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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78. SLU
WOW. Right in time for the Cape Verde season.

Member Since: July 13, 2006 Posts: 12 Comments: 4933
Hey everybody , go to Levi's youtube , tidbit before you write off Dorian !
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To Levi , Your welcome!
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It's either Dorians last hurrah in the next 24 hours or a wink, wink, from it that it's gonna bow up and say "let's do this thing" !!!!
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rip Dorian
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Quoting 67. EyEtoEyE:
Thank You Levi , for your Tropical Tidbit , very informative , keep up the good work you do on youtube .


Thanks, I appreciate it.
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Quoting 56. sar2401:

I never knew Lee's blog existed until today. I just read his last entry on the "clash" of cold and warm air and thunderstorms. Great stuff. Lee must have been a prof in the 70's, when I took my met courses. Sounds like one of the "old timers" (sorry, Lee) that I really enjoyed, because we learned about actual weather instead of arcane formulas that were supposed to teach us how to forecast weather. Always seemed weird to me that you could ever get good at forecasting without knowing a bunch of basic things about weather, no matter how good you got with the numbers.


Lee's blog is great. After reading his blog you won't use the weather vocabulary as you did before (as a lay person, at least), lol. Difficult to find different words, though :) ...

BTW, Sar and nrtiwlnvragn, thanks for your "bogus" answers on the last blog concerning the ECMWF (besides Levi); I've just read back. Appreciate this information.
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Quoting 68. masonsnana:
Thank you Lee!! Been here a long time and I too didn't know about your blog. I will be stopping by from time to time now for sure.


Great! I look forward to talking to you about weather.

Best,

Lee
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Quoting 66. TropicalAnalystwx13:
Thanks for the blog, Lee.


Yet it took us months for you to tell us. -_-


He's an excellent retired professor.

You...eh...a kid who knows a lot in NC. lol.
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Quoting 63. Some1Has2BtheRookie:


Here you go, sar2401 - "24hourprof - Lee Grenci. He is one of WU's own. I visit Lee's blogs often. I lurk to learn there because I am too uneducated on the subject to contribute. Lee is extremely knowledgeable and it is well worth the time spent reading his blogs. ... Great job!, Lee.


Mike,

You too are way too kind. I feel like I'm cultivating relationships with readers, and I'm especially pleased whenever you comment because they're always thought-provoking.

Best,

Lee
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Thank you Lee!! Been here a long time and I too didn't know about your blog. I will be stopping by from time to time now for sure.
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Thank You Levi , for your Tropical Tidbit , very informative , keep up the good work you do on youtube .
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Thanks for the blog, Lee.

Quoting 65. Astrometeor:


Name's Nathan.

I'm perfectly fine with you knowing.

Yet it took us months to get you to tell us. -_-
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Quoting 59. 24hourprof:


You're way too kind. But thanks. It's much appreciated.

I feel like I know you already. Would you mind telling me your first name so I can address you personally? If you feel uncomfortable sharing, no worries here.

Best,

Lee


Name's Nathan.

I'm perfectly fine with you knowing.
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Quoting 53. GTstormChaserCaleb:
Enjoy your lunch and take care, probably won't be around in here much this weekend as I have a busy weekend in store. You all have a great day. And once again thank you Lee for this informative and well written blog post, I may need to read it over a few times before I gain an understanding from this insightful post. :)


You're very welcome.

And thanks!

Lee
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Quoting 28. sar2401:
Well, the blog was signed Lee, not Jeff, so I'm not sure who wrote it. Who ever did, it was one of the most educational I've ever read about the use of steering currents at different levels vs. the intensity of a TC. I'm about as dumb as a rock, so it sure would be nice to see some more simple education blogs likes this.


