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 Bluestorm5:
Isaac is a mess... two centers?




2 embedded vorticie around a mean center.

Definitely has some work to do structurally.

Member Since: August 19, 2005 Posts: 5 Comments: 5458
I just looked at this and found it quite interesting. Out of all the storms that crossed Hispaniola, only one went on to become a major hurricane and make landfall along peninsular Florida, and that was Jeanne from 2004 after she made the loop. There are a much higher concentration of major landfall along the western Gulf, despite the lower density of total storm tracks.



I know each storm is different from each other, but climatology highly does not favor a strong hurricane landfall a strong hurricane landfall along the Florida peninsula after crossing Hispaniola.
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Quoting GeorgiaStormz:


i think the carolinas are quite separate myself too ;)
See, press, that's why they keep putting on that -s-... otherwise they'd just say Carolina.

:o)
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Quoting Bluestorm5:
Isaac is a mess... two centers?

That would explain its slow development.
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The fact that the track of the GFS has moved further west for the 1400 EDT data makes me feel better, since I live near the Cape.

But this storm reminds me so much of Charley in 2004. The TPC prediction on that one was an epic failure. I worked with a meterologist then and the day before the storm struck he said it looked like to him it would come more to the east - based not on models but on observed conditions.

When the storm did turn at Ft Myers the TPC prediction for that day was for the track to move still further west. And when it made that turn the TPC seemed to go catatonic at that point. Local meterologists took over and told us quite accurately what it was going to do.

Can anyone knowledgable here compare the 2004 model accuracy to that of today?
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Quoting Bluestorm5:
Isaac is a mess... two centers?



Please read post 913, that might help you understand what is going on. The old one to the N at 16.1 is the dying one, based on the lack of true westerlies. This one at 15.4 is the dominate circulation that wasn't there the last time recon was in.
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500mb LLC further south..
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The pressure is lower with the northernmost center, as MississippiWx said.
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ok now I really would love to see where the second fix will be
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Quoting floridaboy14:
my euro run is 24 hours out only. that still looks like a gulf storm


Oh it certianly is a Gulf Storm on this run as well..

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932. Relix
*sigh* what a mess. Kinda relieved though... system seems that it will track farther south from PR than expected, and definitely vastly weaker than expected.
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12Z Euro 144 hr

Slightly further north/east.
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Quoting TheMom:
Please remove your peak from my bottom. Thank you


Say what?
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hope she has hurricane insurance!
Member Since: August 28, 2005 Posts: 0 Comments: 303
Quoting BahaHurican:

A local number is 242-322-6081.
Be safe Baha good to see ya again.
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Quoting Grothar:


It's very nice. Have you ever been through it? The first time I was it was 1492. It didn't have a name yet. :)
Probably had a name, but you cannot post it.
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926. SLU
Quoting SLU:
I hope the RECON goes to investigate the suspect rotation near 15n 58.5w. It has much more convection than the one we are tracking.





that's exactly where they're headed
Member Since: July 13, 2006 Posts: 13 Comments: 5367
Martinique Radar


Member Since: July 3, 2005 Posts: 428 Comments: 129903
anyone noticing the CENTER that is near 16.0N is weakining and something is forming a little further south?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
The Euro isn't working for me at all from the raleighwx site..
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Multi-tasking mistake!
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Quoting SLU:
I hope the RECON goes to investigate the suspect rotation near 15n 58.5w. It has much more convection than the one we are tracking.



They just found a wind shift there as well, but the lowest pressure is associated with the center that is farther north.
Member Since: July 15, 2006 Posts: 17 Comments: 10284
Quoting GetReal:
144 hours



00Z run...
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Isaac is a mess... two centers?

Member Since: August 1, 2011 Posts: 28 Comments: 8075
Quoting 954Soxfan:
I was supsrised to see Fort Lauderdale still in the cone at 2pm...

The cone only changes at 5am,5pm,11pm and 11am no changes at intermediate advisories.
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Good... looks like the ULL NE is really helping....

Shifting WSW Isaac and NW TD 10

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p
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Quoting matilda101:
NWS out of Miami has now just updated their forecasts with possible tropical storm conditions for Sunday night into Monday with possible hurricane conditions forecasted for the upper keys on Monday. Not surprised with the wording.
Where are you seeing this. I see nothing of the sort.
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914. NJ2S
Quoting WPBHurricane05:
GFDL with a south/westerly shift.


that wouldn't be good for Miami...and catastrophic for haiti
:(
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Quoting osuwxguynew:
The HH passed through the old center, seem to be finding a broader CoC to the SE with pressure falling again toward this feature. Should see on the next leg.

This agrees with my thoughts based on satellite that the old CoC is going to die and be replaced by a new one. The convective banding STRONGLY suggests this. I guess we'll see over the next 12 hours or so.



Yup, seeing that also. The old CoC at 16.1n is the dying COC with a new one developing at around 15.4W. Very pronounced windshift from the east to the west. That is the main center and it is becoming dominate quickly. That confirms a lot of thoughts around here. Whenever the old CoC dies off completely we probably have intensification.
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Once,long ago,Louisiana went from The GOM to Canada.

And they finally, well, u know.

But we still kept our Parishes.

"Laissez les bons temps rouler"



: )


Member Since: July 3, 2005 Posts: 428 Comments: 129903
Quoting wxchaser97:
My latest forecast, but with recon in there this may change.
Please remove your peak from my bottom. Thank you
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wxchaser97 your forecast track has Isaac making landfall very close to my folks in Palm Coast. They've only ever been through Beryl and Debbie. They don't have a healthy fear of storms of the tropical nature. Not yet at least. Only takes one. Is it just me, or does only a 3% chance of Isaac causing an evacuation during the RNC seem low? Would it take more than a cat1 to cause said evacuation?
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Meanwhile, that Central Atlantic ULL that has been giving Issac such a hard time on his NE quadrant has also turned some of it's attention to TD10......

WV loop:

Link
Member Since: August 8, 2006 Posts: 0 Comments: 9413
Quoting presslord:


You musta been pretty surprised to find out you weren't in India...

HOHOHOHOHO good one!
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Quoting bamagirl1964:
Hello everyone! I desperately need your help! I truly admire the wealth of knowledge you all have and need your input. My husband and I are 48 years old and are taking our first cruise this weekend! We are leaving out of Miami on Friday, going to Nassau, and returning to Miami on Monday. It is getting down to the wire and I don't know what to do. I have been glued to the computer watching the models and I see they are tracking west, which would be good for our cruise. I know that nothing is definite, but do you think this trend will continue? I have the vacation protection package and could reschedule for next week, but I may run into the same scenario as this week! I am very grateful for any insight you can give me and I do truly admire your input! Thanks! Marisa
It's looking like Isaac is likely to get here this weekend after all... Have you contacted the cruise line? because they are probably trying to decide where they will cruise. Depending on what the forecasts say tomorrow night and Friday morning, the entire Bahamas may be off limits, and cruising out of Miami might be a challenge... I'd call them [or your travel agent] ASAP...

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Interesting. 2 circulations?
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904. wpb
Quoting Seflhurricane:
direct hit on the Miami-ft lauderdale metro area as a strong cat 1 hurricane
it will shift again and again hmrf has almost nothing
Member Since: May 28, 2011 Posts: 0 Comments: 573
I will leave again for a while .... carry on....
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Quoting DavidHOUTX:
Euro shifting east..

my euro run is 24 hours out only. that still looks like a gulf storm
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting jeffs713:

The G IV is looking at the strength of the high, and potential dry air influence.


Indeed...
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Product: Air Force Vortex Message (URNT12 KNHC)
Transmitted: 22nd day of the month at 18:19Z
Aircraft: Air Force Aircraft (Last 3 digits of the tail number are 307)
Storm Number & Year: 09L in 2012
Storm Name: Isaac (flight in the North Atlantic basin)
Mission Number: 3
Observation Number: 02
A. Time of Center Fix: 22nd day of the month at 18:02:20Z
B. Center Fix Coordinates: 16°06'N 60°24'W (16.1N 60.4W)
B. Center Fix Location: 86 miles (139 km) to the NE (50°) from Roseau, Dominica.
C. Minimum Height at Standard Level: 1,479m (4,852ft) at 850mb
D. Estimated (by SFMR or visually) Maximum Surface Wind: 35kts (~ 40.3mph)
E. Location of the Estimated Maximum Surface Wind: 71 nautical miles (82 statute miles) to the NW (314°) of center fix
F. Maximum Flight Level Wind Inbound: From 68° at 35kts (From the ENE at ~ 40.3mph)
G. Location of Maximum Flight Level Wind Inbound: 74 nautical miles (85 statute miles) to the NW (314°) of center fix
H. Minimum Sea Level Pressure: 1004mb (29.65 inHg)
I. Maximum Flight Level Temp & Pressure Altitude Outside Eye: 18°C (64°F) at a pressure alt. of 1,525m (5,003ft)
J. Maximum Flight Level Temp & Pressure Altitude Inside Eye: 21°C (70°F) at a pressure alt. of 1,525m (5,003ft)
K. Dewpoint Temp (collected at same location as temp inside eye): 17°C (63°F)
K. Sea Surface Temp (collected at same location as temp inside eye): Not Available
L. Eye Character: Not Available
M. Eye Shape: Not Available
N. Fix Determined By: Penetration, Wind, Pressure and Temperature
N. Fix Levels (sfc and flt lvl centers are within 5nm of each other): Surface and 850mb
O. Navigation Fix Accuracy: 0.02 nautical miles
O. Meteorological Accuracy: 5 nautical miles
Remarks Section:
Maximum Flight Level Wind: 35kts (~ 40.3mph) in the northwest quadrant at 17:39:00Z
Maximum Flight Level Temp: 21°C (70°F) which was observed 8 nautical miles to the NW (314°) from the flight level center
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Quoting seflagamma:


I thought the stronger they were the more broken up they got...

thanks anyway, we will all just have to wait and see...




If Isaac is weaker than expected it will emerge with much less strength taken by the islands, Isaac probably won't be a hurricane my guess by the time it reaches Haiti.
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The HH passed through the old center, seem to be finding a broader CoC to the SE with pressure falling again toward this feature. Should see on the next leg.

This agrees with my thoughts based on satellite that the old CoC is going to die and be replaced by a new one. The convective banding STRONGLY suggests this. I guess we'll see over the next 12 hours or so.

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896. A4Guy
Can anyone tell me when the 12Z late-cycle runs update?
12Z is 8:00 a.m., ET, right? Shouldn't those be done by now???
I never can figure out all the times the various runs complete.
Thanks in advance.
Member Since: June 23, 2006 Posts: 0 Comments: 668
895. SLU
I hope the RECON goes to investigate the suspect rotation near 15n 58.5w. It has much more convection than the one we are tracking.



Member Since: July 13, 2006 Posts: 13 Comments: 5367
Quoting WxLogic:
In case you're wondering here's the Synoptic Pattern being flown by the G-IV/C130-J tomorrow:





They look to be sampling the strength of the ridge to the north. Should help the models a good bit.
Member Since: July 15, 2006 Posts: 17 Comments: 10284
Quoting presslord:


We are by God separated!!!!!!


And shall from here on in be known as North and South Carolina.. :)
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