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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693. SLU
Based on low level cloud lines and Barbados radar imagery, it seems the center is reforming further south east of the previous location near 15n 58.5w. Let's wait to see what the RECON finds.

Member Since: July 13, 2006 Posts: 12 Comments: 4795
Quoting StormJunkie:


Not big on track analogs, but in this case and in regards to intensity it can be useful to look at what happened to other storm that tried to traverse those mountains.

The concept of a car going over a speed bump at 60 comes to mind...
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This is Stephanie when she was good. Boy, does this bring back memories...
img src="">
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Quoting presslord:
I just don't get the obsession with analogs....


Not big on track analogs, but in this case and in regards to intensity it can be useful to look at what happened to other storm that tried to traverse those mountains.
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Quoting MississippiWx:


When analogs are used correctly, they are a very useful tool. I stressed yesterday that taking tracks of past hurricanes in the same location as Isaac means nothing if the pattern is not the same or similar to the storm being used as an analog. You have to compare 500mb anomalies between certain storms as well as take forecast strength into consideration. Choosing a past storm in close proximity to Isaac's coordinates simply does not mean anything without further consideration.

and that is what Presslord is alluding to. Many people who use analogs a lot on here simply go by proximity, rather than similar situations. That is why I posted my (intentionally) contradictory statement in response to Presslord.
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Quoting UnobtrusiveTroll10:
Is this a Google Earth capture? What is link to this imagery? - Thanks
It's Google Earth. You can find them here: Link
Member Since: August 1, 2011 Posts: 28 Comments: 7880
Quoting floridaboy14:
so due west with a bump northward. thats not west north west yet right?


Recon will give us the storm motion. It's hard to say if Isaac was relocating its center for a while or actually moving on a more WNW motion. Current steering would not favor a WNW motion at this time, so I'm leaning towards center-jumping.
Member Since: July 15, 2006 Posts: 17 Comments: 10159
recon decending
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I'd imagine the hurricane hunters will find some strong winds in that recent blow up near the center of Isaac.  Its pretty much perfect timing to head on in there.
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684. A4Guy
Quoting floridaboy14:
so since 11pm it lost .2 latitude you think? interesting


I think ppl are focusing on the way the convection is moving. look at a closeup of the RGB visible floater and you can see the CoC moving westward...not southwest.
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Quoting presslord:
I just don't get the obsession with analogs....


When analogs are used correctly, they are a very useful tool. I stressed yesterday that taking tracks of past hurricanes in the same location as Isaac means nothing if the pattern is not the same or similar to the storm being used as an analog. You have to compare 500mb anomalies between certain storms as well as take forecast strength into consideration. Choosing a past storm in close proximity to Isaac's coordinates simply does not mean anything without further consideration.
Member Since: July 15, 2006 Posts: 17 Comments: 10159
659 presslord I just don't get the obsession with analogs....

Digitals are just SO been there, done that, don'tcher know.
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Recon Inbound!
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Looks like the ULAC has out run Isaac and is providing some shear that has knocked the 850 center Sward a bit from the surface center.



Member Since: August 19, 2005 Posts: 5 Comments: 5020
679. wpb
recon flying just noth of center
Member Since: May 28, 2011 Posts: 0 Comments: 572
Quoting Jedkins01:
Issac is a very large cyclone, you then can derive 2 things from this, first off, generally large tropical cyclones take longer to organize and intensify than a smaller tropical cyclone, and this makes sense given the nature of how and why tropical cyclones intensify.


However, larger tropical cyclones tend to hold together better as they impact a landmass.

If the track verifies, more than likely the largest impact will be rainfall flooding. Especially for north in Central Florida because the ground is heavily saturated here.



I think this is another important component to factor into the whole equation, and it could cause this storm to take a more Westerly path, but that is yet to be seen.
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The Martinique radar seems to show a small vortex like circulation moving west southwest around the broader center
Member Since: August 28, 2005 Posts: 0 Comments: 281
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Is this a Google Earth capture? What is link to this imagery? - Thanks
Quoting Bluestorm5:
Recon plane about to enter Isaac
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I think there are three main components in the possibility of where this storm goes in the long run, and the most fundamental of which is the strength of this storm. If this storm can get its act together fast we could see a storm with a more Eastern path, but if it only slowly gets itself together then most likely we will see a more Western path.

It looks like the trough will dig in pretty deep and fairly far south, so I think that eventually this storm will take a more N/NE turn at some point, its all a matter of when.

If we see it turning earlier we might see it ride up the East Coast of Florida and then turn out to sea potentially. If it goes further West we could be looking at a West coast of Florida event where it rides over the state.


I think the whole catch 22 of this situation is that if it rides over the mountainous regions of Hispaniola and Cuba then it could go further West, and thus enter the Gulf of Mexico, if it survives the trek over the mountains.

Its all a matter of wait and see, although clearly the models seem to be pointing to a landfall somewhere near and around Florida at some point, but we should be able to paint a clearer picture by this weekend.
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Quoting presslord:
I just don't get the obsession with analogs....

Predicting the future with the past that is almost completely different than the present.