Here you go, sar2401 - "24hourprof - Lee Grenci. He is one of WU's own. I visit Lee's blogs often. I lurk to learn there because I am too uneducated on the subject to contribute. Lee is extremely knowledgeable and it is well worth the time spent reading his blogs. ... Great job!, Lee.
Member Since: August 24, 2010 Posts: 0 Comments: 4737
Very informative blog. I often read you blog when you post. This was very informative with a different slant. So do you think Dorian will............?
Member Since: July 17, 2009 Posts: 69 Comments: 25529
Quoting CaneHunter031472:


Let's hope Dorian doesn't make it to the Gulf. I was under the wrong impression that the waters where cool there.

No, the waters in July are never "cool", the have just been lower than average. The surface temperatures are finally starting to recover, but take a look at the the 60 meter depth chart. There's still a lot of 24-25 degree water there when upwelling occurs if a storm gets in the Gulf.
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Quoting 56. sar2401:

I never knew Lee's blog existed until today. I just read his last entry on the "clash" of cold and warm air and thunderstorms. Great stuff. Lee must have been a prof in the 70's, when I took my met courses. Sounds like one of the "old timers" (sorry, Lee) that I really enjoyed, because we learned about actual weather instead of arcane formulas that were supposed to teach us how to forecast weather. Always seemed weird to me that you could ever get good at forecasting without knowing a bunch of basic things about weather, no matter how good you got with the numbers.


I consider "old timers" an "endearing" term, so no need to apologize.

And I completely agree with your last sentence.

Best,

Lee
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Quoting 57. Astrometeor:
Well, this was a pleasant surprise to wake up to! You should take over Dr. Master's blog more often Lee, some of these bloggers just stay glued here like a fly on wallpaper and never visit anyone else's.

I always enjoy your educational blogs Lee, thanks!


You're way too kind. But thanks. It's much appreciated.

I feel like I know you already. Would you mind telling me your first name so I can address you personally? If you feel uncomfortable sharing, no worries here.

Best,

Lee
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Quoting 54. 62901IL:
Where is Dorian's centre?

well looking at it on visible the best "guesstimate" 18.0N 44.2W moving W
Member Since: June 13, 2009 Posts: 2 Comments: 11239
Well, this was a pleasant surprise to wake up to! You should take over Dr. Master's blog more often Lee, some of these bloggers just stay glued here like a fly on wallpaper and never visit anyone else's.

I always enjoy your educational blogs Lee, thanks!
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Quoting SouthernIllinois:
Okay I wanna set the record straight for you all! For anyone who has NEVER been in Lee's blog is SO MISSING OUT. I have read it several times and have been amazed at the detail, effort, and especially PASSION you he puts in. If you don't learn something new from him well...... lol.

In a nutshell a fabulous fill in for Dr. M!!

Nat :)

I never knew Lee's blog existed until today. I just read his last entry on the "clash" of cold and warm air and thunderstorms. Great stuff. Lee must have been a prof in the 70's, when I took my met courses. Sounds like one of the "old timers" (sorry, Lee) that I really enjoyed, because we learned about actual weather instead of arcane formulas that were supposed to teach us how to forecast weather. Always seemed weird to me that you could ever get good at forecasting without knowing a bunch of basic things about weather, no matter how good you got with the numbers.
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Quoting 42. SPLbeater:
Well I dont know about yal, but im ready for LUNCH! And thanks to the last cold front, it is not so warm outside today which makes sitting on the front porch more enjoyable.

Later, taters.


In for lunch as well and its hotter than all get out here so whomever mentioned the margaritas, I'm in for those also. Happy Friday.
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Where is Dorian's centre?
Member Since: June 14, 2013 Posts: 2 Comments: 1594
Quoting 42. SPLbeater:
Well I dont know about yal, but im ready for LUNCH! And thanks to the last cold front, it is not so warm outside today which makes sitting on the front porch more enjoyable.

