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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Not much hope here

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Quoting 1172. moonlightcowboy:


I probably agree with your thinking. But, with an mlc still around (not terribly far from being vertica), even if decoupled, shows some decent stamina with this small system amidst the poor environment and forward speed.


A good point to consider...
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1176. bwi
Quoting 1159. moonlightcowboy:
E D I T

So, are we looking at two circulations here? One at about 47.5w and another at about 45w? LLC outrunning the remnants of a mlc? Or a system trying to get vertical again? Interesting for sure.



My uninformed opinion is that the only low-level circulation is the one at 48w. That's the one to watch for regeneration in my opinion.
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I take that back they appear to have kept as a storm

TROPICAL STORM CENTER LOCATED NEAR 17.8N 45.8W AT 26/2100Z
POSITION ACCURATE WITHIN 30 NM

PRESENT MOVEMENT TOWARD THE WEST OR 280 DEGREES AT 19 KT

ESTIMATED MINIMUM CENTRAL PRESSURE 1010 MB
MAX SUSTAINED WINDS 40 KT WITH GUSTS TO 50 KT.
34 KT....... 60NE 30SE 0SW 60NW.
12 FT SEAS.. 60NE 30SE 0SW 60NW.
WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL
MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT.
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In spite of the convective refire the circulation is open so expect the downgrade at 11 anyway.

If we get heavy convection firing up this may not matter.
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Quoting 1166. TideWaterWeather:


I think the MLC has been spun off for good and that if he is to survive he will build from the LLC up.


I probably agree with your thinking. But, with an mlc still around (not terribly far from being vertica), even if decoupled, shows some decent stamina with this small system amidst the poor environment and forward speed.
Member Since: July 9, 2006 Posts: 184 Comments: 29610
In the meantime while we track the re-firing of convection over Dorian tonight, the latest computer models have come significantly further north.

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Quoting 1158. TimSoCal:
Lots of folks discussing wanting to see the beauty of a major 'cane, so I have a question.

What to you consider the most beautiful/photogenic hurricane you've ever seen? Visual aids would be appreciated.


Category 4 Hurricane Igor, 2010.
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1169. ncstorm
looking at Jim cantore twitter page and nothing about Dorian..I used to post from his twitter account all the time on here when it was hurricane season but I gave up looking after finding nothing..
Member Since: August 19, 2006 Posts: 13 Comments: 16225


This suggest an open wave , but satellite imagery shows clearly a LLC.
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Quoting 1144. KoritheMan:

Don't forget the two week power outages that accompanies major hurricanes.

Gustav left me without power for five days. I can't imagine going longer than that.

I may want a hurricane, and enjoy them, but it's also very wise to understand the full implications of what comes with them. I for one don't like a prolonged lack of electricity, especially not with four annoying little siblings running around the house. :P

You and I are just alike on that statement :) haha
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Quoting 1159. moonlightcowboy:
So, are we looking at two circulations here? One at about 47.5n and another at about 45n? LLC outrunning the remnants of a mlc? Or a system trying to get vertical again? Interesting for sure.



I think the MLC has been spun off for good and that if he is to survive he will build from the LLC up.
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1165. bwi
I'll be watching (ex?) Dorian for a while longer -- any spinner in that area this time of year merits attention.

But meanwhile, I've been having fun following Arctic weather and webcams. This picture is a few days old, but I like the bird on camera effect.

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Hurricanes used to excite me..but that was before surviving Andrew, Opal, Danny(on a boat), Rita , and Ike...The latter 2 flooded me out...Hence the ID I use on here...
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TimSoCal , here's a beautiful hurricane , Hurricane John !
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Quoting 1159. moonlightcowboy:
So, are we looking at two circulations here? One at about 47.5n and another at about 45n? LLC outrunning the remnants of a mlc? Or a system trying to get vertical again? Interesting for sure.



I think it's a decoupled mess.
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Quoting 1146. pottery:


Darn !


Now it's Grothar's turn to come up with an all-alliterative sentence that is on topic and makes sense AND is grammatically correct with 10 words ;)
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The hardest thing to develop is a low level center, he still has this and may be able to build off of it now that he has shed everything else and the water temps and dry air are becoming more favorable. It also looks like the shear might be letting up for the time being but the ULL is still sitting to the NW. Odds are that Dorian is RIP, yet there is still some chance he can make a comeback IMO.
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E D I T

So, are we looking at two circulations here? One at about 47.5w and another at about 45w? LLC outrunning the remnants of a mlc? Or a system trying to get vertical again? Interesting for sure.

Member Since: July 9, 2006 Posts: 184 Comments: 29610
Lots of folks discussing wanting to see the beauty of a major 'cane, so I have a question.

What to you consider the most beautiful/photogenic hurricane you've ever seen? Visual aids would be appreciated.
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1157. Grothar
Quoting 1131. Stormchaser121:

They will be out in FULL FORCE tonight.


We already are. It will be firing up very soon.
But it is not the convection to the SW to look for, rather the convection to the NW. BlobCon don't fail me now.


