Isaac lashing the Keys; an eyewall is building

By: Dr. Jeff Masters , 3:48 PM GMT on August 26, 2012

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Tropical Storm Isaac is steadily organizing as it lashes the Florida Keys with heavy rain and tropical storm-force winds. Sustained winds of 44 mph and 41 mph have been observed at Molasses Reef and Sombrero Key, respectively, this morning. Radar out of Key West shows an increase in spiral banding, and the beginnings of an eyewall. An Air Force Hurricane Hunter aircraft completed its first pass through the center of Isaac near 11:30 am EDT, and did not find the pressure had fallen, or that the peak winds had increased. Infrared and visible satellite loops show that Isaac is a large and increasingly well-organized storm. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow is quite good and increasing to the north, but is lacking elsewhere. Moderate wind shear and dry air to the south are interfering with heavy thunderstorm development on Isaac's south side. Heavy rains from Isaac are lingering over Haiti and the Dominican Republic; flash floods in Haiti from Isaac's torrential rains killed at least four people.


Figure 1. Morning reflectivity image from the Radar out of Key West radar shows the northwest section of an eyewall beginning to form to the southeast of the city.

Latest model runs for Isaac
The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs have diverged significantly, and we can no longer be confident we know where Isaac will make landfall on the Gulf Coast. One camp of models, the UKMET and ECMWF, predict that a trough of low pressure moving across the Southeast U.S. will be strong enough to turn Isaac north to a landfall in the Florida Panhandle. The other set of models, the GFDL, GFS, and HWRF, predict the trough will bypass Isaac, and a ridge of high pressure will build in and force Isaac to a landfall over Louisiana. The official NHC forecast averages out these two extremes, calling for a landfall midway between the two solutions. Odds are, one of the two model solutions will turn out to be the correct one, and the NHC will be forced to make a substantial adjustment in their forecast track to the east or the west. Isaac has the potential to drop torrential rains capable of causing serious flooding and drought relief over the South. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over Southeast Louisiana, where it predicts Isaac will make landfall. The ECMWF model, however, these heavy rains will fall more over the Florida Panhandle, Alabama, and Georgia.


Figure 2. A hurricane forecaster's dilemma: which set of models is correct? The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs have diverged significantly. Our two top models--the GFS and ECMWF--have 72-hour forecasts that are about 350 miles apart. The ECMWF forecast is not shown here, but lies just to the west of the UKMET forecast (white line.)

Intensity forecast for Isaac
Isaac survived passage over Hispaniola and Cuba relatively intact. It's large size aided this. Isaac is over very warm waters of 31°C (88°F) with high total heat content in the Florida Straits, but is encountering moderate wind shear of 10 - 15 knots due to upper-level winds out of the southwest. This shear is predicted to relax to the light range tonight as an upper-level anticyclone becomes established over the storm. This should allow for more substantial intensification after Isaac passes the Florida Keys. However, the total heat content of the ocean decreases for Isaac Monday morning as it encounters a relatively cool ocean eddy in the Southeast Gulf of Mexico. If Isaac takes a more westerly track, passing due south of the Central Louisiana coast, the storm will encounter a modest warm eddy, which would aid intensification. The intensify forecasts from the various models are very divergent. The latest 06Z (2 am EDT) run of the GFDL model keeps Isaac as a strong tropical storm until landfall in Louisiana. Isaac will undergo rapid intensification into a Category 3 hurricane as it hits New Orleans, says the latest 06Z (2 am EDT) run of the HWRF model. The ECMWF model has Isaac as a strong Category 2 storm with a central pressure near 950 mb as it hits near the Alabama/Florida border.

