The Steering of Dorian

By: Lee Grenci , 3:54 PM GMT on July 26, 2013

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The 5 A.M. discussion from the National Hurricane Center indicated that Tropical Storm Dorian "lost organization" as it encountered southwesterly wind shear and middle- to upper-tropospheric dry air (one of the traditions I learned from the late John Hope was to never use "he" or "she" to describe a named tropical cyclone). NHC's discussion also focused on low- to middle-tropospheric winds associated with the the Atlantic subtropical high-pressure system (06Z GFS model analysis of 700-mb heights early this morning) as the primary steering currents for Tropical Storm Dorian (see the 06Z GFS model analysis of 700-mb heights and 700-mb streamlines below (larger image). At the time, Dorian was moving to the west-northwest at 17 knots.



The 06Z GFS model analysis of 700-mb heights and 700-mb streamlines on July 26, 2013. 700-mb wind speeds are color-coded in knots. Larger image. Courtesy of Penn State.

When I was a young forecaster (a long, long time ago), I typically looked at mid-tropospheric winds as a proxy for the general movement of tropical cyclones. That's because mid-tropospheric winds serve as a rough approximation for the mean airflow in the troposphere. More specifically, old timers like me looked at the winds between 700 mb and 500 mb at a radius of approximately five to seven degrees latitude from the center of the storm (one degree latitude equals 60 nautical miles). As it turns out, winds in the layer from 700 mb to 500 mb often tend to correlate best with the movement of tropical cyclones (at these radii, environmental winds are essentially unaltered by the circulation associated with the tropical cyclone).

Obviously, my approach as a young forecaster was old school. Nonetheless, my simple method had some merit. Indeed, research has shown that a deep-layer mean flow (between 1000 mb to 100 mb, for example) can be used as a tool to assess steering currents (this technique captures the spirit of my old-school approach).

Subtropical highs are not the only features that steer tropical cyclones. Indeed, mid-latitude systems (500-mb troughs, for example) can also steer tropical cyclones as they move poleward from the Tropics. At times, two tropical cyclones can steer each other, assuming that they're close enough for their circulations to interact (the Fujiwhara effect...a topic for a future blog). Finally, tropical cyclones contribute to their own steering, especially when steering currents are rather weak (the Beta effect, which is fodder for another future blog).


The variation of the steering layers for tropical cyclones with minimum central pressure. Larger image. Courtesy of CIMSS and Dr. Chris Velden.

That's all well and good, Grenci, but why did NHC specifically reference "low- to mid-tropospheric winds in their 5 A.M. discussion today? Experience gained from the careful observations of operational forecasters eventually prompted further research aimed at establishing the connection between the minimum pressure of a tropical cyclone and the corresponding depth of the steering layer. The bar graph above (larger image), which displays the minimum pressure of tropical cyclones versus the depth of their steering layers in the Atlantic basin, supports the notion that the steering layer for a tropical depression is shallower and resides lower in the troposphere. In contrast, the steering layer for strong hurricanes is much deeper. The simple physical connection for you to take away after reading my blog is that a weak tropical cyclone (like Dorian) is usually associated with a shallow vortex. Thus, the mean wind in a correspondingly shallow and low-level layer serves as the steering current. As a general rule, the deeper the vortex, the deeper the layer mean that steers the tropical cyclone.


The 09Z analysis of the streamlines designating the mean wind in the layer from 850 mb to 700 mb on July 26, 2013. Larger image. Courtesy of CIMSS.

To get a better sense for the movement of Dorian, focus your attention on the first layer on the left of the bar graph above (central pressure between 1000 mb and 1010 mb). The steering current for Dorian and other similarly weak tropical storms boils down to the mean wind between 850 mb (roughly 5000 feet) and 700 mb (10000 feet). The 09Z analysis from CIMSS (above; larger image) indicates the streamlines of the mean wind in the layer between 850 mb and 700 mb. Wind speeds are color-coded in knots.

At the other end of the spectrum, note the deep steering layers for strong tropical cyclones whose central pressures are lower than 940 mb or range from 940 mb to 949 mb.

I should point out that these results do not include the impact of the Beta effect on the movement of a tropical cyclone. Moreover, other factors such as season, latitude, easterly versus westerly environmental flow, the rates at which the intensity of tropical cyclones changes with time, etc., probably can skew these results a bit, but, as a general rule, the bar graph above will get you in the ballpark in all the ocean basins.

We've come a long way since I was a young forecaster.

Lee

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Quoting 2021. 62901IL:
I bet you guys anything that dorian will be an open wave at the next advisory.


