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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1293. WxLogic
Quoting nrtiwlnvragn:
Recon is going to have to alter their normal inbound approach if they are recognizing the SE reforming center. They appear to still be flying a pattern that recognizes the center on the vortex fix.


Noticed that.
Member Since: August 14, 2008 Posts: 4 Comments: 4972
If you look at the water vapor loop, black and white at the NHC the storm is growing exponentially. Now the fly in the ointment is, if you go to canefever and look at the various analysis maps you will see tc prob. formation map shows a strong area to the se of the storm. The convergence and the divergence are centered se of the storm how this is going to resolve itself God only knows. You can see by the radar the system is screwed up for the lack of a better word. Well in closing the area to the se of the storm is robbing the energy from Issac and has been. If it reforms look for a big position change.IMO
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I don't see any surface TS force sustained winds from the recon yet. Plenty of TS force flight level winds, but none from the surface.
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I want to take a minute to thank the Patraps and Levis of this blog. While the NHC is more accurate than almost ALL of the posters on here its guys like y'all who help us understand what is happening. I read the NHC for the predictions and follow posters like you two for the explanation of why the predictions are what they are.

Kudos and keep it up.
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1289. JLPR2
Very easy to see the new circulation in the Barbados radar.

(Click to animate)
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Quoting MississippiWx:
This was the 2pm update. The center is ever farther south than this now due to the reformation. Will be interesting to see how the models respond.


There are 2 things wrong with your statement

1. The center has reformed to the north, not the south, if you looked at the Hurricane Hunter Data you'd know that.

2. the ATCF models havent updated since 9 AM Eastern this morning.
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1287. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 38407
1286. Patrap
TS Isaac RGB Loop

Member Since: July 3, 2005 Posts: 423 Comments: 128231
Quoting MiamiHurricanes09:
School spent the majority of the day today closing shutters and trimming some of the larger trees on the campus. A bit drastic I believe, but good to see they're taking the safe side and keeping a close eye on Isaac.

better to be sfae then sorry
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1284. GetReal


Something is getting itself together there... Beginnings of an eye wall on the SE quad?
Member Since: July 4, 2005 Posts: 204 Comments: 8871
Quoting MississippiWx:
This was the 2pm update. The center is ever farther south than this now due to the reformation. Will be interesting to see how the models respond.



The 2 p.m. NHC path puts Tampa near the coc and on the dirty side of Isaac.
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Quoting Patrap:
# 1220

FOUL language in any form is not Allowed.

That's explicit in the "Rules of the Road".




ok I had edit it down to cut it out the foul
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where do you think the nhc will have the center at 5pm?
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1280. TopWave
Quoting NYCyclone86:
Well let's not discount our very on LEVI then. Lets say Isaac relocates, gets his act together tonight.. This would obviously mean strengthening. Weather or not he forms the new center x amount of miles to the south, he will become stronger which will have him move poleward regardless. So it interacts with Haiti, but goes north/ just skims the cuban coastline. Suddenly Levi isn't crazy after all.


Thats correct. If Issac grows sronger he will move more poleward which supports Levi's forecast. If Issac remains weak. He will follow a more westerly track. It will certainly be interesting to see what unfolds here.
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Quoting DVG:


I was thinking 14.6/8 X 59.? something myself. Shortwave looks like a solid mass growing/spinning from there. How far off from the recon in time are the loops?


30-45 mins
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1278. wpb
with the relocation the storm misses the effects or most of it from hispanols thus a much stronger system thus it would respond more to a weakness to its north
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Quoting NYCyclone86:
BAMM Models shifted east actually and some of the 18Z hurricane model plots are showing a right bend at the end of the run. It is really hard to get "trend information" on here because we have groups that seem like they want a storm.....Never have I seen anything like that before. You think after Katrina and the gulf blows that residents would want nothing to do with a storm. I hope that it goes out to sea, after minimal impacts to Haiti.



Link to that?
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1276. DVG
Quoting SLU:
14.8n 59.5w? Anyone?


I was thinking 14.6/8 X 59.? something myself. Shortwave looks like a solid mass growing/spinning from there. How far off from the recon in time are the loops?
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1275. barbamz

Really massive swirl ball of heavy moisture. But maybe Isaac will have to digest some dry air from the southwest soon?
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1274. JLPR2
Quoting Kowaliga:


By "new" you mean just E of Dominica?


Yep.
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Long,long time lurker here from Scotland on the best weather/climate blog on the planet.

Thoughts on Isaac? Huge system, comparitively fast forward speed, very fine structure so far out and yet to pull itself together mainly due to its sheer size.

Heading straight west right now pretty quick, give or take. I'm far from an expert but this storm has a lot of work to do, latitude-wise, to catch even the western edge of Hispaniola, IMHO.

Maybe worth bearing in mind that fractions of a degree north or south over 200 miles of track now could have a big bearing on where Isaac will be blowing in 4-5 days time...

