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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1528. 7544
hmmm looks like dorian has found a sweet spot just in time for dmax ill stick with the eruo on this one and dejevue lol
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1527. sar2401
Quoting NCHurricane2009:

That's true too...I would say its dry air AND upper wind pattern...

When the storm started out....it was fighting out the dry air because its upper anticyclone stayed stacked over the storm surface center. This upper anticyclone created moistening lift over the ocean surface.

Then last night...the upper anticyclone started lagging behind to the east...which caused the moistening lift to lag behind to the east such that dry air worked into the west half of the storm. Why did the upper anticyclone lag? At first the upper anticyclone (backed by Dorian's t-storm latent heat release) was doing well in pushing out the mid-ocean upper vorticity. That upper vorticity then got jammed up against an upper anticyclone near the Lesser Antilles such that Dorian's upper anticylcone apparently could no longer push it out of its way.

Dorian has now left behind its old upper anticyclone...crossed thru the unfavarable upper vorticity..and now is regaining thunderstorms because it is sliding under the outflow of the Lesser Antilles upper anticyclone.

Those easterly trade winds have been a big deal for Dorian, the same as they were for Chantal. You can see the Easterly's tearing p the outflow from Dorian on the NW side. I think the convect we see tonight on the west side of Dorian, even though it's well displaced from the center, is Dorian's last ditch attempt to change direction more to the WSW and escape the Easterly trades.
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Quoting 1511. moonlightcowboy:


Not seeing that on the shear maps but upper level winds reflect it I think. The strong, low-level easterly flow should let up, too, as it reaches the Greater Antilles.


Best way I seek out upper anticyclones (and all the upper wind pattern) is clicking on HDW-H checkbox in these sort of loops:

Link

By clicking on HDW-H and Lat Lon simultaneously...you can spot the upper anticyclone centered at 20N 52.5W. You can also see upper vortex centered at roughly 67.5W 20N. You can also see with HDW-H the upper riding buidling over the southern US/Gulf of Mexico coast. I expect Dorian will slide under the upper anticyclone at 20N 52.5W...its thunderstorms will flare up and build up the upper anticyclone with latent heat release which will start pushing the upper vortex westward. The cavaet is that the upper vortex will then jam up against the southern US/Gulf upper ridge...so no matter how hard Dorain tries it won't succeed in fully pushing out the upper vortex...so I suspect Dorian will re-strenghten...then re-weaken toward the Bahamas while slipping under that upper vortex...
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good to see ya around Storm, I agree on that. It will be interesting to see if the NHC has any recon flights on tap for later this week if he survives
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Quoting 1498. TideWaterWeather:


Thunderstorms popping near DMAX for a 1010 system (beaten by dry air and shear) that looked no more threatening than a summer breeze a couple hours ago is fairly significant, espec because said storm is reaching more favorable conditions as every hour passes
Dorian isn't near the diurnal maximum, it's in it. The reason it isn't significant, to me at least, is because thunderstorm activity flourishing when it's receiving thermal support is extremely common. If it were to somehow miraculously develop banding features and a closed well-defined surface circulation, then I'd be more than happy to rule out significant progress, but a few disorganized thunderstorms that the system didn't fire on it's own isn't significant. It also isn't moving towards a very favorable environment at all; in it's path is even drier mid-level relative humidity values as well as a TUTT, which albeit is weakening at the present time, it is still going to be impeding plenty of westerly upper-level winds on a struggling system. I don't see much of a chance for intensification until it reaches around 70W in about 72 hours or so.

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I know it's kinda odd to say, with Dorian especially, but the CoC is completely covered by thunderstorms at this time. It's really hard to make it out, but my best guess using extrapolation from movement prior to thunderstorm explosions, would put it around 18N 49W, right under the middle of the deepest thunderstorm activity.
Member Since: March 19, 2010 Posts: 0 Comments: 1147
Quoting 1514. FlaStGuy:


Well to be fair, I've been tempted to post earlier, but have always felt I lacked anything intellectual enough to add to the conversation. AND I've notice more newbies this seasona nd felt the time was right lol


Trust me, having anything intellectual to say is not a requirement here...I've been posting for years ;)
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Quoting 1505. tropicfreak:
It may look like a mess, but Dorian is a fighter. Firing some nice thunderstorms covering the surface circulation.


