Little change to Isaac, but storm poses a serious storm surge threat

By: Dr. Jeff Masters , 3:25 PM GMT on August 27, 2012

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Tropical Storm Isaac has changed little in strength or organization this morning, as the storm heads northwest at 14 mph towards the Central Gulf Coast. There are two hurricane hunter aircraft in the storm, and Isaac's central pressure held steady at 989 mb at the 8:30 am and 9:15 am center fixes. Top surface winds remain near 65 mph. Infrared and visible satellite loops show that Isaac is a very large storm, but isn't very symmetric. Heavy thunderstorm activity is lacking on the southeast side, where 10 knots of wind shear is driving dry air into the circulation. The center is surrounded by a ring of echoes now, which was not the case on Sunday. However, the echoes are weak. The 8:30 am center report from the Hurricane Hunters reported about half of a ragged eyewall, but the 9:15 am report did not mention any evidence of an eyewall. Isaac will have to form an eyewall in order to intensify significantly.


Figure 1. Morning satellite image of Isaac. Note the lack of heavy thunderstorm activity on the storm's southeast side, where dry air and wind shear are combining to interfere with development.

Isaac's rains
Isaac's heaviest rains have fallen along a swath from the east coast of Florida near West Palm Beach to the center of the state, just south of Orlando. West Palm Beach received 7.57" of rain from Isaac as of 10 am EDT this morning. A trained spotter in Western Boynton Beach reported 10" of rain from midnight to midnight Sunday. Heavy rains from Isaac are lingering over Cuba but have ended in Haiti and the Dominican Republic; flash floods in Haiti from Isaac's torrential rains killed at least nineteen, and two died in the Dominican Republic.


Figure 2. Radar-estimated rainfall from Melbourne, Florida radar shows that Isaac has dumped a wide swath of 3+ inches of rain (orange colors) across the state.

Latest model runs for Isaac
The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs are fairly unified taking Isaac ashore near Southeast Louisiana late Tuesday night or early Wednesday morning, but continue to show major differences in what happens after that. It is still uncertain if a trough of low pressure passing to the north of Isaac will be able to turn the storm due north, as the ECMWF model is predicting, or bypass the storm, allowing a more west-northwest motion into western Louisiana, like the GFS model is predicting. In either case, Isaac is likely to slow down as it approaches the coast, which will increase the damage potential fro m its wind, storm surge, and rains. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over much of Louisiana. The ECMWF model predicts that these heavy rains will fall more over Mississippi. It appears likely that Arkansas will see some heavy rains from Isaac late in the week, which would help put a dent in the exceptional drought conditions there.


Figure 3. Predicted precipitation for the 8-day period from 2 am Monday August 27 to 2 am Tuesday September 4, from the 2 am EDT August 27 run of the GFS model. This model is predicting a wide swath of 5 - 10 inches of rain (orange colors) will affect portions of Louisiana. Additional very heavy rains are predicted for the Midwest, as moisture from Isaac interacts with a cold front. Image credit: NOAA/NCEP.

Intensity forecast for Isaac
Isaac is currently crossing over a relatively cool eddy of water, which will keep intensification slow today. By tonight, the total heat content of the waters increases, which should aid intensification. Low wind shear of 10 knots or less is likely until landfall. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow to the north is not as strong as yesterday, which should also slow intensification today. The models forecast the upper-level outflow should improve by Tuesday. A storm this large will have trouble undergoing rapid intensification, and Isaac's most likely intensity at landfall will be as a Category 1 hurricane, which is what most of the intensity models are forecasting.


Figure 4. Track of Hurricane Gustav of 2008, which followed a path very similar to that of Isaac's predicted path.