No, It will still be at 40 with pressure of 1011 mb
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2027. WWPR
Quoting 2001. mikatnight:
Good Morning!

Am I the only one getting bored with these sunrise pics and the Lantana bridge?


I looked forever trying to find what kind of bird this is, but I'm still not sure. Some type of heron maybe? Perhaps a juvenile, dunno. Anybody know?


Dexter's other passion is chasing balls - little action shot here!

Collected 0.6" of rain yesterday in a brief, but intense little downpour, and ended our rainless streak at 4 days. NWS calling for a 60% chance today mostly between 5 & 8 pm. Maybe I'll find a 4x4 and mount that rain gauge today. There's always hope.



Loos like a Juvenile Yellow Crowned Night Heron...but a juvenile for sure.
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Quoting 2019. chrisdscane:



Dorians party about to end

umm not its not
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2025. DDR
Good morning pottery did you get rain last night and this morning?
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2024. LargoFl
GFS has him back in 72 hours
Member Since: August 6, 2011 Posts: 4 Comments: 38423
Quoting 2020. SPLbeater:
Sure is a beautiful mornin. WAVE SPL looks decent...xD


Could try to organize
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2022. Grothar
Coordinates: 18.3N 51.1W 12Z
Coordinates: 18.0N 49.0W 06Z
Coordinates: 17.8N 47.1W 00Z

Wow. Seems to still be moving WNW. Interesting to see what the 18Z will be.
Member Since: July 17, 2009 Posts: 71 Comments: 26117
2021. 62901IL
I bet you guys anything that dorian will be an open wave at the next advisory.
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Sure is a beautiful mornin. WAVE SPL looks decent...xD
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Dorians party about to end
Member Since: July 21, 2010 Posts: 0 Comments: 1163
2018. 7544
looking at where d is now and the high it looks like what ever is left of dorian will wind up in south fla but the ? is will it be strong or weak ?
Member Since: May 6, 2007 Posts: 0 Comments: 6811
Quoting 2009. kmanislander:
Good morning

I just looked at the ASCAT pass from yesterday evening and it shows that Dorian no longer had a closed low at that time.

I wonder why the NHC didnt declassify it as an open wave ?. Was there any discussion about this last night ?

The 5AM discussion mentioned that ASCAT and OSCAT passes suggested that the circulation may no longer be closed, but they resisted declassifying because they wanted to monitor it further before classifying it as a remnant low.
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Quoting 2009. kmanislander:
Good morning

I just looked at the ASCAT pass from yesterday evening and it shows that Dorian no longer had a closed low at that time.

I wonder why the NHC didnt declassify it as an open wave ?. Was there any discussion about this last night ?

yeah but then it closed up and started to pop some convection

I said it would reorganise and well it did do that
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Quoting 2008. StormsWanted:



The spotted bird is a Sandhill Crane, if memory serves. Beautiful photos you take, by the way!


Thanks! I also looked at the sandhill crane...never thought he'd be this hard to ID.
Member Since: October 18, 2005 Posts: 4 Comments: 3052
2014. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 38423
Quoting 2009. kmanislander:
Good morning

I just looked at the ASCAT pass from yesterday evening and it shows that Dorian no longer had a closed low at that time.

I wonder why the NHC didnt declassify it as an open wave ?. Was there any discussion about this last night ?
HI Kman , I am asking myself the same question , on their discussion they think the same way but I believe they are waiting for another pass to confirm and I also believe recon is going to investigate also
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Quoting 2003. pottery:


Striated Heron, I think.


Thanks Pottery. Search results come up with all kinds of images for that name, no wonder I had such a hard time finding it!
Member Since: October 18, 2005 Posts: 4 Comments: 3052
Quoting 2009. kmanislander:
Good morning

I just looked at the ASCAT pass from yesterday evening and it shows that Dorian no longer had a closed low at that time.

I wonder why the NHC didnt declassify it as an open wave ?. Was there any discussion about this last night ?


If I remember, most acknowledged ASCAT but noted visible satellite still showed a robust LLC. OSCAT has had issues as well.
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Quoting 2005. HuracanTaino:
NHC is killing him in 36 hours, saying is his last breath'....lets see what happens...


I see a greater chance of him surviving today than I saw last night...last night looked like its last hurrah
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Good morning

I just looked at the ASCAT pass from yesterday evening and it shows that Dorian no longer had a closed low at that time.

I wonder why the NHC didn't declassify it as an open wave ?. Was there any discussion about this last night ?
Member Since: August 19, 2006 Posts: 0 Comments: 15813
Quoting 2001. mikatnight:
Good Morning!


