforming to the south. We'll see.
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Quoting hydrus:
Absolutely.

Yes and No If Issac is further south then it will likely have less interaction with Hispanola and eastern Cuba. Now you would think that it will have a further west track into GOM etc. But if it is over water alot more then that would increase the intensity forecast and make it more of an issue for Jamaica reguardless. It the weakness in the ridge to the north materializes then a stronger system would tend to be pulled NW and still be a major threat to Florida. My gut feeling is the forecast from the NHC from yesterday may be the more on target as it has Isaac stay over water and move more west til it is pulled NW over central Cuba into the Florida Straits..into south Florida..early Monday morning.
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1081. icmoore
Quoting LargoFl:
true but we..need to prepare, these models can change in a couple of hours..dont get caught offguard, 2-3 sites ive just been to..all Still have the same track as the local mets..monday hugging the sw coast of florida


Yeah I know ... just not quite ready to believe all this could happen yet..I am most definitely watching I don't feel I have much choice in my location..but still..
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End of Euro, Isaac dissapating in Arkansas, while Joyce is stuck under the ridge....is that really going to recurve much with a 1020mb isobar above it?


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1079. Grothar
Full GFS ensemble not out yet, Here is the first.

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Isaac is taking his sweet time getting his act together while cruisin' down the highway....just give him time...
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41 dead now from West Nile with the worst yet to come. Just a friendly heads up. Many thousands will be affected this season.
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Recon heading back to the old circulation now.
Member Since: July 8, 2005 Posts: 259 Comments: 23017
1075. HrDelta
Quoting LargoFl:


Oh, a West-Coast Grazer. That'll screw up everything from Fort Myers to Apalachicola. Including Tampa and the Convention.
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gfs...east.coast.florida..strong.banding.thundersto rms.wi th.tornados..it.aint.going.to.be.pretty
Member Since: September 11, 2010 Posts: 0 Comments: 4018
1073. KEEPEROFTHEGATE (Mod)
T.C.F.W.
10L/TD/XX/CX
MARK
12.11N/36.29W
Member Since: July 15, 2006 Posts: 165 Comments: 52401
1072. scott39
Quoting MississippiWx:
12z Euro is similar to Katrina. Center goes east of New Orleans and makes landfall on the extreme W Mississippi Coast. Thank goodness this is fantasy land.
Yes, still some fantasy, but reality is starting to creep in.
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1071. KEEPEROFTHEGATE (Mod)
T.C.F.W.
09L/TS/I/CX
MARK
15.29N/59.89W
Member Since: July 15, 2006 Posts: 165 Comments: 52401
Quoting islandgirls:
Local met says that Isaac is showing signs of weakening What I am gathering from your discussions is that Isaac is reorganizing. What exactly is going on with this system, strengthening or weakening?
Grateful for your response.


What we are seeing is that there are two center of circulations in Isaac, one at ~16.1N and the other at ~15.3N. The one that is more southern is much better defined per recon, as we've been seeing pages of westerly winds vs. on the to the north is the old center of circulation, which doesn't has a well defined center of circulation anymore. That's what we are seeing anyways, the center appears to be reforming to the SE of the old one.
Member Since: July 8, 2005 Posts: 259 Comments: 23017
1068. LargoFl
...................GFS at 144 hours
Member Since: August 6, 2011 Posts: 4 Comments: 33491
Afternoon everyone. Anyone mind catching me up on the last 6 hours or so?
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Quoting islandgirls:
Local met says that Isaac is showing signs of weakening What I am gathering from your discussions is that Isaac is reorganizing. What exactly is going on with this system, strengthening or weakening?
Grateful for your response.


It's organising. Not likely to strengthen a whole lot over the next 12-24 hours, but assuming it continues to get itself together, it should begin ramping up then.

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to trHUrrIXC5MMX I love the graph you just posted # 960.

Where can I find that model, or something close to it.

mikethefisherman@gmail.com
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very scary scenario for us in nola
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That was quick lol. Appears Isaac is bringing it all in.

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Oh, goody....

Good news (for me): 12z Euro shifts a tad E away from SouthCen LA landfall.

BAD news?? Son of Katrina blasts NOLA/Grand Isle.

IOW...a slight shift E, but still an SE LA/MS/AL potential bomb.

And still, mostly, the outlier, with everyone else targeting FL Penisula.

Still not sold on it, and most likely an S FL event..but that solution should wake some people up here.




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1061. Patrap
Gridded RainBow Top Image

TS Isaac

Member Since: July 3, 2005 Posts: 415 Comments: 125726
Quoting Tribucanes:
If, God forbid, Isaac goes into the Gulf headed NW as a minimum cane; what would stop him from majoring?


Likely, nothing. Scary situation.
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12z Euro is similar to Katrina. Center goes east of New Orleans and makes landfall on the extreme W Mississippi Coast. Thank goodness this is fantasy land.
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Local met says that Isaac is showing signs of weakening What I am gathering from your discussions is that Isaac is reorganizing. What exactly is going on with this system, strengthening or weakening?
Grateful for your response.
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If a new center is forming almost a full degree south, I am seriously doubting the current forecast track and excpect it to miss Haiti to the south and to shift far west of Tampa. We'll see...
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1056. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 33491
1055. JasonRE
Quoting WPBHurricane05:
12Z Euro is faster than 0Z.

Also further north/east in medium range, but landfall still looks like Louisiana.


Where can I see this information?
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If, God forbid, Isaac goes into the Gulf headed NW as a minimum cane; what would stop him from majoring?
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Quoting clwstmchasr:
I don't see where recon has found TS force winds yet?


Haven't dived south into the deeper convection yet, they're checking out the two CoC's that they found.
Member Since: July 8, 2005 Posts: 259 Comments: 23017
Quoting presslord:


that's just an insignificant little Bush League football team nobody cares about....

Speaking of tropical weather, the TIDE might roll in!
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Quoting NICycloneChaser:


Hasn't been in the NE yet. I'd be fairly confident there will be some there.

Had some (suspect) winds 35kt+ in the NW quad on the way in.
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1050. LargoFl
Quoting icmoore:


NOT what I want to hear...
true but we..need to prepare, these models can change in a couple of hours..dont get caught offguard, 2-3 sites ive just been to..all Still have the same track as the local mets..monday hugging the sw coast of florida
Member Since: August 6, 2011 Posts: 4 Comments: 33491
Quoting JasonRE:


Yeah but what percentage of that is possible/probable? That's a good ways off from the current position.


It really is impossible to put percentages of possibility on it. The Euro is the only model showing that, but it is typically one of the best models.

All that can be said is that it is a possibility that people should be aware of.
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I can't wait for recon to visit the SW quad I want to see what the hell is going on there
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when you have the Euro for 6 straight runs(thats right i said 6!) show a gulf major hurricane, take notice.
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1046. scott39
Quoting MississippiWx:
168 hours, the 12z Euro has a strong hurricane sitting SSE of New Orleans, moving NNW.
That is DOOM for us, seeing we will be on the East side....not to mention he will start swinging to the N to NNE to NE at that time.
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floridians.living.in.a.aluminum.structure...plan.no w.for.a.safe.house
Member Since: September 11, 2010 Posts: 0 Comments: 4018
Quoting Bluestorm5:
Recon flew through the center. Going toward one of strong region of Isaac


Looks like the band of storms off to the S is wrapping in...
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1043. Patrap
Hurricane Preparation 2012


Member Since: July 3, 2005 Posts: 415 Comments: 125726

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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.