Storm surge forecast for Isaac
Storm surge is the primary damage threat from Isaac. Isaac is a huge storm, with tropical storm-force winds that extend out 205 miles from the center. For comparison, Hurricane Katrina at landfall had tropical storm-force winds that extended out 230 miles from its center. Isaac's large size will enable it to set a large area of the ocean into motion, which will generate a large storm surge once the storm approaches land on the Gulf Coast. Water levels at Shell Beach, Louisiana, just east of New Orleans, were already elevated by 1' this morning. Conversely, water levels have fallen by 2' this morning at St. Petersburg, Florida, where strong offshore winds due to Isaac's counter-clockwise circulation have carried water away from the coast. The latest 6:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 0.6 on a scale of 0 to 6, but the destructive potential of Isaacs's storm surge was 2.1 on a scale of 0 to 6. I expect this destructive potential will rise above 3 by time Isaac makes landfall, making Isaac's storm surge similar to that generated by Category 2 Hurricane Gustav of 2008, which followed a path very similar to Isaac's predicted path. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5' to Lake Borgne on the east side of the city. A higher Category 2-scale surge occurred along the south-central coast of Louisiana, and was 12.5' high in Black Bay, forty miles southeast of New Orleans. Recent model runs indicate Isaac may slow down to a forward speed under 5 mph on Tuesday evening and Wednesday morning, close to the coast. If Isaac is just offshore at this time, the coasts of Southeast Louisiana, Mississippi, Alabama, and the Florida Panhandle will be exposed to a large storm surge with battering waves for two high tide cycles. This sort of extending pounding will be capable of delivering more damage than the storm surge of Hurricane Gustav of 2008.

The affect of storm size and angle of approach on storm surge
A 2008 paper by Irish et al., The influence of storm size on hurricane surge, found that large storms like Isaac are capable to delivering a 30% larger storm surge to the coast than a smaller storm with the same maximum wind speeds. The angle with which the storm hit the coast is important, too--a storm moving due north or slightly east of north will deliver a storm surge about 10% greater than a storm moving NNW or NW. Consult our Storm Surge pages for detailed information on what the risk is for the coast. I expect that Isaac's storm surge will be about 30% higher than the typical surge one would expect based on the maximum wind speeds.

Isaac's storm surge will provide the first test of the newly-completed New Orleans levee system upgrade. In the wake of the disastrous storm surge flooding from Hurricane Katrina in 2005, Congress appropriated $14.5 billion to upgrade the New Orleans levee system to withstand a Category 3 hurricane storm surge. Katrina was a Category 3 storm at landfall, but the storm passed far enough to the east of the city that its storm surge was characteristic of a Category 1 - 2 storm at the places where the city's flood walls and levees failed. The new flood defenses were only partially completed in time for the arrival of Hurricane Gustav in 2008, which hit Central Louisiana as a Category 2 storm. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5' to Lake Borgne on the east side of the city. Since that time, the imposing 2-mile long IHNC Flood Barrier has been completed to block off the funnel-shaped pair of canals on the east side of the city. I expect New Orleans' new flood defenses will be able to hold back Isaac's surge, but areas outside the levees are at risk of heavy storm surge damage.


Figure 5. A portion of New Orleans' new $14.5 billion dollar flood defenses, as taken from an Army Corps of Engineers map.

New Orleans flood defense info
Army Corps of Engineers map of the new flood defenses
Army Corps of Engineers video showing the flood defenses
New York Times article, Vast Defenses Now Shielding New Orleans

Invest 97L off the coast of Africa
A tropical wave (Invest 97L) is located in the Middle Atlantic, about 1050 miles west-northwest of the Cape Verde Islands. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 30% chance of developing by Wednesday morning. None of the reliable models foresee that 97L will be a threat to the Lesser Antilles Islands, and high wind shear should begin to tear the disturbance apart on Tuesday.

Another tropical wave that moved off the coast of Africa on Sunday is located just south of the Cape Verde Islands. This disturbance is moving west at 10 - 15 mph, and could arrive in the Lesser Antilles around September 2. Several models develop the disturbance into a tropical depression late this week, and NHC is giving the disturbance a 10% chance of becoming a tropical depression by Wednesday morning.

Angela Fritz will have a new post here by 6 pm EDT. I'm in Atlanta to help out The Weather Channel with their on-air hurricane coverage, and will doing a few 3-minute tropical updates at 30 minutes past the hour between 2:30 - 7:30 pm EDT today.

