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 air360:
All of the images posted by Get Real are not current! They are all from last nights 0z Run!

mistakes happen!
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A reformation of the center closer to 15N could throw a big curveball into the long range track.
Member Since: July 15, 2006 Posts: 17 Comments: 10250
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12Z Euro is faster than 0Z.

Also further north/east in medium range, but landfall still looks like Louisiana.
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Quoting MississippiWx:
168 hours, the 12z Euro has a strong hurricane sitting SSE of New Orleans, moving NNW.
Accrding to ummmm, TWC, they have NOLA in the area that shouldn't even "be aware"...fwiw. I don't understand. I'd be aware if I was anywhere from Mexico to the OBX.
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ECMWF 12Z @168 hrs:



From the PSU E-Wall.
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Quoting DavidHOUTX:
Euro has it stronger on this run as it heads towards the Gulf Coast than 00z run



You know, I honestly didn't think that a model run could look much worse than the Euro's 00Z, but evidently it had better ideas...
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986. JLPR2
This seems to support that the strongest surface vort is to the SW of the current one.




Just west of that new blow up of convection.
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Hello, New Orleans.



12Z Euro hour 168, taking a track like that "K" storm...
Member Since: September 3, 2011 Posts: 13 Comments: 3471
Spaghetti loop

Could history repeat?

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3-Hr Precip, Cloud, and Moisture Forecasts
North America 00 UTC cras45naP03-84 hours
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Member Since: July 3, 2005 Posts: 424 Comments: 128344
ok seems my local mets are thinking issac IS going to come up the west coast of florida,they have been pretty good so far..im going to listen to them the next few days or so and prepare for a storm,not going to get caught off guard this time for sure..
Member Since: August 6, 2011 Posts: 4 Comments: 38521
can someone give me a link to the euro run? mine doesnt work :(
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting MississippiWx:


This is why I didn't want to give you a definite answer. Looks very suspicious right now. No true westerlies with the northern-most spin, but the southern-most spin has a higher pressure. It might be lowering, however, due to the fact that it is surrounded by more thunderstorms. Interesting.


Very interesting indeed. They're not that much stronger though, the one at 16.1N is only about 1mb stronger. For all intents and purposes they are the same strength. That being said the fact that this new one is just popping up and has more defined westerlies tells me it is becoming the dominate one. Satellite would support this too.
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Quoting keisa90:
ECMWF still has it heading for Louisiana at 168. Not much of a shift East.


Pretty much have to go with a blend of both GFS and EURO right now with Issac in days 4 and 5.. somewhere from the south coast of Cuba to north Coast of Cuba.. heading NW on Monday into the extreme eastern GOM.. but there are alot of things changing across the southern U.S. by Day 5
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With LLC center trying to redevelop near the MLC this means Isaac is gaining organization and true strengthing will begin in earnest later this pm and tonight.
Member Since: August 28, 2005 Posts: 0 Comments: 301
How strong to nola
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12Z Euro at 168hrs, moving NW towards NOLA...!
Member Since: June 26, 2012 Posts: 0 Comments: 297
Euro has it stronger on this run as it heads towards the Gulf Coast than 00z run

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my ECMWF run at ralieghwx wont work only 24 hours out :( has recon confimed anything about the second center farther south and east?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting mojofearless:


I have been through hurricanes but live inland in Lafayette. So no, I haven't experienced the threat as much as NO or other places. But, to each his/her own.
Member Since: August 6, 2012 Posts: 0 Comments: 122
168 hours, the 12z Euro has a strong hurricane sitting SSE of New Orleans, moving NNW.
Member Since: July 15, 2006 Posts: 17 Comments: 10250
if the center further SE can take over i think it would be better for the storm to strengthen.
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Quoting icmoore:


Say what?
You have your peak in your forecast pointing at me ;-)
Member Since: August 3, 2006 Posts: 5 Comments: 672
12Z Euro @ 168hrs, moving NW towards NOLA
Member Since: June 26, 2012 Posts: 0 Comments: 297
ECMWF still has it heading for Louisiana at 168. Not much of a shift East.
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Quoting WPBHurricane05:


00Z run...