Makes total sense to me.
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Quoting TheMom:
Who got lucky? Cause we had Fay sitting on us for a week and had major flooding. some of my coworkers were even havign to move into work since they were not allowed to use boats to get to thier houses... Luck is relative...


Fay flooded out the Big Bend area (including parts of my house near Tallahasee) and other parts of North Florida........We dodged a bullet in the Big Bend when She weakened in the Eastern Gulf and came ashore as a TS.......This one will be a nail biter for Florida.
Member Since: August 8, 2006 Posts: 0 Comments: 8712
Quoting MississippiWx:


I goofed on my last post, but around 16.0N is about right.
so due west with a bump northward. thats not west north west yet right?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Don't know why my response to Jason in 637 turned out looking the way it did - weird. I tried to reply, and it ended up looking like part of the quote.
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Issac is a very large cyclone, you then can derive 2 things from this, first off, generally large tropical cyclones take longer to organize and intensify than a smaller tropical cyclone, and this makes sense given the nature of how and why tropical cyclones intensify.


However, larger tropical cyclones tend to hold together better as they impact a landmass.

If the track verifies, more than likely the largest impact will be rainfall flooding. Especially for north in Central Florida because the ground is heavily saturated here.
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666. yoboi
Quoting presslord:


When ya look like that...ya don't have to be smart...



as they say, triple d smarts...
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Center of ISAAC is about to make it thorough the islands now, the center is on the NE side of the martinique doppler radar
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The 2 will have Isaac still at 15.9N
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Quoting presslord:
I just don't get the obsession with analogs....


It's wishcasting with a er, "twist".


: O
Member Since: July 3, 2005 Posts: 419 Comments: 127371
charlies.big.brother.
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Quoting floridaboy14:
so since 11pm it lost .2 latitude you think? interesting


Betting recon finds it between 16 and 16.3N
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Quoting floridaboy14:
center is at 16.1N i think


I goofed on my last post, but around 16.0N is about right.
Member Since: July 15, 2006 Posts: 17 Comments: 10159
I just don't get the obsession with analogs....
Member Since: August 13, 2007 Posts: 0 Comments: 10479


Member Since: July 3, 2005 Posts: 419 Comments: 127371
Quoting MississippiWx:


South of there around 15.7N, moving due west for now.
so since 11pm it lost .2 latitude you think? interesting
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting BahaHurican:
- From the 1 p.m. ZNS report in Nassau.

Deteriorating conditions are expected in the SE Bahamas on Friday night / Saturday morning. Major rainfall event especially for Inagua, with 50mph + winds, is expected. Sat / Sun should see TS conditions spread into the Central and NW Bahamas respectively.

NEMA [our emergency agency] is on standby / alert mode. Defence Force is doing their prehurricane checks and planning meetings today.

Well, it does seem like the government agencies are not in denial...


Have been trying - unsuccessfully - for a while now to get into NEMA Bahamas Website - I presume this is the site for official Bahamas Govt Advisories???
Have you got any other appropriate links?
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Sorry for the off-topic but Bartolo Colon of the A's suspended 50 games for testing positive for testosterone. Some very fishy is going in the MLB lately...very windy here as Isaac comes nearer and nearer..yet the rain has stopped for hours now
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Quoting floridaboy14:
center around 16N? is it moving due west?

Seeing that it just got into the radar's view, I don't know.
It's actually actually closer to 16.2N, 60.1W or so.
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Afternoon All.

Isaac definitely has some central issues and a lot of tightening up yet to do.

Member Since: August 19, 2005 Posts: 5 Comments: 5020
Good morning everyone. It's been a while since I've returned to the good 'ol Wunderground chat!
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Quoting popartpete:
I also think Stephanie Abrams is way dumb, and is eye-candy only.

I agree completely with you, although she did do a great job in 2004 covering the hurricanes. That was when she was much younger and innocent, not trying to please with corny jokes and such.
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Quoting GetReal:
center is at 16.1N i think
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Isaac is going to have to SLOW DOWN...to stack up!
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Quoting WeatherNerdPR:

The burst is really expanding now...
What some of you guys thought was a CDO is now a fat spiral band.


Yup exactly right. If anything the center is located about 50 miles to the east of the new convection.
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Quoting floridaboy14:
center around 16N? is it moving due west?


.
Member Since: July 15, 2006 Posts: 17 Comments: 10159
Quoting JeffMasters:


Tropical Storm Fay of 2008. but got lucky that it made landfall over the Everglades near Flamingo.

Jeff Masters
Who got lucky? Cause we had Fay sitting on us for a week and had major flooding. some of my coworkers were even havign to move into work since they were not allowed to use boats to get to thier houses... Luck is relative...
Member Since: August 3, 2006 Posts: 5 Comments: 672
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As many experts have said in the past that sometimes in the formative stages of tropical cyclones there may be a broad area of circulation with several small vortexes circulating around the general center. This is what may be happening today.
Member Since: August 28, 2005 Posts: 0 Comments: 281
Member Since: July 4, 2005 Posts: 204 Comments: 8806

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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.