Isaac lashing the Lesser Antilles; TD 10 forms

By: Dr. Jeff Masters , 3:06 PM GMT on August 22, 2012

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


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

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This was Ernesto at the time it was struggling to find it's LLC, and looked generally a bad storm on satellite through most its life until it reached the Western Caribbean.


And here's Isaac at a similar longitude.
Member Since: December 31, 1969 Posts: Comments:
Quoting tamipeach:


I think one is for TD 10?
yeah thats probably it..watch THAT one northeast US..might just go around the bermuda high looks like
Member Since: December 31, 1969 Posts: Comments:
Quoting GetReal:
An observation with a question... After observing the Barbados radar loops it is clearly visible to anyone that the convection north of Barbados is moving EAST to
WEST. Now if the center of Isaac is currently located at 15.9N as NHC indicated on last advisory, shouldn't the convection on the Barbados radar be moving WEST to EAST. After all doesn't a cyclone rotate counter-clockwise in the N hemisphere?


Link hit the 400km loop.


If the low level center is that far north, Isaac is not stacked, and that is a stronger mid level circulation the Barbados radar is picking up.


If the storm was sitting still then yes, however since the storm is moving at 21mph the storms on the south side would have to be moving at >21 to appear to be moving east.
Member Since: December 31, 1969 Posts: Comments:
Quoting LargoFl:
OK GUYS..WHY DO THEY HAVE TWO WIND CONES UP???
That other cone is TD10 :)
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Quoting LargoFl:
OK GUYS..WHY DO THEY HAVE TWO WIND CONES UP???


I think one is for TD 10?
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One for Isaac, and One for Td 10.
Member Since: December 31, 1969 Posts: Comments:
Quoting LargoFl:
OK GUYS..WHY DO THEY HAVE TWO WIND CONES UP???


Because there are two storms...
Member Since: December 31, 1969 Posts: Comments:
An observation with a question... After observing the Barbados radar loops it is clearly visible to anyone that the convection north of Barbados is moving EAST to
WEST. Now if the center of Isaac is currently located at 15.9N as NHC indicated on last advisory, shouldn't the convection on the Barbados radar be moving WEST to EAST. After all doesn't a cyclone rotate counter-clockwise in the N hemisphere?


Link hit the 400km loop.


If the low level center is that far north, Isaac is not stacked, and that is a stronger mid level circulation the Barbados radar is picking up.
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Quoting Buhdog:


this is for a direct hit... i would say "feel the impacts' at about 20%
Not for a direct hit or feeling impacts. The odds are based on the posit of a mass evacuation.
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Quoting floridaboy14:
thats why i said it will take the southern track of the cone and wont get stronger until it reaches south of cuba. what are your thoughts on the track today drak?


My idea for track is exactly as the NHC has it.
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OK GUYS..WHY DO THEY HAVE TWO WIND CONES UP???
Member Since: December 31, 1969 Posts: Comments:
Impressive to be up to our 10th tropical depression by August 22nd. What is more impressive is the fact that it is an El Nino year. I'd have been less impressed if all these where from like cut off lows off the US East Coast like they where last year, or non-tropical in general. But every storm this month that has developed has done so from a tropical wave off Africa. That's usually a good signal for an active season. The reason though we're not going to see anymore than 15 named this year in my opinion is because the El Nino will probably try to shut off the season in October. We'll see a ramp down probably after peak.
Member Since: December 31, 1969 Posts: Comments:
Thanks, Doc. The model information is just as interesting as the analysis for Isaac.
Member Since: December 31, 1969 Posts: Comments:
Quoting kinase1:
Given the role of the anti-scientific thinking that drives anthropogenic global warming denialism, and extremists who explain some meteorological phenomena as God's punishment for imagined sins, I wonder what these people will say if Isaac disrupts the Republican convention in Tampa?


OUCH !!

heheheheh
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Quoting Drakoen:
Looks like Issac may struggle in the eastern Caribbean to intensify like his predecessors.
thats why i said it will take the southern track of the cone and wont get stronger until it reaches south of cuba. what are your thoughts on the track today drak?
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Looks like the models are coming together on a roll right down the backside of Florida.
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Quoting kinase1:
Given the role of the anti-scientific thinking that drives anthropogenic global warming denialism, and extremists who explain some meteorological phenomena as God's punishment for imagined sins, I wonder what these people will say if Isaac disrupts the Republican convention in Tampa?


I wonder if a storm hits Charlotte if they will blame the rich.
Member Since: December 31, 1969 Posts: Comments:
BAH !!

Heavy rain and 50 mph winds are bad news for the Islands.
Will damage agriculture big time there, especially the Banana crop.

Just had a strong squall from WSW blow through here. Not much rain, but I am expecting some heavy showers later.
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Quoting Maineweatherguy20023:
From the blog above...I put the odds of an evacuation occurring during the convention in the current situation at 3%. Why only 3% ??


