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 Tribucanes:
Knock on wood, this could be great news for Haiti. NHC has had their work cut out for them this year to say the least.


I know everyone is hoping in my area that Isaac hits Haiti to be a weaker storm for us.. but I really hope Isaac moves south of Haiti. I don't want them to suffer anymore, they've had enough. Thousands of people in Haiti have died in the last few years alone from cyclones and let's not even dive into the quake..
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1241. LargoFl
Quoting drs2008:
Well I guess it's time to discuss it. For the past few years we have all been wondering what would happen if an entity were to enter the gulf,under these new, hot conditions. We may soon find out. The gulf is hot and moist. We are all worried.
indeed in some places the water is 90
Member Since: August 6, 2011 Posts: 4 Comments: 36855
Quoting clwstmchasr:


Doubt it. Brings the Panhandle into play and lessens the threat to the East Coast.


Models for the past 24 hours or so show that the Panhandle has been in play, especially with Issac dipping just south of Haiti and DR. If he shoots the gap between Yucatan and Cuba (like Ivan did in 2004) it's definitely problematic for anyone between Biloxi and Panama City.
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Quoting CCkid00:


NEVER safe to rule out anything as close as the Carolinas, when a storm is this close to the US. looking like more of an easter gulf/florida storm, but the Carolinas are by far...not out of the equation yet.
I was just wondering...
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It seems that the circulation of Isaac is twice as big in coverage then that of TD 10.
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Quoting hydrus:
If the center reforms to the south, it does effect the models and forecasts does it not?

Yes, they should shift more towards the west, at least short term.
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1236. SLU
Quoting SLU:


Thanks guys... saw something strange going on and brought it to your attention

Here's the link to follow the RECON. Refresh it ever 10 mins.


Let's see if ISAAC will finally start to consolidate around this new low.
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1235. Patrap
# 1220

FOUL language in any form is not Allowed.

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


Member Since: July 3, 2005 Posts: 420 Comments: 127543
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.

Let's hope so!
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1232. JLPR2
Quoting Kowaliga:
******FRESH RADAR LOOP FROM FDF******

...time to play everybody's favorite quiz sensation: "Pick Out the COC"!



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.
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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.
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Quoting MAweatherboy1:
Pretty tightly clustered around west FL:



Notice the last few points of the TVCN.
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1229. drs2008
Well I guess it's time to discuss it. For the past few years we have all been wondering what would happen if an entity were to enter the gulf,under these new, hot conditions. We may soon find out. The gulf is hot and moist. We are all worried.
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Quoting CCkid00:


wouldn't be pretty here in Louisiana if a hurricane comes in this year. imagine how bad west nile would be if people were without power right now in south Louisiana.

good point CCkid00.  the Mosquito population would explode with all the flood waters to lay eggs in.

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1227. TheMom
Quoting LargoFl:
unless issac is 500 miles off the coast, we sure are going to get flooding, all the ditches and ponds etc are full, many streets have water on them right now..and so much more is still coming in from the gulf right now..geez..enough already mother nature
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.
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Quoting Chicklit:

More time over water + less interaction with land = stronger system.

eww I don't like that a bit
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1224. CCkid00
Quoting Bluestorm5:
So it's almost safe to rule out Irene path toward Carolinas if the location shifted to southern center?


NEVER safe to rule out anything as close as the Carolinas, when a storm is this close to the US. looking like more of an easter gulf/florida storm, but the Carolinas are by far...not out of the equation yet.
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1223. SLU
Quoting ILwthrfan:


NICE CALL SLU!!! Being on the BALL!
Quoting MississippiWx:


I was a bit skeptical at first. Nice job by SLU to pick up on it.


Thanks guys... saw something strange going on and brought it to your attention

Here's the link to follow the RECON. Refresh it ever 10 mins.
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1222. Patrap
GOES Atmospheric Imagery


Below is the latest available GOES GVAR thermal imagery. You can also visit imagery depicting current water vapor, visible, and low cloud (lower atmospheric) conditions.
During hurricane season, the ESL's hurricanes page provides a variety of GOES atmospheric products to help monitor the active storms.
To see our GOES sea-surface temperature imagery, visit our archives. Find more of our oceanography research at our Loop Current Research page.
Member Since: July 3, 2005 Posts: 420 Comments: 127543
Knock on wood, this could be great news for Haiti. NHC has had their work cut out for them this year to say the least.
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I would wait for an official call before handing out the tea and crumpets.
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Looks like that was a spiral band they found those TS winds at.. they are all the way up at 17N.
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Pretty tightly clustered around west FL:

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1216. Patrap
Member Since: July 3, 2005 Posts: 420 Comments: 127543
1215. Matt74
Quoting AllStar17:
It would also appear any kind of center relocation to the south would not be good news...in fact, the alleged new center is about 75 miles SE of the current center.
Maybe the Euro is on to something?
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1214. A4Guy
Quoting HurricaneDean07:

The center is reforming... Its been shown by recon passes.


