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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443. Jaxen
Oh by the way, Friday is the 20th anniversary of Hurricane Andrew's South Florida landfall.
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Quoting interpreter:
Update:

Isaac struggling. The system is still moving on a general due west path at a quick pace of about 20mph which is forecast to continue until it approaches the northern coast of Honduras or Belize in the Yucatan Peninsula as previously forecast. It is being hampered by NE shear which will continue in keeping the system weak and induce it to maintain this westward motion. Also there is SW shear ahead from an ULL that is forecast to move SW that will disrupt the system at about 75 west. This may induce a temporary WNW path then bend back west once again with an increased threat to the Yucatan north of the Belize border. The predicted strength by landfall should not exceed strong tropical storm force.

If the system survives crossing the Yucatan and gets into the GOM steering currents are predicted to weaken and the storm could go stationary but this is too far into the future to predict it's eventual motion and potential second landfall.

Another update tomorrow about this time.


What?! Who's forecast is this? Is this yours? Where is this coming from?
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Quoting Buhdog:


I remember them for how unexpectedly they blew up right in that spot... anyone have an eddy map?

I don't have a way to post the map, but I use the one here...

No real eddy to speak of right now, its pretty much a straight shot via current from Cozumel to Key West.

http://wavcis.csi.lsu.edu/forecasts/forecasts.asp ?modelspec=currents
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I agree with Levi. This is going NORTH. This track will give the storm more time over water and the carolina's will have to deal with a much stronger system!


Thanks Levi for the update
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Quoting Buhdog:


I remember them for how unexpectedly they blew up right in that spot... anyone have an eddy map?


GOM 120 Hour Surface Current "Loop Current" Forecast Model
Member Since: July 3, 2005 Posts: 426 Comments: 129090
Quoting Gorty:
How will Isaac ever turn toward the wnw if he remains so weak?


Someone please correct me if I am wrong, but while it is very weak; it is not an extremely "shallow" system?
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Quoting NICycloneChaser:
Does anybody know when the 12z ECMWF run will start being released?

around 1:00pm CST
Member Since: June 26, 2012 Posts: 0 Comments: 297
Quoting HuracandelCaribe:
16.2N  60.1W  MOVING WNW  PR is in the direct path of Isaac



No it's NOT
Member Since: June 26, 2012 Posts: 0 Comments: 297
Does anybody know when the 12z ECMWF run will start being released?
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man i hope the gfs is wrong. dont need that thing easing up the west coast and into the big bend panhandle. we got spanked with lil ole debby.
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Member Since: August 6, 2011 Posts: 4 Comments: 40960
AL092012 - Tropical Storm ISAAC


2km Storm Relative IR Imagery with BD Enhancement Curve



2km Storm Relative IR Imagery with BD Enhancement Curve

The same infrared imagery shown in the earth relative framework is displayed in a storm relative framework, with a 2km resolution and enhanced with the "BD Curve" which is useful for directly inferring intensity via the Dvorak Enhanced IR (EIR) technique. Scaling is provided by two lightly hatched circles around the center. The two circles have radii of 1 and 2 degrees latitude, respectively.
Member Since: July 3, 2005 Posts: 426 Comments: 129090
Quoting hydrus:
Charley, Katrina and Rita..And I can come up with lots more..


I remember them for how unexpectedly they blew up right in that spot... anyone have an eddy map?
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Geez pouring rain here again!..mother nature..enough already
Member Since: August 6, 2011 Posts: 4 Comments: 40960
WOW.. kudos to Levi - he is NOT budging from his cone taking east of FL's east coast and hitting the GA/SC area..
I believe he is incorrect...
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Quoting CybrTeddy:


Looks like a new blowup is happening directly over that per satellite.

New frame supports that notion.
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Quoting hydrus:
What an ugly picture..:


TRUE DAT!!!
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16.2N  60.1W  MOVING WNW  PR is in the direct path of Isaac


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Quoting GetReal:


Finally found the exposed LLC at 16.1N and 59.7W...


Looks like a new blowup is happening directly over that per satellite.
Member Since: July 8, 2005 Posts: 259 Comments: 24475
i think all of us are waiting for the 12z ECMWF run
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Quoting GetReal:


Finally found the exposed LLC at 16.1N and 59.7W...
looks rather good on the W.V.
Member Since: September 27, 2007 Posts: 1 Comments: 21744
Quoting WeatherCaneFF1331:
when is the HH going out again? ty


According to their schedule 45 minutes ago, but its the air force so you never know.
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AL092012 - Tropical Storm ISAAC

16:15 UTC Enhanced Infrared (IR) Imagery (4 km Mercator)

Member Since: July 3, 2005 Posts: 426 Comments: 129090
419. Gorty
How will Isaac ever turn toward the wnw if he remains so weak?
Member Since: November 8, 2008 Posts: 12 Comments: 1058
Off Topic, but it is time to use the heavy duty bug spray

(CNN) -- The recent West Nile virus outbreak is the largest ever seen in the United States, according to new numbers from the Centers for Disease Control and Prevention.
The number of cases so far this year is the highest recorded since the disease was first detected in the United States in 1999. As of August 21, 38 states had reported human infections. The cases reported to the CDC total 1,118, including 41 deaths.
Member Since: September 6, 2007 Posts: 0 Comments: 6670
Member Since: August 6, 2011 Posts: 4 Comments: 40960
when is the HH going out again? ty
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Quoting Buhdog:


Places Issac in the Sweet spot of the SE gulf... can you name me the 3 storms that come to mind that REALLY liked it there?