this is for a direct hit... i would say "feel the impacts' at about 20%
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Thanks Dr. M. Basically, as I interpret your analysis, the storm is still having short-term intensity issues due to some dry air and the speed is probably contributing to a tilted circulation. Also looks like the best conditions for rapid intensification might be, ironically, after passage over Cuba...........It's going to be a very touchy situation for the Keys/South Florida/Bahamas come next Monday if the storm really intensifies just offshore.
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Quoting Maineweatherguy20023:
From the blog above...I put the odds of an evacuation occurring during the convention in the current situation at 3%. Why only 3% ??
It takes a strong storm coming in at a certain direction to cause an evacuation. There's way too many variables, most notably I think the barrier islands that can effect the storms strength, direction, even survival. 3% is actually pretty high odds, but I think right.
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Good timing on the model performance history Doc. Thanks
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Ok, so if it does run the West coast of FL and enter the Gulf, what would it do after the 5 day forecast, say Wednesday or Thursday of next week? Would it have any possibility to move West or could something else form off of the remnants of this storm in the GOM? When will we see the true path of this thing? If it breaks up a bit over the islands and weakens, could something shear off and start to churn up in the lower parts of the GOM? Just wondering.....in Louisiana here. Thanks to all who respond!
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Quoting Maineweatherguy20023:
From the blog above...I put the odds of an evacuation occurring during the convention in the current situation at 3%. Why only 3% ??
because our shields are up and hurricanes dont come here anymore..geez dont i wish
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Thanks for the awesome graph comparing model accuracies!
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Thank you for the timely update. Just a little typo to fix below (emphasis mine):
Quoting Jeff Masters: 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
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Looks like Issac may struggle in the eastern Caribbean to intensify like his predecessors.
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22Aug.06amGMT's 15.5n54.8w - 15.5n56.5w have been re-evaluated&altered
22Aug.12pmGMT's 15.4n54.8w - 15.5n56.5w - 15.9n58.5w are now the most recent positions
(The previous vectors and straightline projections have been corrected for this mapping)

21Aug.06amGMT: 15.2n50.3w, (267.0*W@17.4knots), 30knots, 1007millibars, TropicalDepression
21Aug.12pmGMT: 15.1n52.0w, (266.8*W@16.5knots), 30knots, 1007millibars, TD
21Aug.06pmGMT: 15.3n53.2w, (279.9*W@11.8knots), 35knots, 1005millibars, TropicalStorm
22Aug.12amGMT: 15.4n54.8w, (273.9*W@15.5knots), 35knots, 1006millibars, TS
22Aug.6amGMT's numbers are below, before 22Aug.12pmGMT's
Derived from NHC_ATCF data for TropicalStormIssac for 22August12pmGMT
MinimumPressure increased from 1003millibars to 1006millibars
MaxSusWinds held to 40knots(46mph)74km/h
Vector changed from 273.7*West@18.9mph(30.5km/h) to 281.9*WNWest@22.7mph(36.5km/h)

ANU-Antigua :: MNI-Montserrat :: DSD-LaDesirade :: FDF-Martinique :: DOM-Dominica

The easternmost dot is where Invest94L became TropicalDepressionNine
The 2nd dot west of that dot is where TD.9 became TropicalStormIssac
The easternmost dot on the longest line is TS.Issac's most recent position

The longest line is a straightline projection through TS.Issac's 2 most recent positions to its closest approach (within 18miles or 29kilometres) to a coastline
21Aug.12pmGMT: TD.9 had been headed for passage ~0.6miles(1.0kilometres)South of Martinque (bottom,FDFdumbbell)
21Aug.6pmGMT: TS.Issac had been headed for passage ~5.8miles(9.3kilometres)North of LaDesirade (top,DSDdumbbell)
22Aug.12amGMT: TS.Issac had been headed for passage ~3.5miles(5.6kilometres)North of Dominica (DOMdumbbell,egg-bottom)
22Aug.6amGMT: TS.Issac had been headed for passage ~5.2miles(8.4kilometres)North of Dominica (DOMdumbbell,egg-top)
22Aug.12pmGMT: TS.Issac was heading for passage ~1.3miles(2kilometres)NNEast of LaDesirade in ~4hours from now (when this was posted)

Copy&paste anu, mni, dsd, 16.432n60.997w, dom-15.691n61.424w, 15.716n61.423w, fdf-14.38n60.867w, 15.2n50.3w-15.1n52.0w, 15.1n52.0w-15.3n53.2w, 15.3n53.2w-15.4n54.8w, 15.4n54.8w-15.5n56.5w, 15.5n56.5w-15.9n58.5w, 15.5n56.5w-16.37n61.005w, 16.349n61.009w-16.367n61.005w into the GreatCircleMapper for a larger map and other information
The previous mapping for comparison
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Be safe up dere MLC.

We know how these Big CV ones CAN go,..and I'm sure yer keeping one Eyeball on Him.
Member Since: December 31, 1969 Posts: Comments:
From the blog above...I put the odds of an evacuation occurring during the convention in the current situation at 3%. Why only 3% ??
Member Since: December 31, 1969 Posts: Comments:
Dry air intrusion to the north seems to be causing the disorganized surface state. However, it is generating good convection and good spiral banding. Suspect 50mph today, 60mph tomorrow, with Friday being the earliest this could become a hurricane.
Member Since: December 31, 1969 Posts: Comments:
where did everybody go???
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Thanks, Doc.


Quoting Patrap:


Roger dat,

U got yer ear's on MLC?

Big Duke NOLA 7 here.

: )



Yup. I'm up mid-state today, beautiful sky and temps. As long as Isaac is sprinting along like it is things could change. Next 24-36 hours will be critical I think. Anything gets in that boiler pot south of us will likely spell trouble somewhere. I keep watching that sfc map, those highs.

Y'all hold the fort down! ;)
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TD 10 has formed!....I just did a special statement on my blog....which states the signficance of this moment (we may tie with 2005 in terms of having 10 named storms by Aug 22!)
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Thanks doc, i see we here in tampa bay are now IN the cone
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Getting interesting now Dr. M.  I think the RNC is doomed.
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Thanks
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Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather