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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In Ike's case I was quite worried 5 days out that South Florida was going to take a beating... but it went SW and we ended up with beautful weather as Ike passed to our south.
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The RGB loop show's the Improving overall organization of TS Isaac.

Member Since: July 3, 2005 Posts: 427 Comments: 129424
240. wxmod
France to England air traffic
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Quoting Patrap:
Its only Afternoon in Fla.

: P

Not even there quite yet! :P

*EDIT: Just noticed you corrected yourself.
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Isaac will have a hard time holding together as it passes over Hispaniola as well as all of the dry air to the north.
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96 hr:

Member Since: September 6, 2007 Posts: 0 Comments: 6697
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Its only Afternoon in Fla...,well, in 2 Minutes

: P
Member Since: July 3, 2005 Posts: 427 Comments: 129424
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232. yoboi
Quoting matilda101:
Have any of you noticed how much smaller the 5 day cone of uncertainly has been shrunked since Ike came thru a couple of years ago...looks like a about a 1/3 smaller.... NHC said that their ability of forecasting that far out has improved remarkably!


did not do so well with debby....
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Quoting BahaHurican:
I'm sure you already saved a blank one so you don't have to go through all that work again... ;o)


Yeah I did... I just cropped the one I already finished :)
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Good afternoon.

Blog update:

Tropical Tidbit for Wednesday, August 22nd, with Video
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217. emcf30

That's 4 days in a row now we have seen the Tampa Strike.
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Quoting NICycloneChaser:


*Facepalm*


LOL!!
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84:

Member Since: September 6, 2007 Posts: 0 Comments: 6697
Quoting hurricanehanna:

Let's not forget the OMGID model (Oh my God I'm doomed)

LOL
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Quoting Patrap:
TS Isaac FunkTop Loop



Funky!
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Quoting jeffs713:
It may just be me, but I'm waiting for the FLUSH model, and the PHMH (Please Hit My House) model.

Let's not forget the OMGID model (Oh my God I'm doomed)
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Quoting Saltydogbwi1:
One has to remember this is a very large system and we shouldn't just focus on where the center is going to pass. It could be affecting people 200 miles either side of the center (more so the north side than south) down the road if it strengthens significantly.

Absolutely !
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Dominican Republic, Isaac comes to you:
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Quoting Patrap:
Those personal Track forecast's and other Formatted looking Tracks are best suited for a Personal Blog as posting them here is like, well kinda distracting and confusing to the general Lurker.

My opine only.

I am differing in opinion, as I very much enjoy looking at each other forecasts in a place where they can be discussed civilly. However, I also think that when they do get posted, a note should be made that it is their own personal forecast and by no means official.

Just how I see it.
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Have any of you noticed how much smaller the 5 day cone of uncertainly has been shrunked since Ike came thru a couple of years ago...looks like a about a 1/3 smaller.... NHC said that their ability of forecasting that far out has improved remarkably!
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78hrs

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SFC MAP
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Quoting Patrap:
TS Isaac FunkTop Loop


Nice burst starting on the eastern side.
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@78HR... following NHCs track:

Member Since: August 14, 2008 Posts: 4 Comments: 5038
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One has to remember this is a very large system and we shouldn't just focus on where the center is going to pass. It could be affecting people 200 miles either side of the center (more so the north side than south) down the road if it strengthens significantly.
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Quoting WxGeekVA:
WxGeekVA forecast... From last blog, fixed to make it not look much like an official track:



UNOFFICIAL
I'm sure you already saved a blank one so you don't have to go through all that work again... ;o)
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Quoting WPBHurricane05:


6Z 57 hours is 51 hours in 12Z…


Yeah, watching Isaac is just driving me to insanity :)
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go watch the local news now.......
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HH should be heading out in 7 min or so.
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Ya keep watching them Run's frame by frame ya liable to drive right off the Pier into the Atlantic.
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Quoting VAbeachhurricanes:


You gotta compare it to 6 hours earlier in the run, as the model started 6 hours earlier :)


*Facepalm*
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Quoting NICycloneChaser:


I disagree, it's ever so slightly faster thus far.

6Z at 57 hours:



12Z at 57 hours:



You gotta compare it to 6 hours earlier in the run, as the model started 6 hours earlier :)
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Quoting NICycloneChaser:


I disagree, it's ever so slightly faster thus far.