Comparing Isaac with Ike of 2008
The current situation with Isaac is similar in some ways to that of Hurricane Ike of 2008. Ike spent considerable time over Cuba, weakening from a Category 4 to a Category 1 storm. The storm couldn't put its energy into building a strong inner core, but it was able to build up its outer rainbands that were over very warm waters. This resulted in a major expansion of its wind field, with tropical storm-force winds extending out 275 miles from the center at one point. Ike was able to intensify into a Category 2 storm on its path towards Texas, and had an unusually low pressure for a Cat 2 storm with 100 mph winds--944 mb. That's a central pressure more typical of a Category 3 storm, but Ike could only manage Category 2 winds, since it had such a large chunk of the atmosphere to keep spinning. With Isaac's TS winds already extending out to 205 miles, maybe we'll see another Ike-type situation as it intensifies--the storm will have an unusually low pressure in order to keep a huge wind field spinning, but never make it above Category 2, since it will take so long to spin up such a large wind field.

Storm surge forecast for Isaac
Isaac is a very large 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. The latest 3:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 0 on a scale of 1 to 6, but the destructive potential of Isaacs's storm surge was 2.1 on a scale of 1 to 6. 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.

Invest 97L off the coast of Africa
A tropical wave (Invest 97L) is located about 650 miles west of the Cape Verde Islands off the coast of Africa. 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 50% chance of developing by Tuesday morning. None of the reliable models foresee that 97L will be a threat to the Lesser Antilles Islands. However, both the GFS and ECMWF models predict that a tropical wave that has not yet emerged from the coast of Africa may develop late this week, and potentially take a more westward track towards the Lesser Antilles, arriving around September 2.

Angela Fritz will have a new post here by 6 pm EDT. For the next few days, I plan to do the morning blog post, and Angela will be doing the late afternoon post. I'm in Atlanta to help out The Weather Channel with their on-air hurricane coverage, and will be on either in the afternoon or evening on Sunday, Monday, and Tuesday.

Jeff Masters

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so there is no chance issac comes up the coast of florida..for sure he goes westward, we dont wake up tomorrow and he's close to the coastline, making for the panhandle of florida?
Member Since: August 6, 2011 Posts: 4 Comments: 39711
Quoting Miamigal:
Ook how is it possible that the FIU weather station basically 2 mile from me has a reading of 73 mph, the Westwood station right next to my house has it 12 mph is that station out or is it the individual band have such strong gust at 2 pm?

tornado?
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I think some of Isaac's energy is going into the spreading of the wind field, which is probably why we haven't seen an increase in winds. The circulation covers a much larger area than it did this morning...
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Quoting pipelines:


RI=Rapid intensification.

While I respect your opinion, you are clearly misinformed.

Ah, my mistake with that term. But I don't see rapid intensification happening with the relatively cool water temps and dry air in the Gulf.

Without strong intensification, Isaac will not, cannot turn north and break the ridge without help from some sort of low pressure trough or jet stream, and it isn't going to get that help. This is looking more and more like a eastern TX/western LA Cat1 hurricane with a late Wednesday landfall. Just MHO...
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Ook how is it possible that the FIU weather station basically 2 mile from me has a reading of 73 mph, the Westwood station right next to my house has it 12 mph is that station out or is it the individual band have such strong gust at 2 pm?
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Isaac seems ready to start some steady strengthening now. He's become more clear of Cuba, which was interfering with southern outflow, and this will go in his favor more as he goes along. I am also seeing a NNW wobble at the moment. Just that though, a wobble.

Reposting this from a couple pages back, great radar view of Isaac; Isaac Flanis loop
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Quoting Grothar:


That actually does not matter. I was demonstrating storms from Florida to New Orleans. No two storms follow the same path. Fairness has nothing to do with it. Demonstrative comparison uses a standard deviation from different points. A particular population does not necessarily negate a derived outcome from a starting point, but of any point taken from a particular line of reference.


It matters here though. With a storm like Isaac, you can't separate out the Caribbean portion of the track because it is the steering currents that brought it out of the Caribbean that are still affecting it now, and it is that pattern that makes this path so improbable, even though it is occurring before our very eyes. You can't just look at the storm every day, take the new position, and draw up a new historical map. The past track matters.
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Quoting LaHeaven:
My local Met. is saying he believes Isaac will take a track similar to Hurricane Gustav of 2008? Can anyone tell me if he is even in the right ballpark?

Thanks...Saint Landry Parish,La.


