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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Lee Grenci is getting so much love on this blog (edit: not, that he doesn't deserve it), our precious Dr. M. may become yellow with envy, lol (that's what we say in German; I just realize that the English saying is: green with envy - but the above colour fits for both). Wonder, when he will show up here, claiming back his reign ... Of course, j/k :)))


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Good Afternoon,

Great post Lee!!, I actually posted and provided a tutorial about this yesterday about going back to the old fashion method in tracking hurricanes..
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well if he doesnt die out, comes wens morning miami and south florida might get a lil nervous, maybe rightly so
Member Since: August 6, 2011 Posts: 4 Comments: 36872
Quoting 115. mikatnight:
Popped in to read the latest post, it turned out to be a good one. That oughta keep the forecasting crew around here busy with pens and plots! I've added the link ('What steers tropical cyclones?') to Lee Grenci's post in the BDA page of the Hurricane Protocol guide.

Don't know if anyone's noticed, but Dorian makes the 3rd TC this year to form in the deep-tropics before August 1st, with a total of 4 storms so far.


From Masters' Blog...
(Monday, July 8)

Chantal: a likely harbinger of an active Atlantic hurricane season
Chantal's formation on July 8 is an usually early date for formation of the season's third storm, which usually occurs on August 13. A large number of early-season named storms is not necessarily a harbinger of an active season, unless one or more of these storms form in the deep tropics, south of 23.5°N. According to Phil Klotzbach and Bill Gray, leaders of Colorado State's seasonal hurricane forecasting team,

"Most years do not have named storm formations in June and July in the tropical Atlantic (south of 23.5°N); however, if tropical formations do occur, it indicates that a very active hurricane season is likely. For example, the seven years with the most named storm days in the deep tropics in June and July (since 1949) are 1966, 1969, 1995, 1996, 1998, 2005, and 2008. All seven of these seasons were very active. When storms form in the deep tropics in the early part of the hurricane season, it indicates that conditions are already very favorable for TC development. In general, the start of the hurricane season is restricted by thermodynamics (warm SSTs, unstable lapse rates), and therefore deep tropical activity early in the hurricane season implies that the thermodynamics are already quite favorable for tropical cyclone (TC) development."


Last year and the year before also had 4 storms form before August, but neither year had 3 pre-August deep-tropic formations, though both years were hyperactive.
The best and worst is yet to come.


Many thanks!
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123. 7544
Quoting 119. Camille33:
Like I said yesterday this was degenerating fast.But this still will be a long term problem.


agree and dont forget the wobbles lol they do count .
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Member Since: August 6, 2011 Posts: 4 Comments: 36872
Quoting 116. Jewelsblues:
Loved this post! As a hurricane or storm hunter/chaser/tracker aficionado (hard to come up with an accurate moniker, since I live in Dominican Republic and the disturbances tend to come to me) I can see myself coming back to this post more times in the future.


Many thanks!
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Quoting 105. SouthernIllinois:

Oops! My bad! I did warn everyone a few days ago that I would slip at least a dozen more times.....I'm on strike 2 now. lol

Nat


LOL, no worries, just messing with you! Still sunny in your neck of the woods? We're finally drying out some in NW Florida!
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Like I said yesterday this was degenerating fast.But this still will be a long term problem.
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Hello CaneHunter, If you listen to Levi's tidbit on youtube , he said if Dorian survives , and gets to the Bahamas , he said hurricane is not out of the question , only if Dorian does survive , which I think he will!
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Loved this post! As a hurricane or storm hunter/chaser/tracker aficionado (hard to come up with an accurate moniker, since I live in Dominican Republic and the disturbances tend to come to me) I can see myself coming back to this post more times in the future.
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Popped in to read the latest post, it turned out to be a good one. That oughta keep the forecasting crew around here busy with pens and plots! I've added the link ('What steers tropical cyclones?') to Lee Grenci's post in the BDA page of the Hurricane Protocol guide.

Don't know if anyone's noticed, but Dorian makes the 3rd TC this year to form in the deep-tropics before August 1st, with a total of 4 storms so far.