Later, taters.
Enjoy your lunch and take care, probably won't be around in here much this weekend as I have a busy weekend in store. You all have a great day. And once again thank you Lee for this informative and well written blog post, I may need to read it over a few times before I gain an understanding from this insightful post. :)
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Let's hope Dorian doesn't make it to the Gulf. I was under the wrong impression that the waters where cool there.
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Quoting 31. GTstormChaserCaleb:
Combination of southwesterly windshear and a fast westward movement due to the ridge is going to make it hard for Dorian to comeback once it passes 60 West. The ridge is strong for this time of the year and we are closing in on August, if this were to be the set-up then there would be a lot of long track Cape-Verde storms. Potentially some dangerous ones too that we will need to keep an eye on.


Could be especially bad for the extreme Northern Leeward Islands and Puerto Rico for this season as well. Seems like over the last couple months most of anything resembling a tropical wave has been drenching those places fairly efficiently.

Also I don't think I have seen the Mid Atlantic that warm ever, especially around Bermuda. I mean it's not august and everything south of 38 degrees North Latitude is at or above 27 degrees Celsius, and the majority of that is at 28 degrees Celsius in the areas south and east.



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WTNT01 KNGU 261500
SUBJ: TROPICAL STORM DORIAN (04L) WARNING NR 010
1. TROPICAL STORM DORIAN (04L) WARNING NR 010
01 ACTIVE TROPICAL CYCLONE IN ATLANTIC
MAX SUSTAINED WINDS BASED ON ONE-MINUTE AVERAGE
WIND RADII VALID OVER OPEN WATER ONLY
---
WARNING POSITION:
261200Z --- NEAR 17.5N 42.4W
MOVEMENT PAST SIX HOURS - 285 DEGREES AT 18 KTS
POSITION ACCURATE TO WITHIN 030 NM
POSITION BASED ON CENTER LOCATED BY SATELLITE
PRESENT WIND DISTRIBUTION:
MAX SUSTAINED WINDS - 045 KT, GUSTS 055 KT
WIND RADII VALID OVER OPEN WATER ONLY
RADIUS OF 034 KT WINDS - 050 NM NORTHEAST QUADRANT
040 NM SOUTHEAST QUADRANT
000 NM SOUTHWEST QUADRANT
040 NM NORTHWEST QUADRANT
REPEAT POSIT: 17.5N 42.4W
---
FORECASTS:
12 HRS, VALID AT:
270000Z --- 18.1N 46.4W
MAX SUSTAINED WINDS - 040 KT, GUSTS 050 KT
WIND RADII VALID OVER OPEN WATER ONLY
RADIUS OF 034 KT WINDS - 050 NM NORTHEAST QUADRANT
040 NM SOUTHEAST QUADRANT
000 NM SOUTHWEST QUADRANT
050 NM NORTHWEST QUADRANT
VECTOR TO 24 HR POSIT: 280 DEG/ 19 KTS
---
24 HRS, VALID AT:
271200Z --- 18.7N 50.3W
MAX SUSTAINED WINDS - 040 KT, GUSTS 050 KT
WIND RADII VALID OVER OPEN WATER ONLY
RADIUS OF 034 KT WINDS - 050 NM NORTHEAST QUADRANT
040 NM SOUTHEAST QUADRANT
000 NM SOUTHWEST QUADRANT
050 NM NORTHWEST QUADRANT
VECTOR TO 36 HR POSIT: 280 DEG/ 19 KTS
---
36 HRS, VALID AT:
280000Z --- 19.2N 54.2W
MAX SUSTAINED WINDS - 040 KT, GUSTS 050 KT
WIND RADII VALID OVER OPEN WATER ONLY
RADIUS OF 034 KT WINDS - 050 NM NORTHEAST QUADRANT
040 NM SOUTHEAST QUADRANT
000 NM SOUTHWEST QUADRANT
050 NM NORTHWEST QUADRANT
VECTOR TO 48 HR POSIT: 280 DEG/ 19 KTS
---
EXTENDED OUTLOOK:
48 HRS, VALID AT:
281200Z --- 19.7N 58.2W
MAX SUSTAINED WINDS - 040 KT, GUSTS 050 KT
WIND RADII VALID OVER OPEN WATER ONLY
RADIUS OF 034 KT WINDS - 060 NM NORTHEAST QUADRANT
040 NM SOUTHEAST QUADRANT
020 NM SOUTHWEST QUADRANT
050 NM NORTHWEST QUADRANT
VECTOR TO 72 HR POSIT: 275 DEG/ 15 KTS
---
72 HRS, VALID AT:
291200Z --- 20.2N 64.4W
MAX SUSTAINED WINDS - 040 KT, GUSTS 050 KT
WIND RADII VALID OVER OPEN WATER ONLY
RADIUS OF 034 KT WINDS - 060 NM NORTHEAST QUADRANT
040 NM SOUTHEAST QUADRANT
020 NM SOUTHWEST QUADRANT
050 NM NORTHWEST QUADRANT
VECTOR TO 96 HR POSIT: 275 DEG/ 13 KTS
---
LONG RANGE OUTLOOK:
---
96 HRS, VALID AT:
301200Z --- 20.5N 70.0W
MAX SUSTAINED WINDS - 045 KT, GUSTS 055 KT
WIND RADII VALID OVER OPEN WATER ONLY
VECTOR TO 120 HR POSIT: 285 DEG/ 12 KTS
---
120 HRS, VALID AT:
311200Z --- 21.5N 75.0W
MAX SUSTAINED WINDS - 045 KT, GUSTS 055 KT
WIND RADII VALID OVER OPEN WATER ONLY
---
REMARKS:
261500Z POSITION NEAR 17.7N 43.4W.
TROPICAL STORM DORIAN (04L), LOCATED APPROXIMATELY 1024 NM
EASTWARD OF BARBADOS, HAS TRACKED WEST-NORTHWESTWARD AT 18 KNOTS
OVER THE PAST SIX HOURS.
261500Z POSTION NEAR 17.7N 43.4W OR APPROX 1432NM E OF ROOSEVELT
ROADS, PUERTO RICO. 12FT SEAS 60NE 45SE 15SW 45NW. NEXT WARNINGS
AT 262100Z, 270300Z, 270900Z AND 271500Z.//
BT
#0001
NNNN
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Quoting 29. Levi32:
From the accumulated precipitation swath of the 12z GFS, you can get a sense of its forecast for Dorian's structure. The beginning of the swatch has a band of precip going off to the northwest associated with the current convective burst east of Dorian's center. Later on, a new band develops to the southwest, separate from the first one. This indicates that the GFS expects a new convective burst closer to Dorian's center later today. Regardless, the model still kills Dorian once he moves north of the greater Antilles.



