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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just so you know..the Euro starts running around 1:45ish est..so if you got to take a bathroom break or eat lunch, do it now..LOL..Im out till later..
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Quoting WxLogic:


Straight for FSU... pretty sure the Met department will have a field day if that verifies.


Wants to make Drak feel at home :)
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Quoting islander101010:
storm.could.be.as.big.as.the.cone

yes.it.could.but.not.very.likely.
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Quoting WeatherNerdPR:

It isn't a CDO. Well, it probably was at one point, but it quickly became displaced from the center. The center is closer to here, due East of Guadalupe:
Looks like the outflow front from the ball of convection on the east of coc may have finally sealed the center from the drier air filtering in from the NE.
This has allowed some convection to fire around the center.

Will be interesting to watch trends these next few hrs...
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Member Since: August 6, 2011 Posts: 4 Comments: 36929
12Z GFS still wants to visit the RNC by way of track up the SW coast.
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Not expecting much difference on the recon, expecting a 45-50mph TS at most. Be rather surprised if otherwise.
Member Since: July 8, 2005 Posts: 259 Comments: 23590
- From the 1 p.m. ZNS report in Nassau.

Deteriorating conditions are expected in the SE Bahamas on Friday night / Saturday morning. Major rainfall event especially for Inagua, with 50mph + winds, is expected. Sat / Sun should see TS conditions spread into the Central and NW Bahamas respectively.

NEMA [our emergency agency] is on standby / alert mode. Defence Force is doing their prehurricane checks and planning meetings today.

Well, it does seem like the government agencies are not in denial...
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Quoting nrtiwlnvragn:


Straight for FSU... pretty sure the Met department will have a field day if that verifies.
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Quoting WPBHurricane05:


Looks the same as the 12Z NOGAPS.

If EURO shifts a little east we'll begin to have a pretty nice model consensus.


But that would make it too easy for us : (
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Quoting MississippiBoy2:
Well Pat what's your thoughts on ole Issac?



Any Large CV TS Storm that has plenty of untapped Potential SST wise downstream,..warrant's ones ultimate respect.

I don't forecast as I am not a met.

But chances are increasing that a CONUS Bound Hurricane may have travel plans is what I will offer.




Member Since: July 3, 2005 Posts: 421 Comments: 127592
Member Since: July 15, 2006 Posts: 169 Comments: 53296
Quoting nrtiwlnvragn:
my guess is this IS what is going to happen
Member Since: August 6, 2011 Posts: 4 Comments: 36929
Quoting Clearwater1:
Looking at Master's model accuracy chart, the cmc is one of the worse performers.


Yes in general it is the worst, only better than the NOGAPS lol.
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Stu Ostro of TWC

Isaac has a huge sphere of influence. If it maintains its very large circulation, that can result in effects across a much larger area than would be the case with a small circulation (such as Helene was).

But how strong Isaac gets will be important in determining how significant the impacts are. (That is for wind/surge/waves; rainfall can be big regardless of how “intense” a tropical cyclone is.)

Below are a current satellite image of Isaac, and one of Allen in 1980 as it came through the Islands. Similar in overall size and appearance … but Isaac is barely a tropical storm at the moment, whereas Allen was already a Category 4 hurricane when it entered the Caribbean ...

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Quoting ncstorm:
12z CMC..hello Florida



Looks the same as the 12Z NOGAPS.

If EURO shifts a little east we'll begin to have a pretty nice model consensus.
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Quoting catastropheadjuster:



could he get as bad as Ike,Wilma,Ivan,Rita or Katrina. I mean could it have enough of time? I'm not wishing this or anything just wondering.

sheri
If Isaac makes it into the Gulf of Mexico, there will be a serious threat of a large and powerful hurricane making landfall on a populated area.
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Quoting moonlightcowboy:


Especially with that forward speed. Ernesto - rinse, repeat.


Yep, extremely reminiscent of Ernesto. That is the analogy I keep coming up with, and can't find a reason it will change. That said, once it makes the turn and slows, if it happens to find a little time over the Gulf or over the Gulf Stream east of Fla, I guess it might be able to pull it's act together.
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TS Isaac RGB Loop

Note the Cluster Blossoming

Member Since: July 3, 2005 Posts: 421 Comments: 127592
Quoting Clearwater1:
Looking at Master's model accuracy chart, the cmc is one of the worse performers.

,and Euro the best :)
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Member Since: August 6, 2011 Posts: 4 Comments: 36929
Quoting Patrap:
Well Pat what's your thoughts on ole Issac?
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Quoting Clearwater1:
Tells me that they (speculators) will leverage the storm if it gulf is merely mentioned. Good excuse to raise the already overprice oil.


They are making bets with real money. If their bet doesn't pan out, they lose real money. They can't simply "leverage" a storm, their bet has to come true to pay off. Unlike us, they have skin in the game. Ignore it at your peril.
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Problems for Haiti will not just be the immediate affects of wind, rain, flooding and possible mudslides.
Down the line, the potential for an increase in water borne diseases could be devastating - cholera is already
a problem.

