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:
Just learning the lessons, that's all :) I never feel the need for attention or taking my frustration out on others. Also it's reasonable that surface winds is lower than flight winds by some.


Nice to have someone polite and wanting to learn and have reasonable discussions on the blog.
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Interesting. As far north the CMC has been all along it has come south with Isaac and West with Joyce! I dont put too much stock in this model but interesting nonetheless.
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Quoting presslord:
I just don't get the obsession with analogs....


I know... I much prefer digitals.
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Quoting Drakoen:
Good agreement among the 12z models so far.


You are one of the better informed ones here. How sophisticated are any of these models when it comes to dealing with rugged land masses like DR and eastern Cuba?
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So what it look like to me is that this is a whole different storms than what we've seen in the past. I don't think I've ever seen storm going over Haiti and then going west of Florida.
Member Since: August 1, 2011 Posts: 28 Comments: 8041
You forgot Moses Presslord....
Member Since: July 14, 2008 Posts: 1 Comments: 9645
Quoting Drakoen:
Good agreement among the 12z models so far.
havent seen them yet but an agreement on a south florida/keys landsfall or glancing blow
Member Since: July 14, 2009 Posts: 0 Comments: 3028
Quoting DaytonaBeachWatcher:


those models seems quite well aligned now...


Really am amazed how well they have been aligned for several days now. I remember Debby though and how aligned "ALL" the other models were with the exception of one lonely GFS. Could be one lonely ECMWF that has had this pegged all along and not the consensus, just food for thought. Can't completely discount a top model, it has been consistent with it's outlier track.
Member Since: August 19, 2005 Posts: 5 Comments: 5452
Quoting Grothar:

Interesting... tries to run the length of Cuba.
Member Since: August 3, 2008 Posts: 16 Comments: 5890
Quoting Patrap:

The water is warm
But it’s sending me shivers
A baby is born
Crying out for attention

The memories fade
Like looking through a fogged mirror
Decision to decisions are made
And not bought,
But I thought this wouldn’t hurt a lot.
I guess not


Beautifully written
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Quoting NICycloneChaser:


Thanks for not being some ignorant kid who gives me an angry response :) lol
Just learning the lessons, that's all :) I never feel the need for attention or taking my frustration out on others. Also it's reasonable that surface winds is lower than flight winds by some.
Member Since: August 1, 2011 Posts: 28 Comments: 8041
Isaac may not have far to go to reach his intensifying period. From what Isaac looked like at 3AM to what he looks like now is impressive. He's tightened up quite a bit from then and the thunderstorms to his SE have gotten much much closer to being wrapped into his overall circulation. Would be surprised to not see Isaac a 60mph storm by 5PM. Jeff opened my eyes with his statement that even five inches of rain would pose a large challenge to the tent city in Haiti. I was thinking it'd take more than that to make it a life threatening situation. That was not what I wanted to read, but I've certainly learned something there.
Member Since: April 18, 2012 Posts: 0 Comments: 2437
This is the ECMWF. Wonder what the new run will look like.

Member Since: July 17, 2009 Posts: 71 Comments: 26814
Quoting Drakoen:
Good agreement among the 12z models so far.


Hey I just said that, LOL :-P
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Quoting BahaHurican:
Unless the viewer is NTIY...

I certainly don't blame them... this would be a real tourism nightmare for the city government...

You know, after you guys posted about the Debbie related flooding, I started thinking about the possibility of a similar scenario during the RNC... but I never thought it would be a real probability...



Remembering Charley, the NHC thought for sure it was going into Tampa but came in far south making landfall near Charlotte Harbor after lashing the barrier islands. That would REALLY suck if that happened again in more ways than one. One thing is for sure, if it stays on more of a GFS type path the Florida Big Bend may be in for another Debby type scenario considering how much rain has already fallen in that region.
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Quoting Bluestorm5:
Thanks for the tip!


Thanks for not being some ignorant kid who gives me an angry response :) lol
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Quoting presslord:


Jesus, Mary and Joseph!!! I wasn't asking you anything...I was trying to encourage Stormjunkie to post a link to his web page...which has what you were asking for...