It could re-organize and develop this week end
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Can't see Dorian surviving shear unless it have anticyclone or something.

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Yes, a little convection is starting to build near the center.



BUT, it is all for naught imho...While the shear is dropping ever so slightly, this is not a system that can ward off the 40kts of shear it is headed towards...



His core is too small and feeble to handle what he's about to head in to. Again, there is a reason you can't find a model that develops him.
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1518. JLPR2
Well now, I guess that so far Dorian's comeback is not bad...
Now, all it needs to do is show some organization, as in banding, outflow, spin, etcetera, looking like a blob isn't exactly strengthening. But let's take it one step at the time. XD

Member Since: September 4, 2007 Posts: 7 Comments: 8735
The LLC is now covered up again... will he continue the flare up? I don't think he could have been any closer to to death than he was this evening..
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NHC has their work cut out for them with the almost impossible task of forecasting what Dorian will be like in the next 24-48.
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Quoting 1495. sar2401:

I wish there were computers and satellites around when I was a teen. :-)
I wish I was a teen...
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Quoting 1508. KoritheMan:


Gee, now they're all coming out of the woodwork!

:D


Well to be fair, I've been tempted to post earlier, but have always felt I lacked anything intellectual enough to add to the conversation. AND I've notice more newbies this seasona nd felt the time was right lol
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Quoting 1508. KoritheMan:


Gee, now they're all coming out of the woodwork!

:D

Wait. There's woodwork?

Has anyone checked it for Dorian's convection?
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Storm Report map for the day:

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Quoting 1499. scott39:
i dont see anyone else(besides NCHurricane) chatting about the upper level anticyclone, that Dorian is about to have. This is going to be key for his survival and further developement. If it lines up like its forecasted too....all systems go.


Not seeing that on the shear maps but upper level winds reflect it I think. The strong, low-level easterly flow should let up, too, as it reaches the Greater Antilles.

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and hello tf..how are you.
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Uploaded with ImageShack.com

he gonna try something like this...lol
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Quoting 1503. FlaStGuy:
Long time lurker, really started in high school and once i got to Florida State when I was a Met major. Just wanted to thank you guys for a bunch of knowledge and some good chuckles regarding trolls.

As a long time lurker, and a bit of a hurricane/tropical weather nerd, I am starting to realize (and this is purely my own naive opinion) that while the last couple seasons haven't necessarily carried on the legacy of '04 and '05 per say, the circulations and systems as a whole seem to be becoming a bit more resilient and are fighting inhibiting conditions much better. What I'm getting at is while these storms have been relatively weak, they are chugging along and holding their ground a lot better than what I remember as a kid in the early 2000's and late 90s.

Thanks again for all your "edumacation" Hope to post a bit more over the season.


Gee, now they're all coming out of the woodwork!

:D
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Quoting 1495. sar2401:

I wish there were computers and satellites around when I was a teen. :-)

I wish weather had been invented when I was a teen.
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It may look like a mess, but Dorian is a fighter. Firing some nice thunderstorms covering the surface circulation.

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Quoting 1500. JLPR2:


I guess the possibility exists that it never really opened up much, I remember the HHs being able to close off a circulation for Chantal when it appeared open in ASCAT passes.


I've learned over time that scatterometer data isn't a substitute for in situ observations, be they from drifting buoys, land stations, or reconnaissance aircraft.
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Long time lurker, really started in high school and once i got to Florida State when I was a Met major. Just wanted to thank you guys for a bunch of knowledge and some good chuckles regarding trolls.

As a long time lurker, and a bit of a hurricane/tropical weather nerd, I am starting to realize (and this is purely my own naive opinion) that while the last couple seasons haven't necessarily carried on the legacy of '04 and '05 per say, the circulations and systems as a whole seem to be becoming a bit more resilient and are fighting inhibiting conditions much better. What I'm getting at is while these storms have been relatively weak, they are chugging along and holding their ground a lot better than what I remember as a kid in the early 2000's and late 90s.

Thanks again for all your "edumacation" Hope to post a bit more over the season.
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Quoting 1496. Astrometeor:


You should be good, happens to all of us at one point or the other. I ! Masters once, lol. By accident of course.
thank you kindly..
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Dorian went from solid TS with the southerly inflow channel to not being able to sustain when it closed. It went in all probability down to an open wave and now is forming into an entirely new storm possibly. Some models showing Dorian roaring back to hurricane status down the road. Wonder if their on to something.
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1500. JLPR2
Quoting 1452. KoritheMan:
Since this pass was at 0z, I wonder what the circulation looks like now? Recent satellite imagery suggests that the bursting convection is covering nearly the entire center, albeit lacking banding or significant organization.