From Masters' Blog...
(Monday, July 8)

Chantal: a likely harbinger of an active Atlantic hurricane season
Chantal's formation on July 8 is an usually early date for formation of the season's third storm, which usually occurs on August 13. A large number of early-season named storms is not necessarily a harbinger of an active season, unless one or more of these storms form in the deep tropics, south of 23.5°N. According to Phil Klotzbach and Bill Gray, leaders of Colorado State's seasonal hurricane forecasting team,

"Most years do not have named storm formations in June and July in the tropical Atlantic (south of 23.5°N); however, if tropical formations do occur, it indicates that a very active hurricane season is likely. For example, the seven years with the most named storm days in the deep tropics in June and July (since 1949) are 1966, 1969, 1995, 1996, 1998, 2005, and 2008. All seven of these seasons were very active. When storms form in the deep tropics in the early part of the hurricane season, it indicates that conditions are already very favorable for TC development. In general, the start of the hurricane season is restricted by thermodynamics (warm SSTs, unstable lapse rates), and therefore deep tropical activity early in the hurricane season implies that the thermodynamics are already quite favorable for tropical cyclone (TC) development."


Last year and the year before also had 4 storms form before August, but neither year had 3 pre-August deep-tropic formations, though both years were hyperactive.
The best and worst is yet to come.
Member Since: October 18, 2005 Posts: 4 Comments: 3052
The last few frames of the satellite loop suggest a due West motion now with the center attempting to consolidate again. The 850 vort update is also stronger than earlier this morning. Down but not out.

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113. 7544
Quoting 107. CaneHunter031472:


I think it will end up hitting Cuba. Dissipating. I guess it all comes down on how strong it is.


hmm but notice the slight northward shift there if it holds its own and misses cuba to the north it will be a very differnt story espeacily if it gets to the bahmmas where the waters are very hot this could change things very fast imo but we wait watch and see .
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Whelp, I'm out till later. Time to do what every kid does during the summer-video games!

:)
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Quoting 108. OviedoWatcher:


I second that. Either regular guest appearaces, or a seperate blog aimed at the level this one was, would be really appreciated.


I post one or two blogs at Weather Underground each week (see the blog directly below this blog).

Lee
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A faster storm is a further north storm, because it will reach the weakness before the high fills in.
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Quoting ILwthrfan:


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.




In looking back over the STT's since 2006, most of the Atlantic is about 1 degree Fahrenheit above average for late July. The last time this anomaly was observed was 2009, so I guess that's a starting point to look at how Atlantic storms behaved. It was a below average year for storms in general, and, from the tracks. it's pretty clear that Atlantic storms were favored and survived much further north than average.
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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.


I second that. Either regular guest appearaces, or a seperate blog aimed at the level this one was, would be really appreciated.
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I think it will end up hitting Cuba. Dissipating. I guess it all comes down on how strong it is.
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Quoting 101. Grothar:
Never write anything off

Ex husbands?¿
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Time to post my beloved TPW again, I think. Weak circulation with Dorian, but still there.

Fortunately I've just got a call from an elderly person who was released from hospital (or better: released himself unpatiently) after he was brought in yesterday as a case of emergency with a total breakdown due to the current heat wave in Western/Central Europe. The heat with very high humidity is really dangerous for a lot of people. Hopefully thunderstorms at Sunday will end this calamity.
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102. wpb
Quoting 73. Levi32:


Thanks, I appreciate it.
x10
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Never write anything off

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Quoting 92. SouthernIllinois:
Well Dorian is clinging to life but may very easily have a lot of life left in her down the road. These next few days will tell a lot about the possibility of re-generation or not if she indeed does bit the dust. Time will tell. If she can overcome that dry air to her west, then perhaps she can build a nice core being that shear is relatively low right now. But once shear kicks up and she still looks wimpy like now, the fat lady will be clearing her throat up at the podium!


