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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Unusually, lol sorry for that grammar misstep last page. Hate making grammatical errors. According to WU spell check unusually is a word, which is why I didn't catch it. :)
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1392. Patrap
xcellent restraint on the er.."visitor"


Kudo's to all.
Member Since: July 3, 2005 Posts: 420 Comments: 127535

I dare find the center of that mess.
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1390. wpb
Quoting StormJunkie:
After looking at Barbados radar, sat imagery, and recon data I can only come to two conclusions...

I have no clue where or if there is a "center"...And Isaac is a hot mess of a storm right now.
u got it. may not e the case on friday
Member Since: May 28, 2011 Posts: 0 Comments: 572
Quoting gustavcane:
then at 240hrs THE END.


Why do you continuously post a model from 15 hours ago?
Member Since: September 6, 2007 Posts: 0 Comments: 6056
yes it will affect Florida whether directly or not...
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The anticyclone is west of the islands and north of the 'new center' if that is what has happened. CIMSS also has a delay, so who knows.

1800 UTC is 2 hrs. ago.
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Quoting GetReal:





There is no LLC this is MLC
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1385. Patrap
AL092012 - Tropical Storm ISAAC

Storm Relative 1km Geostationary Visible Imagery Loop

click image for Loop

click moving GIF to ZOOM

Member Since: July 3, 2005 Posts: 420 Comments: 127535
Quoting ProgressivePulse:
I personally don't think a reformation will alter the 72hr plus track much. It actually may cause the storm to make a harder right turn as it would be a stronger cyclone.


If it doesnt slow then it would be hard pressed to be any stronger than what it currently is
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1382. Chiggy
Quoting JLPR2:
Very easy to see the new circulation in the Barbados radar.

(Click to animate)

If that is the case then I make it at 60.5W and 14N..., is that correct?
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Quoting tropicfreak:
Just got back on, but where has the center reformed?


The blog has declared reformation, the NHC has not (if there is one NHC would recognize at 5 PM) and RECON has not acknowledged reformation via a vortex fix either. To me it is a possibility, but not certain.
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1380. divdog
Quoting THISISAJOKE:
well well well i was laughed at and even called a troll but i think the track of isaac has shifted westward 3 times since last night taking east coast florida out of the equation and im sure by saturday it will get shifted even more to the west probably taking most of florida if not all of florida out of the path altogether. i have been saying fopr 4 days now that florida wont get much and im holding to that by tommorow morning they will have issac in the gulf welll south of south florida so florida will get nothing
Funny this is your post. Go away. Tired of the game.
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1379. hydrus
Quoting MAweatherboy1:
That new convection is pretty intense.



Still doesn't cover a huge part of the storm though. What it really needs is some convection on that dry north side.
Yes..That mean green means business. Wicked updrafts with super cold temps up there. The red and purple is intense on the JSL..
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Post 1371 adios.
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then at 240hrs THE END.
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Quoting RitaEvac:
Just one model



And it's already big

Me getting a little sick feeling.
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1375. GetReal
Quoting tropicfreak:
Just got back on, but where has the center reformed?



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1374. WxLogic
Looks like the center relocated to the SW of the last official center fix.
Member Since: August 14, 2008 Posts: 4 Comments: 4927
Quoting AllStar17:
That recon pass would seem to confirm indeed that the center has relocated farther southeastward.


Wunderkid would probably be jumping out of his chair.
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I personally don't think a reformation will alter the 72hr plus track much. It actually may cause the storm to make a harder right turn as it would be a stronger cyclone.
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Quoting SWLACajun:
Any ideas why the ecmwf shows Isaac going so far west? Some reason it thinks the storm would stay weak and therefore track furthur west?

The ECMWF portrays that a ridge of High Pressure will build further West then the GFS and not allow Isaac to feel the weakness as soon. An outlier at this time but still a possibility.
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After looking at Barbados radar, sat imagery, and recon data I can only come to two conclusions...

I have no clue where or if there is a "center"...And Isaac is a hot mess of a storm right now.
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Quoting 954FtLCane:


Yes the Miami-Dade schools are very proactive when it comes to tropical weather. I guess you have to be when your the 4th largest school district in the US.

The district covers a total of 415 institutions,[28] including:

195 Elementary Schools
56 Middle Schools
23 K-8 Centers
37 High Schools
53 Charter Schools
23 Vocational Schools
5 Magnet Schools
18 Alternative Schools
5 Special Education Centers

With this many campuses they better start early.

*There are over 380,000 students in the district.
That's twice as large as Wake County Public School System in North Carolina... and eight times the size of my school system.
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1367. GetReal
Quoting moonlightcowboy:
Kudos to GetReal. He spotted/anticipated this center reformation on Barbados radar early this morning.