Jeff Masters

Tropical Storm Isaac (chelina)
View from the north side of the Sunshine Skyway Bridge.
Tropical Storm Isaac
our street at noon today (seflagamma)
Aug 26, 2012: Isaac floods our street really bad. Now nearly 11
our street at noon today

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1409. LargoFl
whoa..boomers and lightning with this storm by me now, looks like its going to be an active night
Member Since: August 6, 2011 Posts: 4 Comments: 36899
Quoting trHUrrIXC5MMX:
would advisories be issued every 2 hours at any point?


Did so with Irene, so it seems highly likely to me.
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1407. kwgirl
Quoting ChaseyChasinStorms:
GOOD AFTERNOON ALL. Slidell, LA here. I am happy to say my family and I are much better prepared this time then we were for Katrina. We will be going to my Aunt's house. She has 3 generators and lives right where the three rivers meet in Covington. I agree Isaac will be a hurricane at 5 o'clock.
With the water that is with this system and possible surge, is it wise to stay at a junction of three rivers?
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1406. NSB207
On FL eastcoast... latest word from NWS is the banding over our area is moving further east, taking the training rain maker out to sea.
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70-72kt flight-level winds at the 850mb level, with very lackluster winds being reported by the SFMR instrument in the southern quadrant. However, when these winds work down towards the surface, watch out.

201700 2517N 08600W 8409 01489 //// +183 //// 270069 070 045 001 05
201730 2517N 08558W 8412 01488 //// +175 //// 272071 072 044 002 01
201800 2517N 08555W 8409 01491 //// +172 //// 271071 071 045 001 01
201830 2517N 08553W 8410 01494 //// +166 //// 270072 072 045 001 01
201900 2517N 08550W 8410 01493 //// +159 //// 268070 072 043 002 01
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The "Master" is on TWC
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Quoting kmanislander:


We have the same information here :-)


Just throwing that out there. I believe some of the people in here are much smarter that is why when a storm forms I prefer to come here for information, models, etc. Which by the way I appreciate All you all do.
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1401. bwi
They updated the vortex message to show the higher flight level winds found on the south and west side.

Product: Air Force Vortex Message (URNT12 KNHC)
Transmitted: 27th day of the month at 20:23Z
Corrected: This observation corrected a previous observation.
Aircraft: Air Force Aircraft (Last 3 digits of the tail number are 306)
Storm Number & Year: 09L in 2012
Storm Name: Isaac (flight in the North Atlantic basin)
Mission Number: 27
Observation Number: 10
A. Time of Center Fix: 27th day of the month at 19:30:40Z
B. Center Fix Coordinates: 26°14'N 85°56'W (26.2333N 85.9333W)
B. Center Fix Location: 245 miles (394 km) to the WSW (241°) from Tampa, FL, USA.
C. Minimum Height at Standard Level: 1,280m (4,199ft) at 850mb
D. Estimated (by SFMR or visually) Maximum Surface Wind: 43kts (~ 49.5mph)
E. Location of the Estimated Maximum Surface Wind: 11 nautical miles (13 statute miles) to the NNE (12°) of center fix
F. Maximum Flight Level Wind Inbound: From 100° at 54kts (From the E at ~ 62.1mph)
G. Location of Maximum Flight Level Wind Inbound: 19 nautical miles (22 statute miles) to the NNE (19°) of center fix
H. Minimum Sea Level Pressure: 982mb (29.00 inHg)
I. Maximum Flight Level Temp & Pressure Altitude Outside Eye: 20°C (68°F) at a pressure alt. of 1,543m (5,062ft)
J. Maximum Flight Level Temp & Pressure Altitude Inside Eye: 22°C (72°F) at a pressure alt. of 1,544m (5,066ft)
K. Dewpoint Temp & Sea Surface Temp (collected at same location as temp inside eye): Not Available
L. Eye Character: Open in the north
M. Eye Shape & Diameter: Circular with a diameter of 20 nautical miles (23 statute miles)
N. Fix Determined By: Penetration, Radar, Wind, Pressure and Temperature
N. Fix Level: 850mb
O. Navigation Fix Accuracy: 0.02 nautical miles
O. Meteorological Accuracy: 1 nautical mile
Remarks Section:
Maximum Wind Outbound: 74kts (~ 85.2mph) in the southwest quadrant at 19:52:30Z
Maximum Flight Level Wind: 74kts (~ 85.2mph) in the southwest quadrant at 19:52:30Z
Member Since: September 4, 2007 Posts: 0 Comments: 1381
1400. NESTORM
Jeff is on TV again
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Looking at Google Earth, Isaac has dropped 3mb's in about an hour and has moved NNE. hmmm
Member Since: June 12, 2011 Posts: 0 Comments: 934
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1397. Hoff511
Hey Everyone, on topic but a little bit of a sidetrack. I know storms, but just experienced in Port Saint Lucie, FL thunder and lightning like never before. It was a weird flash, not a bolt, followed by what could only be described as multiple sonic booms occurring simultaneously. To the point of shaking my patio doors. Not thinking conspiracies or anything crazy just wondering if anyone else is aware of or has experienced this phenomenon. I have been through plenty of storms and rolling thunder but this is beyond that exponentially.
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1396. divdog
Quoting kmanislander:
Hang on. We now have an eye. LOL
Nw track still look good to you ? Iam such an amateur but it looks weird in these sw quad. Shape has changed some. Have no idea what that means or am I just seeing things.good luck all from destin
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1395. Xandra
Quoting MississippiWx:

Levi had nearly 60 pluses last week when we first started tracking Isaac. Lol.

929. Neapolitan 07:13 PM GMT on Augusti 27, 2012 +103

Nea has now over 100 pluses ;)
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1394. snotly
What we here in the vernacular call a tropospheric vortex inducing a Burnuli effect in conjunction with the Coriolis effect due to equalization of pressure caused by the evaporation transport of heat above 850 millibars layer inducing a relative wind speed of or above 3308.1 centimeters per second.

Quoting reedzone:
This is what we people like to call ... A Hurricane
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Quoting NSB207:
Appears dry air is training into the south and east side of the TS...could this inhibit further development in the next few hours?


Thanks to Dry Air we don't have a hurricane yet... or higher making landfall....
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Must be a bunch of kids in here.
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1391. Levi32
Southern side has very quickly strengthened:

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1390. Grothar
Quoting txjac:


That's so sweet. It's awesome to see people come together to help each other out!


The blog is like that. Everybody here would help someone else.
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Quoting ChaseyChasinStorms:
GOOD AFTERNOON ALL. Slidell, LA here. I am happy to say my family and I are much better prepared this time then we were for Katrina. We will be going to my Aunt's house. She has 3 generators and lives right where the three rivers meet in Covington. I agree Isaac will be a hurricane at 5 o'clock.


Hi! I am from Pearl River! sheltering in place. wish I had a generator.
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Quoting MiamiHurricanes09:
Ok, when a dropsonde is dropped into the eye of a cyclone for it to be completely valid the wind speed at the surface should be below 9mph to ensure that they truly dropped it in the center of the cyclone and did not miss. When the winds are above 10mph, a 1 millibar reduction should be subtracted from the MSLP found by the dropsonde since it is unlikely that they hit the true center. So the rule of thumb would be a 1 milliar reduction for every 10mph of wind. (E.g: 2 millibar reduction for 20mph surface winds, 3 millibar reduction for 30mph surface winds, and so forth).

Much appreciated for the info! Going to be a long night for the southern Gulf Coast. It still has a little bit of work to do to seal off that last NE quadrant. So the pressure has dropped from 988 to 982 in less than 12 hours today? I'd say we are going to see at least a 970 mb storm before landfall.
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would advisories be issued every 2 hours at any point?
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Quoting sunlinepr:
Who said Panic?!!!!






whats kill with the off topic photos plzs or risk geting ban


i dont think the admins are playing a round today
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Quoting nolacane2009:
Bob Breck Just said that he has information that the NHC should make this Hurricane Issac by the 5PM Advisory.


We have the same information here :-)
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Quoting RitaEvac:


You park your car in a parking garage this time?