He can't 'take a track' like Gustav, as Gustav entered the GOM coming over the western tip of Cuba. What he probably meant to say would be that Isaac might make landfall in the same location as Gustav.
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Another intense cell about to pass right over my area. I don't have access to the current conditions right now but's it's definitely very gusty. Trees bending over towards the west.
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Quoting ringeaux:


Jindal has a press conference later this afternoon. You can also go to the state site:

http://www.gohsep.la.gov/
I do like Jindal ever since the BP spill
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Quoting Articuno:
So Isaac has weakened? WTF?
Was too close to land and should be organizing now as it gain distance from land.
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Live mobile chaser video from Key West:
Live Mobile Feed - Key West FL
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heavy.one
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758. JeffM
Quoting Articuno:
So Isaac has weakened? WTF?


You've got to take into account that its sandwiched between Cuba and Florida right now. Kind of hard to get going with all the land interaction.

Wait til it moves away from land and it's going to get stronger.
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Quoting Joanie38:
UT OH, I am in Central LA...so far, it isnt looking good...has any of the officials in LA say anything yet????


Jindal has a press conference later this afternoon. You can also go to the state site:

http://www.gohsep.la.gov/
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So much for a bust hurricane season?.I thought we were suppose to have 10-12 T.C's this year?.
Member Since: August 14, 2010 Posts: 10 Comments: 17161
It is amazing how much better the forecast cones have gotten in the last couple of years. Especially since 2005 still seems like yesterday. These are the 11 AM advisories on August 26 2012 and 2005, with the sole purpose being to illustrate the improved forecast cones. The clips from the two discussions, which are exactly seven years apart, illustrate about the same level of model disagreement. Also the lead time on the warnings is notable.




DYNAMICAL
MODELS DEPICT THE CYCLONE MOVING INTO A BREAK IN THE SUBTROPICAL
RIDGE NORTH OF THE GULF COAST IN ABOUT 72 HOURS. HOWEVER THERE IS
A LARGE SPREAD AMONG THE MORE RELIABLE TRACK MODELS. FOR EXAMPLE...
THE ECMWF FORECAST IS ABOUT 300 N MI EAST OF THE GFS SOLUTION AT DAY
3. SINCE THE DYNAMICAL MODEL CONSENSUS HAS SHIFTED WESTWARD...THE
OFFICIAL TRACK FORECAST IS MOVED A BIT TO THE WEST OF THE PREVIOUS
ONE.




THE TIMING OF THE
EROSION OF THE RIDGE AND AN INDUCED NORTHWARD MOTION OF KATRINA IS
THE MAIN DIFFERENCE BETWEEN THE MODELS...WHICH HAS RESULTED IN A
LARGE SPREAD AFTER 48 HOURS. THE NOGAPS AND GFDN MODELS HAVE MADE A
LARGE JUMP TO THE WEST OVER LOUISIANA...WHEREAS THE MAJORITY OF THE
NHC MODELS TAKE KATRINA INLAND OVER THE NORTHEAST GULF COAST.
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Quoting Stormchaser121:

My forecast.

Intensity?? BTW i agree
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Quoting GetReal:
What do we have here coming out???



If Isaac's "coming out", Key West will suit him just fine!

Blowin' 'bout 35 and gusty here at MM24.
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I guess for the most part we were pretty lucky here in south Florida... Please be careful in the north gulf coast in the next few days.......Take care and God Bless
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I doubt Isaac has weakened, considering how much he has organised. Quite suprised the NHC bumped the winds down, even if this recon plane is yet to find 65mph winds.
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Quoting Hurricanes101:


to be fair Gro

none of those came through the Caribbean too


That actually does not matter. I was demonstrating storms from Florida to New Orleans. No two storms follow the same path. Fairness has nothing to do with it. Demonstrative comparison uses a standard deviation from different points. A particular population does not necessarily negate a derived outcome from a starting point, but of any point taken from a particular line of reference.
Member Since: July 17, 2009 Posts: 71 Comments: 26558
Down to 60mph, so it did weaken some. Probably due to continued dry air intrusion and Cuba. Not significantly though, will probably again intensify soon. Pressure did drop 2mb though.
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Approximately 1,190 Palm Beach County residents are without power as Tropical Storm Isaac sweeps across the state, said Max Macon, Florida Power & Light spokesman.
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looks like isaac has been moving due west for some time. um wow
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Quoting Grothar:



There have been numerous storms that have taken this similar path from Florida to New Orleans. This is not unusual.