C, K, R were the letters.
Charley, Katrina and Rita..And I can come up with lots more..
Member Since: September 27, 2007 Posts: 1 Comments: 21744


Finally found the exposed LLC at 16.1N and 59.7W...
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Quoting SFLWeatherman:
Any idea how long, before HH are up


Should be v. shortly.
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Just went to Walmart and Publix in Tampa. Pallets of bottled water hitting the lobbies!


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big.bend.landfallers.are.usually.homegrown.systems. ..dont.know.any.cv.ers
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Quoting Patrap:


Actually the last 4 Days GFS has pinged Tampa.

So it bears scrutiny ,,but the end lines and such on the Forecast ensembles will be affected West, if the trend West continues.
Yep. Every mile Isaac moves west now increases the chance of a U.S. strike..I certainly hope this does not make the gulf. That would most likely be a catastrophe of near epic proportions.
Member Since: September 27, 2007 Posts: 1 Comments: 21744
Quoting Patrap:


Tampa Bay Cat 3 Surge Potential
ty Pat
Member Since: August 6, 2011 Posts: 4 Comments: 40960
Member Since: July 3, 2005 Posts: 426 Comments: 129090
Any idea how long, before HH are up
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GFS 12z matches the NHC forecast track well. Let's see what the other models bring in.
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Storm's characteristics have improved everywhere except the NE quadrant. Upper level low and dry air up there are giving it problems still.

It may get away from it in another 6 to 12 hours or so though...
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Quoting Clearwater1:


I'm sure you know, but the 0z is out, past 144 hour and has it closer to the coast, near us.
yes at the moment but so many changes coming down the pike, this could..get destroyed if it goes right over those high mountains in haiti..we'll see, a week away yet
Member Since: August 6, 2011 Posts: 4 Comments: 40960
Quoting alvarig1263:
GFS Model at 114 Hours:



Notice how Joyce is stronger as Isaac trends westward...
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Tampa Bay Cat 3 Surge Potential
Member Since: July 3, 2005 Posts: 426 Comments: 129090
Ascat is hinting that Isaac could be open on the SW
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Quoting SFLWeatherman:
From Levi

He moved west.
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Quoting wakd3Xn04:


Just had that conversation with my mother who lives in Sarasota. Wanted to make sure she is aware of the storm and is getting extra water and batteries, ect. just in case.
good thing. it still could very well hit us hard here in florida. even though the models are shoewing it moving more west. we not out of danger.
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Quoting StormJunkie:


Afternoon CT, good to see ya and thanks for the update. It is a pretty rough looking system right now. Will be interesting to see what recon finds.


Hey SJ, yea just kind of on standby right now. The land interaction is obviously going to be the X-factor with Isaac. Worst case scenario for us, is a CAT 1 Isaac crossing central Cuba and moving NNW parallel to Florida. This will of course allow for RI and maybe a CAT 3 landfall somewhere from Savannah to the OBX. Another grim set up for us would be an eastern Gulf storm, over water and parallel to the west coast of Florida and then bending back NE. Even though a landfall would be in the big bend of Florida, we would eventually be in the NE quadrant and the storm wouldn't weaken all that much before arriving here. By the way the crow flies, Charleston to Apalachicola is only 400 miles.
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When is the 1200Z ECMWF published?

The 0000Z scares the hell out of me...
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From our local Met here in Tampa (Denis Phillips):
It's really simple folks. There is a giant bubble of air protecting the Southeast. It's called a ridge. That ridge is expected to crack by Saturday. The LOCATION of the crack is key. The models are completely split on where that crack occurs. Some say it cracks to the right, and Issac missed Florida to the East. The Gfs cracks the ridge more to the left, and Issac crosses Cuba and hits Florida on Monday and Tuesday. The GFS says the ridge cracks late and brings Isaac into the Gulf to hit LA or Tx as a major hurricane. As we said yesterday, we won't know until late Friday or Saturday. So until then, the models will flip flop, so might the forecasts...but THAT is the forecast is a nutshell. Here is what you need to do. Finish off your hurricane kits. Have everything you need in case the GFS is correct. IF it is, we will likely have strong tropical storm force winds or possibly hurricane force winds later Monday or Tuesday. If the other 2 possibilities are correct, we won't see a thing. If the GFS is correct, I would expect winds between 60 and 80 mph on Tuesday. IF it's right. There is a better chance it isn't.
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Quoting Levi32:


You pegged the center on the nose.
the center is around 16N moving off to the west. If the 12z Euro still shows the same solution, what should we make of this?
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Quoting yoboi:


did not do so well with debby....
They seem to have more problems with the TSs than the hurricanes... wonder why that is...
Quoting mcluvincane:
I don't understand why people post the precipitation map instead of the vorticity map when posting model runs
Precip maps work for me because it gives some idea how much rain we're likely to see, which could inform re: flooding potential...
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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.