6Z at 57 hours:



12Z at 57 hours:



6Z 57 hours is 51 hours in 12Z…
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Quoting BahaHurican:
The problem with just about any Florida strike, as others have said, is just that it's so narrow. Tampa rarely gets direct hurricane strikes, but "crossover" strikes from the east coast are fairly numerous. Fortunately the danger is normally considerably less that way...

Even if they do believe God is punishing them, would it change their future behaviour any? I think people who are set on doing what they are set on doing will interpret any comments made by God through natural phenomena to suit their own desires.

Besides God only punishes other people this way... when it happens to you, the devil did it...

Ditto my comments above.

I must say, however, it would be absolutely phenomenal if Isaac devastates the RNC in Tampa next week, and Joyce does the same in Charlotte the following week...

Not a laughing matter, but I wonder if it would be enough to make Americans put politics aside long enough to work together for their common good?

From your mouth to God's ear Baha.
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Quoting Relix:
Convection blowing up over the actual COC?


No, that is a new band developing on its east and southeast side.
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69 hrs.

Trough in texas, Isaac over Haiti
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195. FOREX
Quoting txwcc:
GFS also suggesting it slows down a bit too...


I really enjoy reading your posts and learn a lot from them. I believe in the end Isaac will make landfall at Mobile Bay after crossing Western Cuba.
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Quoting Relix:
Convection blowing up over the actual COC?


Is having some success this time at getting a bit more convection on its E and soon NE quadrant.
Member Since: August 14, 2008 Posts: 4 Comments: 5038
Quoting Patrap:
A Large Hurricane dosen't have to strike the Tampa Convention site for a impact.

It happened 4 years ago almost ,and for the life of me I dont know why Jeff Masters hasn't made that correlation by now.

History is a fine teacher, as Terrance McKenna has showed us.

Gustav prompts major changes to RNC agenda
JOHN MCCAIN


August 31, 2008


The Republican National Convention will cut back most of its activities Monday because of Hurricane Gustav, Sen. John McCain said Sunday.

"This is a time when we have to do away with our party politics and we have to act as Americans. We have to join the 300 million other Americans on behalf of our fellow citizens. It's a time for action. So, we're going to suspend most of our activities tomorrow except for those absolutely necessary," said McCain, speaking from St. Louis, Missouri.
Campaign manager Rick Davis said Republicans would meet in an abbreviated fashion, conducting only what was necessary to constitute a convention, such as calling the convention to order, receiving a report from the credentials committee and adopting the party platform.

"Tomorrow's program will be business only and will refrain from any political rhetoric that would be traditional in an opening session of a convention," he said.

The convention will begin at its regularly scheduled time, 3 p.m. central time, and adjourn around 5:00 or 5:30, he said. iReport.com: Share your view of the RNC convention

The GOP's four-day program was originally scheduled to begin Monday in St. Paul, Minnesota. Davis said the rest of the week's schedule would be determined on a day-by-day basis.

Earlier Sunday, McCain said it wouldn't be appropriate to hold a political celebration during the storm. Learn more about Gustav

"We must redirect our efforts from the really celebratory event of the nomination of president and vice president of our party to acting as all Americans," he said, adding that it was likely the event would change into a "call to the nation for action."

"I pledge that tomorrow night, and if necessary, throughout our convention ... to act as Americans, not Republicans, because America needs us now no matter whether we are Republican or Democrat,'' he said.

McCain will attend some small events Monday in Pennsylvania, a senior aide told CNN.

The aide, though, noted that McCain doesn't "have a really busy day" as he keeps a close eye on Hurricane Gustav. The senior aide also said the campaign asked the state parties "to be all very respectful" given the situation and to help raise money for various relief agencies.

Also on Sunday, the White House announced President Bush and Vice President Dick Cheney would not be in attendance.



Patrap makes a great point. While it's too early to tell if Isaac will directly effect the RNC, it's almost certain now to have an indirect effect.

If Isaac heads toward Florida as CAT 1 or greater, the RNC will likely scale back activities. Current path puts Isaac in or near Tampa by Tuesday. Only a significant shift to the East in projected path in the next 48 hours would spare Florida and the RNC.
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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.

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