I looked forever trying to find what kind of bird this is, but I'm still not sure. Some type of heron maybe? Perhaps a juvenile, dunno. Anybody know?




The spotted bird is a Sandhill Crane, if memory serves. Beautiful photos you take, by the way!

No, I take that back, a poster above was right, Striated Heron. Google image comparisons show I misremembered.
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2007. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 38423
Quoting 1962. wunderkidcayman:

yeah I know how you feel I use to be like that until 01 when the M storm passed here the waves where massive like waves you would only see in movies anyway my mother was outside when this this was passin us and the sea came up and took my mother dragged her up our lawn slapped her on the concrete wall surrounding our Front and sides and dragged her towards the sea luckily for her me and a few other jumped in to save her and then more so in 04 with Ivan and well that was self-explanatory what Ivan done to Cayman


That's a terrible picture, thanks God you guys managed to save her.. Hope it'll never happen again.

We did experience severe hurricanes as well... and I would fear a strong Cat 2 and higher... but minimal hurricanes and TS are fun and very exciting.
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Quoting 1976. Hurricanes305:
Sea surface temperature increase to 28C along with moisture around 55W. So its bulking up till it gets there by 2pm. After that it has a chance of strengthening just in time for the 5pm advisory. If this happens Tropical storm watches may be issued for the Northern Hispaniola and/or SE Bahamas (Turks and Caicos) by 11pm tonight granted it survive today. Next 24-48 hours is crucial!
Quoting 1976. Hurricanes305:
Sea surface temperature increase to 28C along with moisture around 55W. So its bulking up till it gets there by 2pm. After that it has a chance of strengthening just in time for the 5pm advisory. If this happens Tropical storm watches may be issued for the Northern Hispaniola and/or SE Bahamas (Turks and Caicos) by 11pm tonight granted it survive today. Next 24-48 hours is crucial!
NHC is killing him in 36 hours, saying is his last breath'....lets see what happens...
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Quoting 1913. washingtonian115:
Let's see what shear has to say about Dorian..Still not optimistic..I wanted it to be like Danielle from 2010.That's all I asked!.

You do realize Danielle was almost a month further into the season then Dorian. The Cape Verde hurricanes don't really ramp up until the meat of the season. The fact that Dorian did form out near the Cape Verdes in July is something that is still a likely harbinger for an active Cape Verde season.
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2003. pottery
Quoting mikatnight:
Good Morning!

Am I the only one getting bored with these sunrise pics and the Lantana bridge?


I looked forever trying to find what kind of bird this is, but I'm still not sure. Some type of heron maybe? Perhaps a juvenile, dunno. Anybody know?


Dexter's other passion is chasing balls - little action shot here!

Collected 0.6" of rain yesterday in a brief, but intense little downpour, and ended our rainless streak at 4 days. NWS calling for a 60% chance today mostly between 5 & 8 pm. Maybe I'll find a 4x4 and mount that rain gauge today. There's always hope.



Striated Heron, I think.
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looks like we are going to keep Dorian a TS at 11:00
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Good Morning!

Am I the only one getting bored with these sunrise pics and the Lantana bridge?


I looked forever trying to find what kind of bird this is, but I'm still not sure. Some type of heron maybe? Perhaps a juvenile, dunno. Anybody know?


Dexter's other passion is chasing balls - little action shot here!

Collected 0.6" of rain yesterday in a brief, but intense little downpour, and ended our rainless streak at 4 days. NWS calling for a 60% chance today mostly between 5 & 8 pm. Maybe I'll find a 4x4 and mount that rain gauge today. There's always hope.

Member Since: October 18, 2005 Posts: 4 Comments: 3052
2000. 7544
morning all looks like dorian had a good dmax last night .still a ts at this hour tia ?

Member Since: May 6, 2007 Posts: 0 Comments: 6811
Hey y'all from Ft Laud. Hot and very humid already. Dorian still hanging in there for the moment. Per NHC they are dissipating the storm but many on here are in the other camp. Reality check moment. Half of us will be right and the other half will be wrong. Time will tell.
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Mid-Level Wind Shear



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Quoting 1988. LargoFl:
good morning,thanks for the breakfast..you did say to send some your way yesterday lol..looks like some good storms out your way huh


Yes and thank you! We'll take what we can get. We had .08 inches last night. The big storms were/are east of us, but rain is rain! Surprisingly, highest gusts were only 17 mph. With the electric out I thought they would have been higher.
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1996. LargoFl
FLOOD ADVISORY
NATIONAL WEATHER SERVICE RALEIGH NC
929 AM EDT SAT JUL 27 2013

NCC067-271630-
/O.NEW.KRAH.FA.Y.0139.130727T1329Z-130727T1630Z/
/00000.N.ER.000000T0000Z.000000T0000Z.000000T0000 Z.OO/
FORSYTH NC-
929 AM EDT SAT JUL 27 2013

THE NATIONAL WEATHER SERVICE IN RALEIGH HAS ISSUED AN

* URBAN AND SMALL STREAM FLOOD ADVISORY FOR HEAVY RAINFALL AND MINOR
FLOODING IN...
FORSYTH COUNTY IN CENTRAL NORTH CAROLINA...