I guess the possibility exists that it never really opened up much, I remember the HHs being able to close off a circulation for Chantal when it appeared open in ASCAT passes.
Member Since: September 4, 2007 Posts: 7 Comments: 8735
1499. scott39
i dont see anyone else(besides NCHurricane) chatting about the upper level anticyclone, that Dorian is about to have. This is going to be key for his survival and further developement. If it lines up like its forecasted too....all systems go.
Quoting 1494. MiamiHurricanes09:
Get's an A for effort. But as Kori pointed out, thunderstorm activity flourishing in the middle of the diurnal maximum isn't anything ground breaking.



Thunderstorms popping near DMAX for a 1010 system (beaten by dry air and shear) that looked no more threatening than a summer breeze a couple hours ago is fairly significant, espec because said storm is reaching more favorable conditions as every hour passes
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Quoting 1460. theyoungmet:
Hey guys! Long time member of other weather forums, but I only realized WU had a blog this season. And I got to say, this is the sassiest and most funny weather forum I've ever read. The fights you guys have are beyond hilarious. Anyway, I'm a 16 year old weather enthusiast that's striving for a metrology degree in college. But don't worry I'm not gonna be posting ASCAT 24/7 or wish cast. I'm here to now cast. I hope you guys accept me into your community. This season promises to be one to remember.

Quick Dorian comment: he never really lost his low level vort. and he still has some sort of circulation (no mid level spin and it's a very small core). Dmax should be good to him as he makes his way past the 40 kt shear and dry air. Shear is letting up in front, but he is without an anticyclone and will take the full force of the shear the ULL is producing. He may die at some point soon, but the wave will continue to be steered towards the Bahamas. Keep am eye on him because Dorian is the storm that surprises us when you least expect it. Hopefully those cold tops grow on him tonight over the weak LLC (should be around 1010 mb relative to the heights over the Atlantic currently)


Another teen here (17) and I aspire to also pursue a career in meteorology, hopefully at Virginia Tech (only college in my state that offers a meteorology degree). Great to have another youngster on here that is eager to learn as I am. I am also a nowcaster, and some of my pet peeves on here are when people try to speculate on where it will head when it is thousands of miles from the east coast! I look forward to discussing weather with you in the future as this season is just about getting started. Welcome to Wunderground! And trust me, I don't have much of a sense of humor but being on here and seeing these goofballs make my day.

And in regards to Dorian, I definitely agree. He will likely succumb to the wind shear in the near future. Maybe not this morning given DMAX is around the corner but his future isn't looking too great. But like you said, Dorian is a small system and will react quickly to the changing environment surrounding him, so we will just see what happens as he heads into progressively warmer waters.

Watch it as it heads into the Bahamas though assuming it dies out, for regeneration.
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Quoting 1485. mrsalagranny:
Oh no I tried to plusv your comment and it reported it I think. How do I fix it? I am so sorry.


You should be good, happens to all of us at one point or the other. I ! Masters once, lol. By accident of course.
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1495. sar2401
Quoting DavidHOUTX:
I wish this blog was around when I was a teen :(

I wish there were computers and satellites around when I was a teen. :-)
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Get's an A for effort. But as Kori pointed out, thunderstorm activity flourishing in the middle of the diurnal maximum isn't anything ground breaking.

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Quoting 1484. Doppler22:

Baby steps for Dorian, baby steps :p


While they are baby steps, I would like to be able to give some updated shots of the system. I am not one of those who wants the system to reform, I am in the path of it, should it survive. IE the name I have.
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Quoting 1487. BaltimoreBrian:


No---let em run wild. :)


Yo Brian. I need you to read my comments on Max's blog. I think I scared him a bit.
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Last Coriolis image on Dorian:


Last Geo Vapor Image of Dorian
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I wish this blog was around when I was a teen :(
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1489. scott39
Quoting 1479. NCHurricane2009:

That's true too...I would say its dry air AND upper wind pattern...

When the storm started out....it was fighting out the dry air because its upper anticyclone stayed stacked over the storm surface center. This upper anticyclone created moistening lift over the ocean surface.