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Quoting southfla:


I think we need to wait until the NOAA jet has flown tomorrow and its data has been ingested into the models. The 11pm advisory tomorrow will most likely show the GFS and European models converging on one solution. Then we will have a better idea if this will be trending toward an eastern or western Florida impact. After that the million dollar question will be how the interaction with the islands will tweak that track.


Most definitely as the G-4 Sniff will greatly enhance the Models to solute some consensus downstream.
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Quoting NICycloneChaser:


Not likely. The flight level winds along with that reading were 32 mph, so surface winds are highly unlikely to be higher than that.

It's always worth remembering that SFMR readings are estimations only.
Thanks for the tip!
Member Since: August 1, 2011 Posts: 28 Comments: 8041
Good agreement among the 12z models so far.
Member Since: October 28, 2006 Posts: 57 Comments: 30602
Quoting ProgressivePulse:


Hoping they stay mis-aligned myself.

Be a windy & wet few days for a lot of folks...





those models seems quite well aligned now...
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The water is warm
But it’s sending me shivers
A baby is born
Crying out for attention

The memories fade
Like looking through a fogged mirror
Decision to decisions are made
And not bought,
But I thought this wouldn’t hurt a lot.
I guess not


Member Since: July 3, 2005 Posts: 426 Comments: 129089
Quoting gustavcane:
what do you guys think about the ECMWF track. Jeff Masters posted on this Blog today that the single best-performing model over the past two years has been the ECMWF (European Center model). The GFS model has been a closebr


I think we need to wait until the NOAA jet has flown tomorrow and its data has been ingested into the models. The 11pm advisory tomorrow will most likely show the GFS and European models converging on one solution. Then we will have a better idea if this will be trending toward an eastern or western Florida impact. After that the million dollar question will be how the interaction with the islands will tweak that track.
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Quoting Grothar:
The GFS shifted just a little to the right on the last run.


That's interesting because other models trending off the the East have moved a little to the left (West)Weather channel has it over Key West (close to) as an 80mph storm monday AM
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I like this little guy!


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Thanks pat.
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24 hour

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It's all about timing
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Quoting BahaHurican:


Seems NHC is forecasting Joyce by tonight...

I'm also a little leary of this forecast if something freaky happens with Isaac... wouldn't more west for Isaac = more west for Joyce too?

I think the NHC is right on Joyce. She will pull more WNW due to being stronger than Issac was at this point.
Member Since: August 3, 2008 Posts: 16 Comments: 5890
Quoting wolftribe2009:


No I like to have the models to better get a feeling of what the storm is going to do.

Asking if I am a storm junkie is like asking me if I want a big hurricane? I am an hour from the coast where I live. Why would I want it to come up here?

Ummm...Stormjunkie is a poster on here who could probably provide a link to what you are asking for..
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Quoting Bluestorm5:
40 mph SURFACE winds found in NW part of Isaac.


Not likely. The flight level winds along with that reading were 32 mph, so surface winds are highly unlikely to be higher than that.

It's always worth remembering that SFMR readings are estimations only.
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What a mess.......SHEESH!!!

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Quoting NICycloneChaser:


Half expecting him to move back across the US and redevelop in the north Atlantic.

I would love it if that happened and the NHC just renamed it Ernesto.


Im certain that Bongevine's PLANFALF model puts it on a bus in San Diego, takes it to Mobile where it redevelops and moves SW...back over Mexico. Ernesto is THE KING. Joyce and Pre (Captain) Kirk cant hold a candle to it.

I would also request that when we get Kirk on Sunday, that the folks here refer to it as THE CAPTAIN.

Thank you.m
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ECMWF is running 24 hours out 1004mb moving due west
Member Since: July 25, 2011 Posts: 0 Comments: 1102
757. SLU
Quoting MississippiWx:


Radar doesn't really support that, but I'm not going to give you a definite answer since the center has been very jumpy/ill-defined.