Sorry for an confusion...
Member Since: July 4, 2005 Posts: 204 Comments: 8874


Looks like the center is north of that small elongated blow up between the two lobes.
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All of the images posted by Get Real are not current! They are all from last nights 0z Run!
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but I have a gut feeling that the one at 16N is dying out and a new one has formed further S
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Quoting GetReal:
192 hours



Doc's current blog put the ECMWF at the top of the chart. And, while we're all thinking outlier, let's hope ECMWF's bell curve falls steeply soon; otherwise, well, you all know. Been a couple weeks since I looked, but the Loop and Loop Eddy extend far northwards into the GoM and maybe covered nearer 1,000 square miles in totality.
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LATEST FORECAST FOR ISAAC
_______________________________

With the latest runs of many models, current and forecast patters, here is my prediction.
NOTE: Give or take 5 mph from current wind intensities...

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The entire envelope is circulating around a "mean" center, and very interestingly within that CoC mean, are 2 vortices ..with the Southern one consolidating better, but to my view they will shake out a true center later this afternoon/tonight.

When that happens, Isaac will start to build that "Tower Of Power" slowly, but confidently if the Intensity models pan out currently.




Member Since: July 3, 2005 Posts: 424 Comments: 128344
Quoting SLU:


that's exactly where they're headed


This is why I didn't want to give you a definite answer. Looks very suspicious right now. No true westerlies with the northern-most spin, but the southern-most spin has a higher pressure. It might be lowering, however, due to the fact that it is surrounded by more thunderstorms. Interesting.
Member Since: July 15, 2006 Posts: 17 Comments: 10250
Quoting CybrTeddy:
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.


Maybe so, but it still seems like a very week storm. La Desirade, Guadeloupe is reporting 2 mph variable winds, a pressure of 29.74, light rain, and a maximum wind gust of 16 mph. At least, for the Windwards, it looks like this is not going to be a big deal for them.
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.
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Quoting DavidHOUTX:


Oh it certianly is a Gulf Storm on this run as well.
Albeit shifted a bit eastward...more like just E of NOLA now rather than Morgan City...

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Well I see the projected track has moved from Miami to Fort Myers in the span of a few hours. This makes me less concerned, being in Fort Myers ... these things are never accurate five days out.
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Quoting GetReal:
192 hours



That is last nights
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952. SLU
Quoting MississippiWx:


They just found a wind shift there as well, but the lowest pressure is associated with the center that is farther north.


They're not quite there yet. This "swirl" could be the area of conflict between the 2 circulations .....
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Quoting WindshearWillie:
If the Carolinas are sperated explain this:







So which team is Carolina? The North or the South one?


Now back to weather: It's like a nice Fall day here in Dallas!


that's just an insignificant little Bush League football team nobody cares about....
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Pressures are lower in the circulation at 16.1N, but the one at 15.4 is a lot tighter and more pronounced. Wasn't there at the last recon, I suspect the WSW motion on the old CoC we've been seeing all day on radar was from it dancing around the new developing COC.
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A centre relocation a full degree in latitude is gonna result in a sizeable westward shift in track.
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Quoting GetReal:
192 hours



Weaker storm...further west?
Member Since: August 16, 2007 Posts: 1 Comments: 2348
Quoting sporteguy03:

The cone only changes at 5am,5pm,11pm and 11am no changes at intermediate advisories.


that kinda thing right there is what this blog is truly good for
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even though the pressure is 1004mb with the center north, the one farther south is competing with it and looks to be making a little shift. lets wait for RECON to confirm this. this has a LONG WAY TO GO TO BECOME A HURRICANE
Member Since: July 25, 2011 Posts: 0 Comments: 1102
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Quoting JasonRE:
So in 3 days, it's supposed to turn that much North? Sorry I'm not a big weather person, just anxiously awaiting the storm here in Louisiana. Hoping for a turn this way.......FASCINATED with storms. I wonder what it will do after it hits all of the islands with it's severity/structure. Wonder if something can take off from a piece of this storm and get into the GOM.

Also wonder if it has any ability to turn West after hitting the islands instead of making such a dramatic turn Northwesterly. Recently (this year) all the storms have either gone West into Mexico or turned back off into the Atlantic.

JasonRE , I take it you must not live near the coast of Louisiana to be wanting this storm to hit your location. This storm could be a massive monster by the time it comes to your doorstep. if it comes your way at all.
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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.