I saw you talking about this earlier.

It's not the hit on the central gulf coast from the southeast that's unusual. We've had Georges, Betsy, 1947, Elena, you name it. It happens a lot. The uniqueness of this track is how it came out of the Caribbean, over the islands, and now straight NW towards the central gulf coast. None of the storms I mentioned above came from that far south. They all started either over the islands or east of Florida. They did not come up from the south. This kind of a track, if Isaac takes it, has not happened since the beginning of the hurricane database that now extends back to 1842.
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Quoting BiloxiBuoy:
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NEW ORLEANS LA
506 AM CDT SUN AUG 26 2012

...
AT THAT POINT...MOST OF THE RELIABLE MODEL GUIDANCE IS STILL IN
DECENT AGREEMENT WITH THE ECMWF...THE GFS...THE NAM AND THE CMC
ALL IN REASONABLE AGREEMENT WITH THE CANADIAN THE FURTHEST WEST.
THAT IS WHERE THINGS START TO GET A LITTLE...OFF.


LOL, I'll say...
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Quoting TomballTXPride:
11 AM he was at 23.9N
2 PM has him at 23.9N

3 hours of moving due west I guess....from 80.8W to 81.5W.


thats not right, I think they had him too far north at 11am, it is clear on radar he has been moving between WNW and NW since then
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Quoting MiamiFL305:
Key West


Stunning photograph. Ominous.
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Quoting floridaboy14:
pressure down winds down.


Well, that doesn't make musch sense. That will probably be fixed in the Post-Season Report.
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My local Met. is saying he believes Isaac will take a track similar to Hurricane Gustav of 2008? Can anyone tell me if he is even in the right ballpark?

Thanks...Saint Landry Parish,La.
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Quoting MiamiFL305:
Key West


Wow, what a great picture.
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Quoting CybrTeddy:
Quite a bit of misinformation flying around today, so I will try to assist in helping people understand. I hope this doesn't come off rude or anything, but it does get annoying seeing people post that somehow this storm is badly organized.

A) This system appears to be developing a decent central core per recon.
B) The system is not weakening, the last VDM that came in is showing 65mph with a *slightly* higher pressure of 996mb. NHC will keep it 995 for consistency.
C) The system is on a steady course.
D) Dry air intrusion is happening, but is no where near as bad as to too badly inhibit intensification in the future.
E) Inflow is being restricted from Cuba, once we have it clear Cuba intensification will probably resume.
F) Satellite presentation isn't too badly disorganized, noted spiral banding improving.

G) Large pressure gradient tells me that Isaac will probably gain size as well as intensity. Could be another Ike/Alex situation at peak. We will see.
H) The models are extremely consistent with forecast intensity and track, at least the global models. Also the ECMWF has a good initialization , showing a slightly elongated low from the NE-SW, still bombs it out in the Gulf. Current disorganization seems to be only a temporary thing.

I) The NHC has had the intensity forecast pegged for a long time as the models shifted north, they called for most of the time a 75-80mph hurricane hitting Haiti, Isaac hit as a 70mph TS.

There's a lot more technical aspects to it, but I wanted to cover the basics to help people who are new here. Hope this helps.
Thanks Teddy. This is worth repeating.
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So Isaac has weakened? WTF?
Member Since: October 22, 2011 Posts: 0 Comments: 2467
Quoting JeffM:
Isaac just needs time to move further away from Cuba/Florida and then get its huge size moving before it begins to intensify.


