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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And because of this 18 hour weakening/disorganizing trend, I will likely post a new blog this evening(if anyone cares).
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177. JLPR2
Lower convergence is hanging on, unlike yesterday.

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Dorian close to breathing its last breathe. Almost an open wave now.
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12z Euro running
00 hours


24 hours
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I aint one to change my view with the wind, but looking at satellite loops...I cant find a good circulation. To me...

Dorian looks dead.
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Quoting 24hourprof:


Many thanks!

I'm an ensemble guy.

In the short term, model consensus for its track forecast is pretty tight, so lots of confidence for the first several days.

Lots of forecast uncertainty with regard to intensity, so I'm reserving an opinion.

Dang! An actual forecaster, based on what he sees and knows, not on some whacky models. Lee, can you hang around here for...say...the next four months, at least? :-0
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Quoting 152. 62901IL:

I'm not.


I am too, I think my ISP is blocking most .gov websites for some reason. I thought it was just my internet at first, but I call a friend who has the same ISP as me and he could access the NHC site either. But I know it is up and running fine as I was able to access it through Verizon internet on my phone...I'm not sure whats up with that.
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Really good rotation showing up again unlike earlier today when it looked like an open wave. Pretty amazing actually considering its size and the environment.

EWall loop
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Quoting 164. HurricaneAndre:
I'm suprised it's not in the TWO.

I'm not one bit surprised in fact I was expecting it
Member Since: June 13, 2009 Posts: 2 Comments: 11837
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12Z GFS has a little something by Caymans in 129 hours



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Quoting NasBahMan:


'IF' it makes it to The Bahamas don't expect any quick blow up, I can tell you the water is colder than I can ever remember them being for the month of July.

Indeed. With all the rain and clouds the last month, the average water temperatures have taken a dive from the Gulf to the Bahamas. The water is still plenty warm to support a TC, but the fabled "rocket fuel" we had two years just isn't the case today.
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Quoting 136. Envoirment:
The wave behind Dorian is almost fully into the Atlantic



It's looking quite good. Its convection was good a few hours earlier



It looks as if it has increased some more since then.
I'm suprised it's not in the TWO.
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Quoting 156. 24hourprof:


Tough question.

Assuming upwelling of cool water weakens the storm (depends on factors such as time spent over cooler water, etc.), I suspect that the propagation factor (the Beta effect) might play less of a role, turning more responsibility for steering over to the environmental flow (mean wind in the steering layer determined by minimum central pressure). Since the environmental flow is responsible for 80-90% of the motion of a tropical cyclone, any change in path caused by upwelling would probably be pretty subtle.

In the final analysis, it's tough to say for sure because there are so many factors at play.


Thank you sir. That makes sense. It seems like it would be a catch 22 in the fact that any upwelling significant to impact steering currents would likely have to be so strong and would impact the heat engine first anyways. Now on to reading about the Beta effect. I have learned quite a bit today and that makes for a very awesome Friday :)
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00Z ECMWF OPER has a bit of something by S Florida in 168 hours

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GEM at 180 hours....a little itty bitty something in the gulf left from Dorian

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Quoting 142. EyEtoEyE:
I believe that Dorian , is playing us as fools !
Lmao off! South Florida should particularly keep a close eye on it. The Atlantic has turn on the switch with upward motion returning if Dorian gets in the Bahamas and slow down.
Member Since: May 25, 2012 Posts: 0 Comments: 2043
Quoting 113. 7544:


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 .


'IF' it makes it to The Bahamas don't expect any quick blow up, I can tell you the water is colder than I can ever remember them being for the month of July.
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Quoting 148. 24hourprof:


Many thanks!


Nice entry, Lee! We need more of those kinds of blogs. :)
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Quoting 155. sugahcane:
been lurking in the background for years.... Levi's tropical tid-bit on Dorian is quite impressive

Welcome to the blog!
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Quoting 135. calkevin77:
Hi Lee,
This may be a dumb question but what effect, if any, on steering currents does upwelling have? Not just on intensity of the center of circulation. If I recall somewhere a while back I read satellites measure the intensity of upwelling energy through the tropospheric level. This would make me think that depending on a previous storm's path, a wake-like path could influence a system that follows behind in essence. Much like the wake turbulence one would find if another plane followed in the path of a 757.




Tough question.