Unfortunately, no garage to park it in. But it's ok. If it gets flooded this time I have GAP insurance baby and I will be able to get my new Camaro. :) LOL
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1383. NSB207
Appears dry air is training into the south and east side of the TS...could this inhibit further development in the next few hours?
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The area of subsidence behind Issac is just off the E coast of Florida and is following Issac towards the NW. It should help put an end of the flooding rains in eastern Florida from the training storms along the feeder band.
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Bob Breck Just said that he has information that the NHC should make this Hurricane Issac by the 5PM Advisory.
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Ok. No, really. What is going on with that Eastern Blob/tail. It's like Issac placed an anchor out there. Is that outflow?
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1378. JasonRE
Quoting hurricanehanna:

I called LCG - not giving them out yet. I'm thinking they are going to wait until they declare a State of Emergency, then they will start offering them. Youngsville is doing so now, so maybe Lafayette will follow shortly. KATC.com is updating info as it comes in so check there too later today.


Thanks, just got a call from my sister saying that as well. I thought they had declared LA a state of emergency already. Oh well...thanks though.
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Hey everyone. Just checking in. It is still raining here in West Boynton Beach in Palm Beach County. Most streets including mine are rivers, the lakes are overflowing and there is flooding just about everywhere. Schools are closed again tomorrow (I am going to go insane another day in the house with the kids). I can't believe 2 days off of school for a tropical storm that we didn't even get a direct hit from. It seems rain/water is the major problem with this storm. I wish all of those in its path good luck.
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1376. Grothar
Quoting NICycloneChaser:


My plan for the week:

Mon: Isaac
Tue: Isaac
Wed:Isaac
Thu: Day off
Fri: Day off
Sat: Kirk?
Sun: Kirk

Expect Kirk to consume much of next week.


Maybe, I wish they would allow us to see the long range EURO models, but they are restricted for some reason.
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Eric Fisher@EricFisherTWCWell as soon as I wrote that, this Vortex message came out. Yes, #Isaac will be a #hurricane at 5p EDT: nhc.noaa.gov/recon.php
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Did somebody say Captain Kirk???

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1373. txjac
Quoting hurricanehanna:

Jason - are you in Lafayette or Lafayette Parish? I can try to find out for you where to get the sandbags.


That's so sweet. It's awesome to see people come together to help each other out!
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1372. airmet3
Quoting STXHurricanes2012:

I meant his forecast yesterday at 5 pm but thats different now! Btw I didn't no mail to him lol


The numbers I quoted were from his 5pm yesterday. You can check the NHC archive to verify if you wish.
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1371. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 36899
Quoting weatherh98:
A lot of kids at school are expecting a strong tropical storm if this is really RI then they will be in for at least a cat 2.

Im thinking a strong cat1 to moderate cat2 peak. I dont think they will just get a strong TS.
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Quoting ChaseyChasinStorms:
GOOD AFTERNOON ALL. Slidell, LA here. I am happy to say my family and I are much better prepared this time then we were for Katrina. We will be going to my Aunt's house. She has 3 generators and lives right where the three rivers meet in Covington. I agree Isaac will be a hurricane at 5 o'clock.


You park your car in a parking garage this time?
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Hurricane Central@twc_hurricanePrelim data from NHC suggest #Isaac not becoming a hurricane at 5p - but that was before the 982mb recon fix, so stay tuned...
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1339. There is a difference in making simple comments, and making derogatory comments that also bring into question someone's ability to do the job. They started out somewhat creative and non-derogatory, and quickly went south...
Member Since: August 3, 2008 Posts: 16 Comments: 5871
Quoting reedzone:
This is what we people like to call ... A Hurricane


But it is still going to have to find a way to get rid of that persistent dry air intrusion before it will strengthen significantly.
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1365. LargoFl
Quoting Levi32:


Nothing, just enhanced outer spiral bands in a higher convergence zone.
..ok ty
Member Since: August 6, 2011 Posts: 4 Comments: 36899
Isaac's southern semicircle is very vigorous; consistent hurricane-force flight-level winds being reported by Recon.
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Levi what are the odds this comes in a little further east of where they are saying? In mobile here, any little shifts east gives us more weather to deal with
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Quoting Grothar:
Coming soon to a PC near you!



whoooaaaaa..... whaddaya call that Gro?
Member Since: September 5, 2007 Posts: 0 Comments: 3481
1360. Grothar
Quoting WalkingInTheSun:


Looks like W Florida storms forming outer bands for much larger size forming on this storm. Is that my imagination or for real?


For real.
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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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