If this is indeed the case, staying a bit south could result in a stronger storm 9i.e., less interaction with Haiti), and be pulled more northwards anyway. Everyone is just guessing at this point - still too much spread in the models.
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1213. hydrus
Quoting MAweatherboy1:
Remember, just because the center reforms doesn't mean Isaac has to go more south, but it increases the chances... We may see the cone shift down towards Jamaica at 5PM.

If the center reforms to the south, it does effect the models and forecasts does it not?
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Quoting mcluvincane:
I can see a shift south and west on the next advisory for sure. Like it or not the euro has done pretty well with Isaac so far. I do believe there will be a center relocation to the south. Isaac will likely miss a lot of land during its path through the Caribbean and clip the western tip of Cuba heading into the boiling pot


If it stay further south it will RI into a Major then move NW to NNW. Don't see it staying weak like the Euro if thats the case.
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1211. yoboi
Quoting gustavcane:
at 144hr ECMWF  Isaac does not hit any land areas going thru the Yucatan Channel.


DR M said sunday the yucatan has been the pattern this yr.....
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Quoting OldLeatherneck:
I generally just lurk on this blog, since I don't have the knowledge and expertise that many of you seem to have. However, if Isaac does form a new center to the south, what does that do to the potential track of Isaac for the next 3/4 days?

More time over water + less interaction with land = stronger system.
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Need to keep an eye out for any unexpected ULLs spinning up ahead of this thing, they always seem to come outta nowhere
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1208. trey33
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.


He's rarely IN the actual storm......
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Quoting clwstmchasr:


I believe you were one of them. The next discussion should be interesting.


I'm just an observer with a coke in hand lol.. didn't even see it until the recon showed it. Pat and SLU got that one first via satellite..
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1205. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 36855
Definitely still a TS

From 105° at 42 knots
(From the ESE at ~ 48.3 mph)
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Time: 19:10:30Z
Coordinates: 17.0333N 59.3833W
Acft. Static Air Press: 843.1 mb (~ 24.90 inHg)
Acft. Geopotential Hgt: 1,566 meters (~ 5,138 feet)
Extrap. Sfc. Press: 1009.3 mb (~ 29.80 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 103° at 41 knots (From the ESE at ~ 47.1 mph)
Air Temp: 17.0°C* (~ 62.6°F*)
Dew Pt: 17.0°C* (~ 62.6°F*)
Peak (10s) Flt. Lvl. Wind: 41 knots (~ 47.1 mph)
SFMR Peak (10s) Sfc. Wind: 26 knots (~ 29.9 mph)
SFMR Rain Rate: 2 mm/hr (~ 0.08 in/hr)
(*) Denotes suspect data
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1202. CCkid00
Quoting Tribucanes:
41 dead now from West Nile with the worst yet to come. Just a friendly heads up. Many thousands will be affected this season.


wouldn't be pretty here in Louisiana if a hurricane comes in this year. imagine how bad west nile would be if people were without power right now in south Louisiana.
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ECWMF 168hr out Isaac is emerging out into gulf.
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This is my Afternoon Update video blog Link
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1199. Patrap
Animation mosaïque Antilles Radar
Member Since: July 3, 2005 Posts: 420 Comments: 127543
1198. hydrus
Quoting MississippiWx:
If Isaac ever fills in the N part of his circulation, he's going to be massive.

Very unsettling image....I wonder what this image will look like in three days.
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1197. TheMom
Quoting WXGulfBreeze:
When Jim Cantore shows up anywhere between Panama City and Pensacola Beach, then I'll start to get worried. He was here earlier this year for Debbie, but she had a mind of her own. What's most interesting to me is that Issac seems to be following a track and development similar to Ivan in 2004, and for anyone on the Fl. panhandle, that bears watching.
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.
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1196. Gorty
Quoting LargoFl:
ok ty


np.

Tracks and intensities can change a lot from advisory to advisory.
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Quoting GeorgiaStormz:
End of Euro, Isaac dissapating in Arkansas, while Joyce is stuck under the ridge....is that really going to recurve much with a 1020mb isobar above it?


that's not joyce, that is the K storm. look at entire model run
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Quoting tamipeach:
So...does a new center help move the path away from Florida? I went to grab a couple of cases of water and missed like 30 pages lol. thanks!

not really. if it allows it to get stronger it will feel the weakness more than a weak storm like the Euro shows going thru the caribbean.
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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.