Exactemundo he needs space and some moisture to wrap round

Cdo is expanding to west rightnow
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Euro starting, again with good initialization.
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Quoting nrtiwlnvragn:
12Z GFDL



HOUR: .0 LONG: -80.19 LAT: 23.26 MIN PRESS (hPa): 994.54 MAX SURF WIND (KNOTS): 43.89
HOUR: 6.0 LONG: -81.33 LAT: 23.79 MIN PRESS (hPa): 991.93 MAX SURF WIND (KNOTS): 44.39
HOUR: 12.0 LONG: -82.47 LAT: 24.63 MIN PRESS (hPa): 988.16 MAX SURF WIND (KNOTS): 61.25
HOUR: 18.0 LONG: -84.02 LAT: 25.25 MIN PRESS (hPa): 982.72 MAX SURF WIND (KNOTS): 57.28
HOUR: 24.0 LONG: -85.21 LAT: 25.73 MIN PRESS (hPa): 984.32 MAX SURF WIND (KNOTS): 52.83
HOUR: 30.0 LONG: -86.28 LAT: 26.33 MIN PRESS (hPa): 983.16 MAX SURF WIND (KNOTS): 49.98
HOUR: 36.0 LONG: -87.38 LAT: 26.71 MIN PRESS (hPa): 981.39 MAX SURF WIND (KNOTS): 53.10
HOUR: 42.0 LONG: -88.47 LAT: 27.00 MIN PRESS (hPa): 979.09 MAX SURF WIND (KNOTS): 56.28
HOUR: 48.0 LONG: -89.29 LAT: 27.29 MIN PRESS (hPa): 975.40 MAX SURF WIND (KNOTS): 65.58
HOUR: 54.0 LONG: -89.73 LAT: 27.60 MIN PRESS (hPa): 967.77 MAX SURF WIND (KNOTS): 83.72
HOUR: 60.0 LONG: -90.07 LAT: 27.89 MIN PRESS (hPa): 965.85 MAX SURF WIND (KNOTS): 78.54
HOUR: 66.0 LONG: -90.57 LAT: 28.22 MIN PRESS (hPa): 963.71 MAX SURF WIND (KNOTS): 79.95
HOUR: 72.0 LONG: -91.13 LAT: 28.60 MIN PRESS (hPa): 962.48 MAX SURF WIND (KNOTS): 83.59
HOUR: 78.0 LONG: -91.59 LAT: 28.90 MIN PRESS (hPa): 963.61 MAX SURF WIND (KNOTS): 72.71
HOUR: 84.0 LONG: -91.97 LAT: 29.20 MIN PRESS (hPa): 964.21 MAX SURF WIND (KNOTS): 74.29
HOUR: 90.0 LONG: -92.66 LAT: 29.74 MIN PRESS (hPa): 968.42 MAX SURF WIND (KNOTS): 61.25
HOUR: 96.0 LONG: -93.30 LAT: 30.36 MIN PRESS (hPa): 974.61 MAX SURF WIND (KNOTS): 54.08
HOUR:102.0 LONG: -93.97 LAT: 31.20 MIN PRESS (hPa): 980.17 MAX SURF WIND (KNOTS): 44.85
HOUR:108.0 LONG: -94.44 LAT: 32.23 MIN PRESS (hPa): 982.54 MAX SURF WIND (KNOTS): 33.01
HOUR:114.0 LONG: -95.06 LAT: 33.42 MIN PRESS (hPa): 984.70 MAX SURF WIND (KNOTS): 34.28
HOUR:120.0 LONG: -95.26 LAT: 34.80 MIN PRESS (hPa): 987.47 MAX SURF WIND (KNOTS): 32.05
HOUR:126.0 LONG: -95.11 LAT: 36.34 MIN PRESS (hPa): 989.33 MAX SURF WIND (KNOTS): 25.09


Thats pretty far west
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Quoting interstatelover7165:
...ISAAC HAS NOT STRENGTHENED... ...U.S. Warnings in Effect...
2:00 PM EDT Sun Aug 26
Location: 23.9°N 81.5°W
Moving: WNW at 18 mph
Min pressure: 994 mb
Max sustained: 60 mph


Isn't the wind speed lower than last night?
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You would think the NWS would show longitude-latitude coordinates on its Key West radar. Sounds like a no-brainer to me.
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Key West
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Another east caribbean Storm...let's see if he finds more instability in the atlantic than Isaac

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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.