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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Member Since: July 4, 2005 Posts: 204 Comments: 8898
Quoting BahaHurican:
Wonder if mosquitos care whether the blood they suck or the person who dies from West Nile virus is Democrat or Republican...

There should be some things that all Americans have: health care, retirement security for workers, and public systems [like weather forecasting] that save lives.

Phenomenal: remarkable, extraordinary.

Not a wish, but a comment on how outrageously impossible such a thing would seem until it happened.

I certainly wouldn't wish a major hurricane strike on a flea. I do wish this storm away from ME, but I wouldn't wish it on anybody else just to get it away from me, either.

My point again is that Americans are so polarized about these parties that they sometimes forget the basic underlying humanity of every member of the opposing party. That is wrong.


Right on regarding the safety net for all Americans and good, public weather service.
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THE average of the last hours is WNW or 282 degrees. The last 12 hours is W or 278 degrees. We will see if the WNW trend continues. Although 282 degrees has vey little N lattitude in it.
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Recon plane about to enter Isaac
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Quoting StormJunkie:
That deep convection is very, very close to the center...That said, it is still a very disorganized system and will take time to try and stack.


Yep. The 500mb MLC is displaced some to the south.
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Quoting MississippiWx:
Center placement was fairly uncertain from the NHC as it was difficult to pinpoint with such a ragged core. It seems like it has improved slightly. Plenty of west winds coming out of the Southern islands.



As soon as it's eastern rain band wraps around the north side of the system which is seems to be doing so rather quickly this afternoon, I bet we see both dry and shear subside. When that happens we are going to get a true center fix and possible an attempt at organizing even further. Seems like his circulation is very broad and elongated as someone else has already pointed out earlier today on here.
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Quoting JasonRE:
So in 3 days, it's supposed to turn that much North? Sorry I'm not a big weather person, just anxiously awaiting the storm here in Louisiana. Hoping for a turn this way.......FASCINATED with storms. I wonder what it will do after it hits all of the islands with it's severity/structure. Wonder if something can take off from a piece of this storm and get into the GOM.



So I take it that you don't actually OWN property, a business, or live in New Orleans. I take it you've never been through a hurricane where you found the reeking corpse of your dead neighbor, right? Or lost your home? I'm fascinated with storms, too, but would never in a million years find myself "hoping for a turn this way."
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That deep convection is very, very close to the center...That said, it is still a very disorganized system and will take time to try and stack.

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Since some of you missed my edited post, here is the Martinique radar.

Member Since: July 15, 2006 Posts: 17 Comments: 10284
The current state of Issac reminds me so much of Ernesto...
Member Since: June 26, 2012 Posts: 0 Comments: 297
Quoting MississippiWx:


Directly under the new blowup of convection. Radar from an island proves this.



Martinique Radar:

center around 16N? is it moving due west?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
632. wpb
Quoting wpb:
recon desending lots of work for those onboard trying to located a center.looks ne sw same as this a m
yes
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Quoting LargoFl:
Tampa is getting worried...............Tampa won't hesitate to pull the plug on the Republican National Convention next week if Tropical Storm Isaac threatens the Tampa Bay area as a major storm, mayor Bob Buckhorn told CNN this morning


Assumuing the track forecast holds for 36 hours, this is going to be a mess. It almost seems they have to call it off by Friday. You've got people flying in from all over starting Friday. Also, a lot of folks with businesses will have to get all their biz storm prep done on Friday. Mon am is too late. Folks will need the weekend to get their personal stuff together.
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Member Since: August 6, 2011 Posts: 4 Comments: 40995
629. Relix
Quoting WeatherNerdPR:

D:


Irene's Brother. Have said it for 2 days now haha
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Quoting MississippiWx:


Directly under the new blowup of convection. Radar from an island proves this.


The burst is really expanding now...
What some of you guys thought was a CDO is now a fat spiral band.
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All quadrants are excellent in outflow, convection, and structure... Except for the northeast. Once that fills in, which it is doing now, Isaac will take off.
Member Since: September 3, 2011 Posts: 13 Comments: 3476
Quoting Bluestorm5:
I'm back from *in Levi's voice* annoying appointment...


welcome back
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Quoting Patrap:
Fretting over 10 th's of degrees and a Center fix does not change the overall Motion as its been West around 280 true over the last 6 hours.