Dorian is a he or an it but definitely not a she!
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Quoting 92. SouthernIllinois:
Well Dorian is clinging to life but may very easily have a lot of life left in her down the road. These next few days will tell a lot about the possibility of re-generation or not if she indeed does bit the dust. Time will tell. If she can overcome that dry air to her west, then perhaps she can build a nice core being that shear is relatively low right now. But once shear kicks up and she still looks wimpy like now, the fat lady will be clearing her throat up at the podium!
Going to come down to that ULL to Dorian's NW if it survives the dry air. It appears to be filling in right now and may be weakening. The question then will become will it move southwest away from Dorian as Dorian approaches the vicinity of the islands, that may be its greatest chance of restrengthening.
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Quoting 69. 24hourprof:


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
So Lee..What will Dorian do?.. What kind of dressing to you like on your salad..SUV or Pickup?...Good Morning America or Today Show..American Idol or The Voice? French Fries or Onion Rings...Just trying to get all the facts straight...Glad to hear from you again Lee, It's a pleasure
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what we need is dorian to slow down some,20mph is too fast to grow...
Member Since: August 6, 2011 Posts: 4 Comments: 36872
96. SLU
Quoting 88. Tropicsweatherpr:


And the favorable MJO pulse will be on the basin in the next two weeks.



I expect that the models will latch on to something in the next 5 days or so.
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Thanks Lee for the information. I had the pleasure of taking some online meteorological classes at PSU and learned a great deal of information from your lessons. I'm glad to see you are still contributing to my learning.
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Quoting 86. LargoFl:
dont rip dorian just yet in a day or so he'll be in favorable waters..


Do they take every factor into consideration? If so I wonder if rapid intensification is a possibility. that is, if Dorian doesn't crash against Cuba.
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Quoting 90. chrisdscane:



For the first time the Gulf looks like a likely landing spot, anyone on the Gulf Coast and FL needs to watch it, the threat to the Carolinas is all but gone atm


I thought the gfs kills it in 4 or 5 days?
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according to the NHC Dorian has a 54% chance to be a TS next wens..until he's gone finished..poofed..keep watching him
Member Since: August 6, 2011 Posts: 4 Comments: 36872



For the first time the Gulf looks like a likely landing spot, anyone on the Gulf Coast and FL needs to watch it, the threat to the Carolinas is all but gone atm
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Quoting 80. 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.

You are very kind for saying this, Lee. However the primary thought that I provoke with people is something along the lines of, "Good to see you. I am sorry that you have to leave so soon. .... You are leaving, aren't you?!?!"

Mike


LOL!!!
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Quoting 78. SLU:
WOW. Right in time for the Cape Verde season.



And the favorable MJO pulse will be on the basin in the next two weeks.

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Quoting 79. auburn:
Nice blog Lee,Very informative and easy to understand.


Many thanks!

Lee
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dont rip dorian just yet in a day or so he'll be in favorable waters..
Member Since: August 6, 2011 Posts: 4 Comments: 36872
Quoting 81. maeko:
WoW!! Great blog post, Lee. I really learned something today! I'm just a layperson that lurks around because I live on the coast. Most of the jargon and maps go over my head, but today I was able to follow along and understand the graphics. Thanks!


You're quite welcome!

Best,

Lee
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Quoting 82. SouthernIllinois:

Yep. Totally good stuff. Definitely raw old school weather stuff like when weather used to be weather and not commercialized the way it is now. And yeah, the math requirement side of forecasting is really intense...and probably over-reaching. I'll admit I can tell you what any cloud is in the sky but don't know a lick about trig or calc or that stuff....when it comes to math Natalie = EPIC FAIL. :)


Thanks!
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83. SLU
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WoW!! Great blog post, Lee. I really learned something today! I'm just a layperson that lurks around because I live on the coast. Most of the jargon and maps go over my head, but today I was able to follow along and understand the graphics. Thanks!
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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.
Quoting 69. 24hourprof:


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


You are very kind for saying this, Lee. However the primary thought that I provoke with people is something along the lines of, "Good to see you. I am sorry that you have to leave so soon. .... You are leaving, aren't you?!?!"

Mike
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79. auburn (Mod)
Nice blog Lee,Very informative and easy to understand.
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78. SLU
WOW. Right in time for the Cape Verde season.

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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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