If you look closely you can see 2 well defined areas of rotation. The one near Guadeloupe that you and the NHC have been tracking all day and another one about 120 NNE of Barbados near the burst of deep convection from earlier this morning on the eastern side.

Also, I may be wrong and it could be due to the extreme elongation of the overall circulation.
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Quoting gustavcane:
what do you guys think about the ECMWF track. Jeff Masters posted on this Blog today that 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.


I will put my bets on the GFS for 2012. Euro scrwd the pooch bad on TS Debby in June and was *way* too far west.

Land interaction with DR and Cuba is the big variable here. That could make quite a difference when and where Isaac is eventually able to tap into an energy source and rapidly intensify. GFS currently show this beyond 114 hours.
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Quoting BahaHurican:


Seems NHC is forecasting Joyce by tonight...

I'm also a little leary of this forecast if something freaky happens with Isaac... wouldn't more west for Isaac = more west for Joyce too?


Not necessarily. The only land mass that should have to worry with Joyce is Bermuda. It's early, so things change. Isaac and Joyce could have an interesting interaction down the road if Joyce is strong enough and close enough to Isaac.
Member Since: July 15, 2006 Posts: 17 Comments: 10284
Quoting StormJunkie:


Hey pulse, good to see ya. That is a big IF; but I do agree that if they can get together it would be trouble. I am not sure they will though.


Hoping they stay mis-aligned myself.

Be a windy & wet few days for a lot of folks...



Member Since: August 19, 2005 Posts: 5 Comments: 5452
wolftribe...

The most used model pages can be found on the Quick Links page along with imagery, wind, and buoy info.
Member Since: August 17, 2005 Posts: 26 Comments: 16874
Pressure is likely at 1006 mb. Almost to the center right now.
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Quoting wolftribe2009:


No I like to have the models to better get a feeling of what the storm is going to do.

Asking if I am a storm junkie is like asking me if I want a big hurricane? I am an hour from the coast where I live. Why would I want it to come up here?


I believe presslord was referring to StormJunkie.com

Re-Lax, and enjoy a nice Cold Fresca maybe.
Member Since: July 3, 2005 Posts: 426 Comments: 129089
Quoting BahaHurican:


Seems NHC is forecasting Joyce by tonight...

I'm also a little leary of this forecast if something freaky happens with Isaac... wouldn't more west for Isaac = more west for Joyce too?
and its not even really showing a turn, just NW all the way through.
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Good initialization of Euro... 12z
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Member Since: July 17, 2009 Posts: 71 Comments: 26814

1004mb
24 hrs

Isaac approaching southeast of Puerto Rico
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Quoting wolftribe2009:


No I like to have the models to better get a feeling of what the storm is going to do.

Asking if I am a storm junkie is like asking me if I want a big hurricane? I am an hour from the coast where I live. Why would I want it to come up here?


Jesus, Mary and Joseph!!! I wasn't asking you anything...I was trying to encourage Stormjunkie to post a link to his web page...which has what you were asking for...
Member Since: August 13, 2007 Posts: 0 Comments: 10492

Quoting mojofearless:


Gustav, I'm pretty concerned about the EMCWF, to be honest.


I am very
concerned too Mojofearless. and I know Patrap and anyone that lives on the northern gulf coast does too.

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If the other trough, expected in 72 hours moves down, it will bring Isaac NW and North. Now, the reason for the model spread is only the speed of Isaac. It will depend on where it is at the time, so the NHC have the West coast and East Coast of Florida both covered. I think Isaac may shift back for an East Coast strike. If this scenario is the one, it could be as high as a Cat 2 somewhere in Florida.

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Quoting MississippiWx:


Radar doesn't really support that, but I'm not going to give you a definite answer since the center has been very jumpy/ill-defined.



i see 2 rotations on this...a weak rotation in the old CDO that is now passing the antilles, and a more defined one with little precipitation up in the NE side of the radar image.....
Apparently the NE side is the dominant one...

What is it with 2012 and false CDO/COC correlations?
These storms look all perfectly lined up and then they arent what they seem when the COC falls apart.....
I dont think its trade winds but what is it?
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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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