16:45 Enhanced Infrared (IR) Imagery (4 km Mercator)




Little north of due west...
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624. wpb
recon desending lots of work for those onboard trying to located a center.looks ne sw same as this a m
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Quoting MississippiWx:


Directly under the new blowup of convection. Radar from an island proves this.


Well radar off Barbados also shows there is a MLC centered around Barbados..
Member Since: June 26, 2012 Posts: 0 Comments: 297
Quoting VAbeachhurricanes:
Off Topic, but it is time to use the heavy duty bug spray

(CNN) -- The recent West Nile virus outbreak is the largest ever seen in the United States, according to new numbers from the Centers for Disease Control and Prevention.
The number of cases so far this year is the highest recorded since the disease was first detected in the United States in 1999. As of August 21, 38 states had reported human infections. The cases reported to the CDC total 1,118, including 41 deaths.
Wonder if mosquitos care whether the blood they suck or the person who dies from West Nile virus is Democrat or Republican...

There should be some things that all Americans have: health care, retirement security for workers, and public systems [like weather forecasting] that save lives.

Quoting gulfbeachgal:


"Devastates?"..."Absolutely phenomenal"?...Well that's just not very nice - never mind your persuasion - to wish devastation is rather extreme don't you think?
Phenomenal: remarkable, extraordinary.

Not a wish, but a comment on how outrageously impossible such a thing would seem until it happened.

I certainly wouldn't wish a major hurricane strike on a flea. I do wish this storm away from ME, but I wouldn't wish it on anybody else just to get it away from me, either.

My point again is that Americans are so polarized about these parties that they sometimes forget the basic underlying humanity of every member of the opposing party. That is wrong.
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Quoting MississippiWx:


Directly under the new blowup of convection. Radar from an island proves this.



That would make more sense.
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Isaac is moving WEST not west north west. seriously people just because its bumping north doesnt mean its moving wnw. center around 16N
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting clwstmchasr:


Me too.


me three
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I'm back from *in Levi's voice* annoying appointment...
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Quoting GetReal:



Interesting loop... Which one is the real dominate LLC of Isaac.... Link

I think the way I am seeing it Issac has several little vortices some south of 15N.. recon will tell us more..!?
Member Since: June 26, 2012 Posts: 0 Comments: 297
Quoting sar2401:


The Day 7 Elipse (or cone) shows a margin of error that stretches all the way from the Central Gulf to Virginia. I can make that kind of forecast all day long, without any maps or models, and have a high probability that Isaac will end up somewhere within that cone. Until the models start coming together at Day 7, this is still a crapshoot. We now resume our normal arguing about who's forecast is correct. :)


I guess we can cancel the Hurricane Forecast Improvement Project and just hire you, you seem to have all the answers.
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Man...when is Isaac going to put on the breaks?!?! I look for him to take the more southern route, following the EURO. Look for the NHC track to shift a little more to the left on each run. It will be interesting to see how much land interaction there is, and how he holds up in the Eastern Caribbean.
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Fretting over 10 th's of degrees and a Center fix does not change the overall Motion as its been West around 280 true over the last 6 hours.

16:45 Enhanced Infrared (IR) Imagery (4 km Mercator)


Member Since: July 3, 2005 Posts: 426 Comments: 129093
Quoting Gearsts:
So where do you have the center at?


Directly under the new blowup of convection. Radar from an island proves this.



Martinique Radar:

Member Since: July 15, 2006 Posts: 17 Comments: 10284
Quoting JasonRE:
So in 3 days, it's supposed to turn that much North? Sorry I'm not a big weather person, just anxiously awaiting the storm here in Louisiana. Hoping for a turn this way.......FASCINATED with storms. I wonder what it will do after it hits all of the islands with it's severity/structure. Wonder if something can take off from a piece of this storm and get into the GOM.

Also wonder if it has any ability to turn West after hitting the islands instead of making such a dramatic turn Northwesterly. Recently (this year) all the storms have either gone West into Mexico or turned back off into the Atlantic.