I guess that means we will have a good weekend lol!!
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Quoting 41. sar2401:

Seriously, that was a great job. I took a few met course in college before I realized the math was going to make my brain explode. ;-) If you're not a teacher, you should be, since that's one the best written and logical non-technical pieces I've seen. Do you have a blog where you plan on or are already have pieces like this? As much as I like and respect Dr. Masters, I'd really like to follow your blog as well.


Thanks so much.

I usually post my blog once or twice each week here at Weather Underground.

I taught for 42 years. I retired July 31, 2012, but I miss teaching folks like you who want to learn. So Dr. Masters graciously gave me an opportunity, and I'm really enjoying it. So many nice people here.

I am really lucky.

Best,

Lee
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FIM-9 shows Flossie over the Big Island of Hawaii and Dorian on the north coast of Hispaniola:

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Quoting RitaEvac:


I know, but it's still all gonna melt regardless, you'll be using the melted pieces of ice to put in your glass in your beverage

Maybe we should get up there now, grab the few pieces of ice that are left, and freeze them. If they can sell water from Fiji (the only place I ever got giardia), we could make a fortune off selling "Genuine North Pole Ice". :-)
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Quoting 43. SouthernIllinois:
Okay I wanna set the record straight for you all! For anyone who has NEVER been in Lee blog is SO MISSING OUT. I have read it several times and have been amazed at the detail, effort, and especially PASSION you he puts in. If you don't learn something new from him well...... lol. Long story short a great fill in for Dr. M!!