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Quoting wunderkidcayman:

if it e nd us further south it won't get as much of an effect from that weakness as it would if it was to be further N




you shut the freaking hell up yes I bloody do care I had one of my closest friends almost got his head sliced off from flying Zinc in Ivan I my self nearly died in hurricane so unlike you I do give a crap on how this affects peoples lives

and NO FOR THE 1000TH TIME I DON'T WANT A FREAKING MAJOR IN CAYMAN SO SHUT THE HELL UP ALREADY



I doubt being a couple miles south of Land as a deepening system will cause it the lose the trough it could easily come to a stop and making a sharp NNW as the HWRF model track-wise.
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Quoting JLPR2:


I'll go with 15n-15.2n 59.2w.



I had put it at 15.2N and 60.0W about an hour ago
Member Since: August 28, 2005 Posts: 0 Comments: 301
I think the key may be Joyce. GFS has J helping the trough to erode the ridge. ECMWF has J racing away and not moving the ridge any.
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anyone.know.of.a.major.that.landfalled.big.bend...i .dont
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1268. JLPR2
Rather healthy line heading inland.

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Quoting JLPR2:


Old coc, or what's left of it, is on a collision course with Guadeloupe.
This could be bad, the old circulation was rather dry, but the new one seems to be wrapping a fair amount of convection around it.


By "new" you mean just E of Dominica?
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School spent the majority of the day today closing shutters and trimming some of the larger trees on the campus. A bit drastic I believe, but good to see they're taking the safe side and keeping a close eye on Isaac.
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I'm not sure a relocation to the SE means a more southern track, it means more time for the trough to erode the edge of the high.
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Ok so Issac is prob going for W. Florida but ow strong does everyone think he'll get b4 he makes his final landfall
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1262. intampa
so all the models predicting doom for florida may follow the tradition of dean ,ivan etc.... in other words all this commotion about the tampa/central florida hit with the RNC next week is probably not going to happen if this thing is further south or relocatiing south than previously thought. i know i will get in trouble for stating it but every year two or three times a year florida is in the cone and it stays that way for days. there are predictions of death destruction mayhem etc... then some unexpected ULL or som trough etc etc changes everything..... lets hope!
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1261. wpb
will be interesting opps from recon on this leg. should give relocation a y/n
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This was the 2pm update. The center is ever farther south than this now due to the reformation. Will be interesting to see how the models respond.

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1259. 2ifbyC
All of that is just skirting by me in west Bradenton. You sure have had a bunch today!

Can ya say 'ground saturation...?

Quoting LargoFl:
gee is this ever going to end? more boomers coming in from the gulf
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Quoting FOREX:


Haven't seen Jim Cantore here in Panama City Beach, however, an NBC Truck with Georgia plates is parked in the oversized parking area at the Holiday Inn Sunspree Resort. Just coincidence maybe.


That would be TWC crew - perhaps Roker for tomorrow morning's Today show.
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1257. JLPR2
Quoting SLU:
14.8n 59.5w? Anyone?


I'll go with 15n-15.2n 59.2w.
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1256. Relix
Quoting SLU:
14.8n 59.5w? Anyone?


I'd have the new one at around 15.2 59.3W
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where you guys have the center?
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1253. LargoFl
Quoting TheMom:
That stuff you sent over from this morning had a couple schools holding kids in about 20 mins ago for a cell possible funnel .. thanks. We are really soggy on this side of the state too so a Floyd would be a lot better option here than a Fay even. I'd far prefer the western side of this storm than the eastern.
yes for sure, with this soaking wet ground..real strong hurricane winds will bring the tree's down on us..hoping it goes somewhere else..but so far..gee
Member Since: August 6, 2011 Posts: 4 Comments: 38407
Quoting CosmicEvents:
You don't care how many people get killed downstream do you? Just as long as you get your major near the Caymans.


Yes he is king wishcaster. Giving the legend/former wishcaster JFV a true run for his money on that front.
I can only read his quotes when other quote him cause of that.
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Quoting SLU:
14.8n 59.5w? Anyone?



Nah, the other recon circulation was at about 15.5N so thats the lowest I think it could be.
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1250. FOREX
Quoting TheMom:
Umm I thought Jim showing up a few days before was a good sign that you were safe and then he moved on to the actual hit area.


Haven't seen Jim Cantore here in Panama City Beach, however, an NBC Truck with Georgia plates is parked in the oversized parking area at the Holiday Inn Sunspree Resort. Just coincidence maybe.
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1249. Patrap
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1248. GetReal


964mb
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Foul language, dude... Post #1220
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1246. Patrap


Member Since: July 3, 2005 Posts: 423 Comments: 128231
1245. SLU
14.8n 59.5w? Anyone?
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Quoting hydrus:
Very unsettling image....I wonder what this image will look like in three days.


Dry air starting to shift from Issac NE quadrant towards TD9. Once it's south of Peurto Rico it should have no real problems with dry air

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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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