Speaking with various Aid Agency colleagues there - they are ALL taking it very seriously - some facilities, such as the MSF hospital, are as robust as one could hope for in Haiti.
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Quoting Levi32:


CMC is correcting its poleward bias now as it usually does after a storm develops, but it is still the farthest east of the globals.
Looking at Master's model accuracy chart, the cmc is one of the worse performers.
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As far south as that trough is sinking, for anyone who thinks of a high possibility that Isaac heads west of Florida is a little out of touch with the current conditions. East of Florida is my call with him. Again i think the GFS is under doing strength. The stronger the storm is the more influence that trough will have. Also the flow is definitively to the NE keeping that high across the continental from blocking Isaac. Watch out East Coast Florida to the Carolinas.
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Quoting Drakoen:


Farther west. Sometimes the models have to be forced into reality.


LOL...
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GFS keeps predicting the trough to come later and later, which is more in favor with the Euro. Shall the trend continue?
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Quoting Kowaliga:


Then that would suggest its still getting NE shear (?)

It had(has?) a displaced anticyclone. Seems to be recovering though, as convection is bursting over the center again.
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And we're off....this image will update all aircraft wind observations during this mission into Isaac.

More obs here if you like tracking the plane and its findings.

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Of course the TWC is covering this like every two seconds.
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Quoting StormJunkie:


That is one of them...The low level. Looks like the mid might be 30-50 miles ESE of there? One jacked up system that will have an extremely hard time getting stacked.


Especially with that forward speed. Ernesto - rinse, repeat.
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Member Since: August 6, 2011 Posts: 4 Comments: 36929
storm.could.be.as.big.as.the.cone
Member Since: September 11, 2010 Posts: 1 Comments: 4344
This is going to get interesting
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Quoting Drakoen:


Farther west. Sometimes the models have to be forced into reality.


CMC is correcting its poleward bias now as it usually does after a storm develops, but it is still the farthest east of the globals.
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Quoting AllStar17:
Where I see the center:


That is one of them...The low level. Looks like the mid might be 30-50 miles ESE of there? One jacked up system that will have an extremely hard time getting stacked.
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Quoting sar2401:
Just another data point from the CME (Chicago Mercantile Exchange) model. Both oil and gas have reversed their downward trend and are now up sharply. This tells me the met consultants that are hired by the oil companies and commodity brokers are starting to look at the HWRF models, that bring Isaac into the East/Central Gulf, more seriously than they were before the last update.
Tells me that they (speculators) will leverage the storm if the gulf is merely mentioned. Good excuse to raise the already overprice oil.
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Quoting hydrus:
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.



could he get as bad as Ike,Wilma,Ivan,Rita or Katrina. I mean could it have enough of time? I'm not wishing this or anything just wondering.

sheri
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Quoting AllStar17:
Where I see the center:



Yep, me too
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Interesting that the 12z GFS has now shifted more southerly with instead of a landfall in the Dominican Republic like in the 06z, it sends it into the western tip of Haiti. Also note the GFS initialized the system rather well, with a slightly elongated 1007mb low.
Member Since: July 8, 2005 Posts: 259 Comments: 23590
I have a question about the models. If the idea is to make the models more accurate in predicting future behavior of storms, wouldn’t it be better to do something other than average predictions?

I’m not a weather expert, but it seems to me that the models predict something about the physics of a storm and the effects large weather systems have on the storms. The tracks aren’t evenly distributed, they fall into groups of tracks corresponding to the assumptions of underlying the models.

An observation: if one uses 2 models for predicted track and only one is correct, doesn’t averaging almost assure inaccurate prediction?

Prediction cones have to be kept large to accommodate all the variations in track until “wrong” models are “dropped” (predicted storm behavior don’t materialize).

Does averaging models result in a fundamental limitation to the accuracy of long term predictions? Wouldn’t it be better to come up with a way to decide which model deserves more confidence for each storm its environment?
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Quoting WeatherNerdPR:

It isn't a CDO. Well, it probably was at one point, but it quickly became displaced from the center. The center is closer to here, due East of Guadalupe:


Then that would suggest its still getting NE shear (?)
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498. Gorty
Wow TD 10 is looking to be another big cyclone.
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Quoting Clearwater1:
No, it's a great post. Get rid of standing water, it a death trap in your own back yard.
yes im afraid that west nile is going to explode here with all the standing water and flooded ponds etc and around me there is alot of standing water in every roadside ditch..this rain really needs to stop....now im waiting for the sinkhole reports to start coming in
Member Since: August 6, 2011 Posts: 4 Comments: 36929
Quoting BahaHurican:
Waiting patiently for the 1 p.m. news, because I want to hear what our local mets are saying about possible impacts here this weekend...


And unfortunately, I think you will get them.
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Quoting WINDSMURF:
I am concerned that Issac may only cross a small portion of eastern Cuba allowing the system to hit the water sooner than expected allowing it more time to get to a category 2 storm before landfall somewhere in the US.
And your concern is a good one. Mine as well
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Quoting StormJunkie:
Man the mid and low level circulations seem to be extremely displaced.

yup agree...850, 700 and 500mb are FAR from aligned!
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I am beginning to think the storm is in the process of moving more southern and westward than the NOAA forecast is showing.
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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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