You're WAY too kind, but I'll take it!!!! :-)

Best,

Lee
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Quoting 38. TideWaterWeather:


This is a very refreshing and informative blog entry.. thank you!


You're very welcome.
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Well I dont know about yal, but im ready for LUNCH! And thanks to the last cold front, it is not so warm outside today which makes sitting on the front porch more enjoyable.

Later, taters.
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Quoting 24hourprof:


Many thanks for your kind words. You made my day because I was indeed trying to teach.

Lee

Seriously, that was a great job. I took a few met course in college before I realized the math was going to make my brain explode. ;-) If you're not a teacher, you should be, since that's one the best written and logical non-technical pieces I've seen. Do you have a blog where you plan on or are already have pieces like this? As much as I like and respect Dr. Masters, I'd really like to follow your blog as well.
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I stand corrected. Thanks Lee!
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Quoting 37. sar2401:

That's not the North Pole. That's a picture from a floating buoy considerably south of the North Pole.


I know, but it's still all gonna melt regardless, you'll be using the melted pieces of ice to put in your glass in your beverage
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Quoting 34. 24hourprof:


Many thanks for your kind words. You made my day because I was indeed trying to teach.

Lee


This is a very refreshing and informative blog entry.. thank you!
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Quoting RitaEvac:
Pretty soon we'll all be having cocktails and margaritas at the north pole


That's not the North Pole. That's a picture from a floating buoy considerably south of the North Pole.
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If the track takes it west like steering currents are to GOM, Dorian...call me maybe!! we do need rain
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Quoting 28. sar2401:
Well, the blog was signed Lee, not Jeff, so I'm not sure who wrote it. Who ever did, it was one of the most educational I've ever read about the use of steering currents at different levels vs. the intensity of a TC. I'm about as dumb as a rock, so it sure would be nice to see some more simple education blogs likes this.


I get told that i'm as sharp as a cue ball.
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Quoting 28. sar2401:
Well, the blog was signed Lee, not Jeff, so I'm not sure who wrote it. Who ever did, it was one of the most educational I've ever read about the use of steering currents at different levels vs. the intensity of a TC. I'm about as dumb as a rock, so it sure would be nice to see some more simple education blogs likes this.


Many thanks for your kind words. You made my day because I was indeed trying to teach.

Lee
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Thanks Doc! Always look forward to your updates!
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Quoting 22. OviedoWatcher:
As a non-met, I must say this was a very interesting explanation of steering prediction. I am sure those on the blog with met training consider it basic stuff, but as I don't have that background I found it very helpful information that will give me a better idea of what to look out for.


That's what I was trying to do.

Lee
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Combination of southwesterly windshear and a fast westward movement due to the ridge is going to make it hard for Dorian to comeback once it passes 60 West. The ridge is strong for this time of the year and we are closing in on August, if this were to be the set-up then there would be a lot of long track Cape-Verde storms. Potentially some dangerous ones too that we will need to keep an eye on.
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for all people that missed the link posted a few minutes ago on the old blog by Levi....

Link


again ty levi.
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From the accumulated precipitation swath of the 12z GFS, you can get a sense of its forecast for Dorian's structure. The beginning of the swatch has a band of precip going off to the northwest associated with the current convective burst east of Dorian's center. Later on, a new band develops to the southwest, separate from the first one. This indicates that the GFS expects a new convective burst closer to Dorian's center later today. Regardless, the model still kills Dorian once he moves north of the greater Antilles.

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Well, the blog was signed Lee, not Jeff, so I'm not sure who wrote it. Who ever did, it was one of the most educational I've ever read about the use of steering currents at different levels vs. the intensity of a TC. I'm about as dumb as a rock, so it sure would be nice to see some more simple education blogs likes this.
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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.