* UNTIL 1230 PM EDT

* AT 922 AM EDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED
THUNDERSTORMS WITH HEAVY RAINFALL OVER FORSYTH COUNTY. DOPPLER
RADAR ESTIMATED UP TO THREE QUARTERS OF AN INCH OF RAIN HAS
FALLEN OVER NORTHWESTERN FORSYTH COUNTY. TRAINING SHOWERS AND
STORMS WILL SLOWLY MOVE EAST THROUGH THE COUNTY THROUGH 1230 PM
RESULTING IN A POSSIBLE 2 TO 3 INCHES OF ADDITIONAL RAINFALL.

* RUNOFF FROM THIS HEAVY RAINFALL MAY CAUSE MINOR FLOODING TO
OCCUR...ESPECIALLY ALONG SMALL STREAMS...IN LOW LYING AREAS...AND
AREAS OF POOR DRAINAGE. SOME LOCATIONS THAT MAY EXPERIENCE MINOR
FLOODING INCLUDE...RURAL HALL...STANLEYVILLE...LEWISVILLE...BELEWS
CREEK...WINSTON-SALEM...AND CLEMMONS.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

EXCESSIVE RUNOFF FROM THIS STORM MAY CAUSE MINOR FLOODING OF SMALL
CREEKS AND STREAMS...URBAN AREAS...HIGHWAYS AND UNDERPASSES...COUNTRY
ROADS...AND LOCATIONS ALONG THE BANKS OF CREEKS AND STREAMS AND OTHER
LOW LYING AREAS.

DO NOT DRIVE YOUR VEHICLE INTO AREAS WHERE THE WATER COVERS THE
ROADWAY. THE WATER DEPTH MAY BE TOO GREAT TO ALLOW YOUR CAR TO CROSS
SAFELY. TURN AROUND...DO NOT DROWN.

PLEASE REPORT FLOODING TO YOUR LOCAL LAW ENFORCEMENT AGENCY. THEY
WILL RELAY YOUR REPORT TO THE NATIONAL WEATHER SERVICE.

&&
Member Since: August 6, 2011 Posts: 4 Comments: 38423
Quoting 1974. wunderkidcayman:

it is decreasing


deserves double LOL LOL and Bah Bah

I know its decreasing. I can read.
But the shear above it is still stronger than where it's currently at.
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1994. LargoFl
folks in North carolina listen to your local warnings,stay safe folks up there.
Member Since: August 6, 2011 Posts: 4 Comments: 38423
1993. pottery
Quoting aislinnpaps:
Good morning, afternoon and evening, everyone. It's 75 degrees with a dew point of 75. We did get some rain last night, and lost electric for five hours. I think it's time for the power companies to once again go along the lines and cut down some trees and branches.

I see Dorian is working to pull himself back together? You have to give him credit for persistence.
br>Breakfast's on the sideboard: beignets smothered in powdered sugar, cheesy grits and shrimp, Corn Waffles With Cajun Vegetables and Shrimp, whole wheat English muffins with poached egg whites, low fat cheese, ham, tomato and sauce, thick slices of fried ham, bacon, cheese danishes, yogurt, fresh fruit, orange, apple and cranberry juice. Enjoy!


URP....

Thank you.
I was planning some tasks today, but after eating that breakfast I think I'll take a Siesta.
Nothing quite like a Siesta before 10: am.....
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good morning. Wow I hate to say it but dorian looks much better this morning.
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1991. LargoFl

THE NATIONAL WEATHER SERVICE IN GREENVILLE-SPARTANBURG HAS ISSUED A

* FLASH FLOOD WARNING FOR...
NORTHWESTERN LINCOLN COUNTY IN THE PIEDMONT OF NORTH CAROLINA...
WESTERN CATAWBA COUNTY IN THE PIEDMONT OF NORTH CAROLINA...