Then last night...the upper anticyclone started lagging behind to the east...which caused the moistening lift to lag behind to the east such that dry air worked into the west half of the storm. Why did the upper anticyclone lag? At first the upper anticyclone (backed by Dorian's t-storm latent heat release) was doing well in pushing out the mid-ocean upper vorticity. That upper vorticity then got jammed up against an upper anticyclone near the Lesser Antilles such that Dorian's upper anticylcone apparently could no longer push it out of its way.

Dorian has now left behind its old upper anticyclone...crossed thru the unfavarable upper vorticity..and now is regaining thunderstorms because it is sliding under the outflow of the Lesser Antilles upper anticyclone.
Your spot on with the upper anticycloe both times. The anticyclone is what could save Dorians hide.
theyoungmet:

Just be sure to ignore trolls. There is an ignore list you can use.
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Quoting 1486. Some1Has2BtheRookie:


Well, this old guy is out for the night. .... Brian, are you going to watch the young uns so that they do not knock over any lamps? :)


No---let em run wild. :)
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Quoting 1469. BaltimoreBrian:


East winds to the north

West winds to the south


Well, this old guy is out for the night. .... Brian, are you going to watch the young uns so that they do not knock over any lamps? :)
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Quoting 1438. aislinnpaps:
Off topic: I know this is off topic, but I'm excited about it. Today I was asked to put in the paperwork to help train service dogs for active duty and veterans who have PTSD and TBI. I'd be training them to train their dog, and best of all, there is no cost to the soldier or vet, unlike most programs. It's volunteer and I'm really looking forward to it.

To try to keep on topic, all that rain north of me? Well, I think Pat will see it and it's going to slide right to the east of me. I'll need my hose tomorrow morning to water the garden again.
Oh no I tried to plusv your comment and it reported it I think. How do I fix it? I am so sorry.
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Quoting 1481. plywoodstatenative:
Looking at the latest images from the Navy Site, I do see some form coming back to dorian. Albeit that he is a bit ragged looking. Vapor shows some serious firing near the center and it appears to be forming. Going to post a couple updated shots from Navy.

Baby steps for Dorian, baby steps :p
Member Since: February 13, 2012 Posts: 11 Comments: 3779
Quoting 1473. theyoungmet:
Thanks guys! I feel welcome already :)


Welcome to the blog. I don't always post, but when I do I... nevermind.



Looking forward to hearing more of your input. The more the merrier. Especially when it is youngins that like to learn and express their opinions on weather in a civil way.
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Convergence improving again, good outflow still lacking according to charting but loops show it improving.
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Looking at the latest images from the Navy Site, I do see some form coming back to dorian. Albeit that he is a bit ragged looking. Vapor shows some serious firing near the center and it appears to be forming. Going to post a couple updated shots from Navy.
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Jeez, I half wish the blog dawdled over me like this when I first joined.

Oh, who am I kidding, welcome to WU theyoungmet. Join us in chat sometime, get to know the others who are your age and share your same fascinations. Happy blogging!
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Quoting 1462. scottsvb:


upper winds have nothing todo with Dorian weakening over the past 24hrs.. it's been the dry air

That's true too...I would say its dry air AND upper wind pattern...

When the storm started out....it was fighting out the dry air because its upper anticyclone stayed stacked over the storm surface center. This upper anticyclone created moistening lift over the ocean surface.

Then last night...the upper anticyclone started lagging behind to the east...which caused the moistening lift to lag behind to the east such that dry air worked into the west half of the storm. Why did the upper anticyclone lag? At first the upper anticyclone (backed by Dorian's t-storm latent heat release) was doing well in pushing out the mid-ocean upper vorticity. That upper vorticity then got jammed up against an upper anticyclone near the Lesser Antilles such that Dorian's upper anticylcone apparently could no longer push it out of its way.

Dorian has now left behind its old upper anticyclone...crossed thru the unfavarable upper vorticity..and now is regaining thunderstorms because it is sliding under the outflow of the Lesser Antilles upper anticyclone.
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Quoting 1473. theyoungmet:
Thanks guys! I feel welcome already :)

Yes I agree with Bluestorm5, come to the chat :) it'd be nice to meet another teen on here
Member Since: February 13, 2012 Posts: 11 Comments: 3779

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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.