Assuming upwelling of cool water weakens the storm (depends on factors such as time spent over cooler water, etc.), I suspect that the propagation factor (the Beta effect) might play less of a role, turning more responsibility for steering over to the environmental flow (mean wind in the steering layer determined by minimum central pressure). Since the environmental flow is responsible for 80-90% of the motion of a tropical cyclone, any change in path caused by upwelling would probably be pretty subtle.

In the final analysis, it's tough to say for sure because there are so many factors at play.
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been lurking in the background for years.... Levi's tropical tid-bit on Dorian is quite impressive
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Quoting 149. chrisdscane:



Dorian is alone now, this fight is up to him, atleast for now.


Outside the rain begins
And it may never end
So cry no more on the shore a dream
Will take us out to sea
For ever more.. for ever more
Close your eyes Ami
And you can be with me
'Neath the waves through the caves of hours
Long forgotten now
We're all alone..we're all alone
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Hello Enviroment , that TW behind Dorian really looks like it wants to be Erin , at some point !
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Quoting 139. 19N81W:
anybody else having problems opening the nhc site these days?

I'm not.
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Quoting 128. barbamz:


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 :)))


Dr. M. may become HULK too.:)
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Quoting 149. chrisdscane:



Dorian is alone now, this fight is up to him, atleast for now.


That's cos he's a hobbit storm.
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Dorian is alone now, this fight is up to him, atleast for now.
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Quoting 145. hydrus:
Thank you Mr. Grenci...Dorian looks weak, could get interesting in the Bahamas as water temps gradually increase in that direction.


Many thanks!
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Quoting 144. SouthernIllinois:

Chantal's twin.
. Not even close , to Chantal !
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Quoting 98. PalmBeachWeather:
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


Many thanks!

I'm an ensemble guy.

In the short term, model consensus for its track forecast is pretty tight, so lots of confidence for the first several days.

Lots of forecast uncertainty with regard to intensity, so I'm reserving an opinion.
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Thank you Mr. Grenci...Dorian looks weak, could get interesting in the Bahamas as water temps gradually increase in that direction.
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when will this dry spell end in the Caribbean basin...seems void of any convection..
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I believe that Dorian , is playing us as fools !
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Both visible satellite imagery and a recent ASCAT pass this afternoon both suggest that Tropical Storm Dorian is attempting to re-organize as the low level circulation has become more pronounced and convection seems to be reforming over the circulation center. Looks like Dorian may be responding to the gradually warmer waters it is now encountering after having spent the past couple days over waters generally non-supportive for tropical cyclones. Also you could see that Tropical Storm Dorian is doing a fairly good job at moistening the environment around itself.
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anybody else having problems opening the nhc site these days?
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Quoting 133. SouthernIllinois:

haha it's all good! :) Right now yes....but, cirrus deck beginning to lower now as the blow-off tops from the deeper convection (where moderate is falling) to our west in Central MO is beginning to drift in. It should cloud up here completely very soon and rain chances will increase. Yay!!

SUPER glad you are finally getting a break. A nice even trade with you getting my sun and me getting your rain would be fab right now!




Trade approved, take all the rain you can handle and we'll take as much sunshine as we can get and just maybe we can call it the Sunshine State again!
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137. 7544
whatever dorian lost its now gaining it back this should be the story on ,of this track across the atl - and + s.
Member Since: May 6, 2007 Posts: 0 Comments: 6794
The wave behind Dorian is almost fully into the Atlantic



It's looking quite good. Its convection was good a few hours earlier



It looks as if it has increased some more since then.
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Hi Lee,
This may be a dumb question but what effect, if any, on steering currents does upwelling have? Not just on intensity of the center of circulation. If I recall somewhere a while back I read satellites measure the intensity of upwelling energy through the tropospheric level. This would make me think that depending on a previous storm's path, a wake-like path could influence a system that follows behind in essence. Much like the wake turbulence one would find if another plane followed in the path of a 757.


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Quoting 101. Grothar:
Never write anything off



Ever
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132. FOREX
Quoting 119. Camille33:
Like I said yesterday this was degenerating fast.But this still will be a long term problem.


Dorian looking slightly better on visible you think?
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Stronger winds,still closed.
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130. 7544
Quoting 125. LargoFl:
well if he doesnt die out, comes wens morning miami and south florida might get a lil nervous, maybe rightly so


i will go for monday it could pick up in speed and move faster after that .largo
Member Since: May 6, 2007 Posts: 0 Comments: 6794
Quoting 111. 24hourprof:


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

Lee


Thanks. I had never noticed the link. I shall certainly be reading your blog from now on.
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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 :)))


Member Since: October 25, 2008 Posts: 53 Comments: 5918

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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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