* UNTIL NOON EDT

* AT 856 AM EDT...EMERGENCY MANAGEMENT REPORTED A MAJOR FLASH FLOOD
EVENT UNDERWAY ACROSS MUCH OF CATAWBA COUNTY. NUMEROUS ROADS ARE
IMPASSABLE DUE TO FLOODING OF STREAMS AND POOR DRAINAGE AREAS. AS
MUCH AS 9 INCHES OF RAIN HAS FALLEN ACROSS THE WARNED AREA SINCE
ABOUT 4 AM...AND THUNDERSTORMS WITH VERY HEAVY RAINFALL WILL
CONTINUE THROUGH AT LEAST 10 AM. IN ADDITION...MORE THUNDERSTORMS
ACROSS SOUTH CAROLINA MAY PRODUCE ADDITIONAL HEAVY RAINFALL ACROSS
THE AREA LATE THIS MORNING. ADDITIONAL RAINFALL AMOUNTS OF AN INCH
OR MORE APPEAR LIKELY.

* THE ADDITIONAL HEAVY RAINFALL WILL RESULT IN AN INCREASINGLY
SEVERE FLASH FLOOD EVENT ACROSS WESTERN CATAWBA AND NORTHWEST
LINCOLN COUNTY THIS MORNING. IN ADDITION TO SEVERE URBAN FLOODING
ACROSS PORTIONS OF HICKORY...NUMEROUS STREAMS WILL CONTINUE TO
RISE WELL ABOVE BANKFULL. AFFECTED STREAMS INCLUDE BUT ARE NOT
LIMITED TO...POTT CREEK...JACOB FORK...LITTLE INDIAN CREEK...AND
BUFFALO CREEK. UNLESS YOUR HOME IS THREATENED BY FLOOD WATERS...
RESIDENTS OF WESTERN CATAWBA AND NORTHWEST LINCOLN COUNTY ARE
ADVISED TO STAY IN THEIR HOMES UNTIL FLOOD WATERS RECEDE.
Member Since: August 6, 2011 Posts: 4 Comments: 38423
Quoting 1956. Grothar:
What I find remarkable about Dorian, is that is has never once gone due west. It has maintained a continuous WNW motion the entire time.






Odd run.. like Dorian decided to pull over in the Keys for some conch fritters and Key Lime Pie. Right angles are something we don't see in forecast tracks.
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Quoting 1982. Bluestorm5:
I believe Dorian is the most impressive comeback I've seen 2 years of tracking the storms closely on WU. I'm sure there are some more impressive comebacks in the past 10 years I've been watching hurricanes on TWC, but my memory sucks.


Why the sudden change of heart?
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1988. LargoFl
Quoting 1986. aislinnpaps:
Good morning, afternoon and evening, everyone. It's 75 degrees with a dew point of 75. We did get some rain last night, and lost electric for five hours. I think it's time for the power companies to once again go along the lines and cut down some trees and branches.

I see Dorian is working to pull himself back together? You have to give him credit for persistence.

Breakfast's on the sideboard: beignets smothered in powdered sugar, cheesy grits and shrimp, Corn Waffles With Cajun Vegetables and Shrimp, whole wheat English muffins with poached egg whites, low fat cheese, ham, tomato and sauce, thick slices of fried ham, bacon, cheese danishes, yogurt, fresh fruit, orange, apple and cranberry juice. Enjoy!
good morning,thanks for the breakfast..you did say to send some your way yesterday lol..looks like some good storms out your way huh
Member Since: August 6, 2011 Posts: 4 Comments: 38423
convection is starting to wane down , ULL to its west is still there and IMO will tear this weak system apart , but I still believe Dorian will hold
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Good morning, afternoon and evening, everyone. It's 75 degrees with a dew point of 75. We did get some rain last night, and lost electric for five hours. I think it's time for the power companies to once again go along the lines and cut down some trees and branches.

I see Dorian is working to pull himself back together? You have to give him credit for persistence.

Breakfast's on the sideboard: beignets smothered in powdered sugar, cheesy grits and shrimp, Corn Waffles With Cajun Vegetables and Shrimp, whole wheat English muffins with poached egg whites, low fat cheese, ham, tomato and sauce, thick slices of fried ham, bacon, cheese danishes, yogurt, fresh fruit, orange, apple and cranberry juice. Enjoy!
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He never bothered anyone. Brought good weather to all. He will be missed.
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And with that... I'm out for real. Have fun tracking Dorian.
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1983. ryang
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I believe Dorian is the most impressive comeback I've seen 2 years of tracking the storms closely on WU. I'm sure there are some more impressive comebacks in the past 10 years I've been watching hurricanes on TWC, but my memory sucks.
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The surface low near near NC is more organized than Dorian currently, just lacks strong deep convection.
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20% for me
Quoting 1977. Hurricanes305:


What about Monday :)
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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.