BARBADOS RADAR



Thanks, but even a blind squirrel can find an acorn every now and then!
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Quoting TropicalAnalystwx13:

Not a circulation? So I guess the pressure dropped to 1005 millibars for no reason?

This southward shift in the center is not good.
i didnt expect this cody but the euro still shows a gulf track.
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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.


Goodness, I hope so. I am in Hattiesburg.
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1364. divdog
Quoting LargoFl:
oh boy thanks for the info..sure hope that changes
It will. 500 mile error rule in effect
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but this is ECMWF before at 216hrs out. DOOM!!!
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Quoting 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


So far this storm has had plenty of open water and not strengthened much so don't count on that. Seems this storm wants to stay weak and relocate a center to the SE, that will push it further West before it feels any weakness IMO. The longer this storm stays weak the further West it's going. The Doc hinted on this and he might just be right, he's been doing this a long time!
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Pretty interesting how a redeveloping center to the north increased its pressure by over a millibar in one pass. Also, it has SE winds over that "center."
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Quoting MAweatherboy1:
Recon got 1004.9mb... Still not very strong winds though.


...no west winds near the northern circulation, though.
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A reason for the size of Isaac storm could be a different jet stream.

Changing Jet Streams May Alter Paths of Storms and Hurricanes
Link

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if Issac makes it into the gulf everyone in New Oreleans have to leave!!!! or it could be a very deadly situation there
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Recon got 1004.9mb... Still not very strong winds though.
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Just got back on, but where has the center reformed?
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Quoting jcpoulard:


Even if Isaac pass south of Haiti rain band will still have a lot of impact on the country.


its a loose loose situation... Either Issac blows through/ heavely affects Haiti or it misses them and comes for the USA
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That recon pass would seem to confirm indeed that the center has relocated farther southeastward.
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1353. Patrap
Those 2 Centers are giving way to a "Overall" very quickly Now and the HH have it well in Hand I do believe.


Member Since: July 3, 2005 Posts: 420 Comments: 127535
1351. kwgirl
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.

Phenomenal: remarkable, extraordinary.

Not a wish, but a comment on how outrageously impossible such a thing would seem until it happened.

I certainly wouldn't wish a major hurricane strike on a flea. I do wish this storm away from ME, but I wouldn't wish it on anybody else just to get it away from me, either.

My point again is that Americans are so polarized about these parties that they sometimes forget the basic underlying humanity of every member of the opposing party. That is wrong.
It's all about money and power. The politicians have gotten greedy and fight really hard to hang on to their jobs. They have no skills to do anything else, so it is their livlihood. That is the main cause of inhumanity to man. Greed!
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1004.9 mb
(~ 29.67 inHg)

from old center, rising pressure, its weakening.
Member Since: September 6, 2007 Posts: 0 Comments: 6056

Quoting SWLACajun:
Any ideas why the ecmwf shows Isaac going so far west? Some reason it thinks the storm would stay weak and therefore track furthur west?
That, and the Bermuda ridge rebuilding back enough to the W to delay northerly recurvature until it reaches Yucatan Channel/W Cuba.

GFSand the other models  sees a more definite break in the Bermuda ridge, plus the TX/OK Panhandle "Mini Death Ridge" building SE'wrd into LA, making the weakness for Issac to pass through further eastward.

Personally, I'd split the difference and go Mobile Bay to W FL Panhandle; I don't see how this can go as far W.  But, we will see.
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I also think the center fix is poor.

If you combine radar and RGB satellite, it's clear the center can't be north of about 15.5N, else the coulds would not be moving that way.

The automated tool on the RAMMB site sucks and is off by about half a degree or more...
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Quoting MississippiWx:


Where are your west winds with that northern circulation? And considering the northern one has been there the entire time, how did it re-locate to that location? Satellite imagery is increasingly obvious, as well as recon data, that the center is reforming to the south.

the west winds are there, they are flying back through to assure it
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1346. hydrus
Quoting barbamz:

Really massive swirl ball of heavy moisture. But maybe Isaac will have to digest some dry air from the southwest soon?
Very true...Isaac is not embedded in that immense moisture field we see with some storms this time of year. He will have to pick some up from the ocean. I should mention that the CIMSS water vapor loop you posted does a poor job picking up low level moisture content, so it may not be as bad as it looks. The NOAA GOES is a better indicator..
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Just one model



And it's already big

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Quoting LeithScot:
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...



How is Leith Walk, these days? Did a pub crawl or two down there in the 80s. Some well dodgy bars, and a few not so bad.

Starting to look as if Isaac might go west of the models. Hope so, as that could keep it south of Haiti, although they'd still get some flooding.

I think it'll take its time ramping up, but if it gets into the GOM, expect a major.
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Quoting dfwstormwatch:
Link

Not a circulation? So I guess the pressure dropped to 1005 millibars for no reason?

This southward shift in the center is not good.
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About

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.