It is supposed to move w to wnw then turn NW in around 72 hours
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Quoting Patrap:
TS Isaac Rainbow Loop



it's getting that look...
Member Since: September 3, 2011 Posts: 13 Comments: 3476
Quoting WxGeekVA:
So I took time and plotted the last few NHC center fixes on a map, a trend appeared...


D:
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607. AVL
I love reading the comments up in here. I just wish it would quit putting the JAWS theme song in my head...
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Quoting WxGeekVA:




If you say it enough times maybe Isaac will hear it.
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So in 3 days, it's supposed to turn that much North? Sorry I'm not a big weather person, just anxiously awaiting the storm here in Louisiana. Hoping for a turn this way.......FASCINATED with storms. I wonder what it will do after it hits all of the islands with it's severity/structure. Wonder if something can take off from a piece of this storm and get into the GOM.

Also wonder if it has any ability to turn West after hitting the islands instead of making such a dramatic turn Northwesterly. Recently (this year) all the storms have either gone West into Mexico or turned back off into the Atlantic.
Member Since: August 6, 2012 Posts: 0 Comments: 122
TS Isaac Rainbow Loop

Member Since: July 3, 2005 Posts: 426 Comments: 129093
Quoting MississippiWx:
Center placement was fairly uncertain from the NHC as it was difficult to pinpoint with such a ragged core. It seems like it has improved slightly. Plenty of west winds coming out of the Southern islands.

So where do you have the center at?
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Quoting clwstmchasr:


It would be terrible for the area if Isaac hits and they are forced to cancel the RNC. All along, I've been thinking East of FL and that the Carolina's are at risk. There is still a chance that Isaac could miss FL to the East but it is becoming less likely. All of FL is at risk right now.
yes the bay area was counting on this convention to boost its economy etc..yes i hope it misses us also, not a good time to be reparing the house etc, for me anyway
Member Since: August 6, 2011 Posts: 4 Comments: 40995



Interesting loop... Which one is the real dominate LLC of Isaac.... Link


As already pointed out the more dominate LLC may indeed be at 15N AND 60w.
Member Since: July 4, 2005 Posts: 204 Comments: 8898
Quoting popartpete:
I also think Stephanie Abrams is way dumb, and is eye-candy only.


When ya look like that...ya don't have to be smart...
Member Since: August 13, 2007 Posts: 0 Comments: 10492
Quoting SykKid:
isaac look horrible lmao

Whats so funny about that? Give it time!
Member Since: June 4, 2012 Posts: 0 Comments: 767
Quoting GetReal:


Very rarely, if ever, have I downcast a system. Currently I have to downcast Isaac. This system is poorly organized by any standard. The LLC, located on 60W near 15.3N is clearly exposed and fighting to retain TS status. IMO it will take Isaac to slow down to a more reasonable speed (15mph) in order to literally come together. I do not see this occurring for at least another 48 hours.


I'm in your camp. Take the dry air away and Isaac will still have to slow down to get vertically stacked.
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Quoting LargoFl:
OK GUYS..WHY DO THEY HAVE TWO WIND CONES UP
Because one is for 10L
Member Since: August 3, 2006 Posts: 5 Comments: 672
Center placement was fairly uncertain from the NHC as it was difficult to pinpoint with such a ragged core. It seems like it has improved slightly. Plenty of west winds coming out of the Southern islands.

Member Since: July 15, 2006 Posts: 17 Comments: 10284
Quoting weathermanwannabe:
There is no telling exactly what the storm will do intensity wise after leaving Cuba and a close call between the general a/b/c scenario on the table right now (just West of FL in the Gulf-Into South Fl-on the Eastern Side of Florida). All folks in Florida need to prepare for this storm because of the huge size/impacts.

Post-Florida,there is going to be some last minute scrambling by lots of folks, on either side of the equation (from Gulf Side to Eastern Seaboard), depending on what happens on Monday with the updated 3-Day track if the storm stays offshore and does not make initial Florida landfall right after crossing Cuba.
yeah probably up the eastern seaboard not too many are paying attention to this storm..oh thats a florida storm
Member Since: August 6, 2011 Posts: 4 Comments: 40995

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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.