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Heck everybody , what if he's a CAT 1 in two days ? Who knows anything is possible !
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Quoting 1126. moonlightcowboy:


There's a fascination about them, sure. Last year, Isaac came through here, not even a Cat 1, just a TS and it was miserable. Lingering rain and wind, flooding. Work stops, traffic stops, etc. Twas not fun at all.


That was a nightmare here! 20" of rain...
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Quoting 1134. washingtonian115:
Note: "abandon island"..Definition..

a·ban·don
/əˈbandən/
Verb
Give up completely (a course of action, a practice, or a way of thinking).
Noun
Complete lack of inhibition or restraint.
Synonyms
verb. leave - relinquish - forsake - quit - give up - desert


;) W115, I gotcha: an (with past participle) abandoned island. I sort of agreed with you, but noting that even the tropical storms can be a unique and unwelcoming experience.
Member Since: July 9, 2006 Posts: 184 Comments: 29610
1153. ncstorm
Quoting 1143. washingtonian115:
I didn't know stating the facts was downcasting.Like stating how the environment is dry with shear also being a issue.Oh well..

I can tell where I'm not wanted I will leave.so Pat can get back to blogging laters.


LOL..Wash, you have been downcasting this storm since day one..I personally have been wishcasting it since day one..its nothing wrong with that..its part of WU..
Member Since: August 19, 2006 Posts: 13 Comments: 16225

Quoting 1141. sar2401:

That's the reason I asked you to clarify your statement. I never labeled you as anything. I posed a question and asked you to clarify your position. Your answer is you want to see a hurricane and all your friends know you want to see a major hurricane. If your friends know you want to see a major hurricane, but somewhere out to sea, where there won't be death and destruction, why would that make them hate you? It's not that most people want destruction, but some people don't care if there's destruction, as long as their wish id=s fulfilled. When you're young and you have no family (as in wife and kids) or property that can be destroyed, you tend to have a different view of hurricanes. I don't even know where you live and if your own life is at risk. You may not realize it, or may not even care, but it's disrespectful to ever want to see a hurricane that will cause destruction and death. They will do so, regardless of what you or I want, but that's still different than wanting to see one that will cause death and destruction. I would gladly take an entire season of really ugly cat 1's out to sea than ever see another Katrina. It's not how beautiful she looked from above, which is undeniable. It's how ugly she looked at ground level. Unless your were there, you can never understand.

We'll agree to disagree on this part.

As for the rest of your post, I think it was spot on, and I have nothing bad to say about it.
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Quoting 1148. JLPR2:


That is interesting, a sign that waters are warmer a little to the west?


Waters get warmer as Dorian heads westward; its gone through the coolest waters of its lifetime.
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1150. JLPR2
Quoting 1145. nrtiwlnvragn:
ASCAT B



It needs a good amount of convection to close up and tighten. Though with this evidence the NHC should declare Dorian a remnant low, it just doesn't meet the requirements of a tropical cyclone.
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If and only if when Dorian , does comeback to full vigor , he be singing Michael Jackson's song , to all the naysayers , Just Beat It !
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1148. JLPR2
Quoting 1140. cchsweatherman:
Very interesting to note that the deepest convection building thus far is in a cluster to the southwest of the circulation center. If this were indeed a sheared environment, it would not be building in this portion of the circulation center.


That is interesting, a sign that waters are warmer a little to the west?
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1147. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 178 Comments: 56141
1146. pottery
Quoting BaltimoreBrian:
Dorian's Degenerative Death Disappoints Droves; Doubters Dismiss Dorian's Demise.


Darn !
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ASCAT B

Member Since: September 23, 2005 Posts: 15 Comments: 11343

Quoting 1126. moonlightcowboy:


There's a fascination about them, sure. Last year, Isaac came through here, not even a Cat 1, just a TS and it was miserable. Lingering rain and wind, flooding. Work stops, traffic stops, etc. Twas not fun at all.
Don't forget the two week power outages that accompanies major hurricanes.

Gustav left me without power for five days. I can't imagine going longer than that.

I may want a hurricane, and enjoy them, but it's also very wise to understand the full implications of what comes with them. I for one don't like a prolonged lack of electricity, especially not with four annoying little siblings running around the house. :P
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Quoting 1119. ncstorm:
convection starting to fire again..oh downcasters??
I didn't know stating the facts was downcasting.Like stating how the environment is dry with shear also being a issue.Oh well..

I can tell where I'm not wanted I will leave.so Pat can get back to blogging laters.
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Dorian's Degenerative Death Disappoints Droves; Doubters Dismiss Dorian's Demise.
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1141. sar2401
Quoting KoritheMan:
Yes. All of my friends know that I want to see a major hurricane. If admitting that to people makes them hate me, I really don't care.

Nobody wants destruction, even those who want a storm. And anyone who suggests they do is not thinking rationally, as is typically the case during controversial topics like this.

But "I don't understand your mentality, therefore you're sick and twisted" is not justification for saying what you said, nor will it ever be.

I'm not really going to try and debate the point any further, because it'll just be an emotional pissing contest between both of us.

You, me, or anybody else, have no right to label people with differing opinions as insane, sick, twisted, or any other random derogatory label. People need to get off their soapboxes and realize that; the sooner the better.

EDIT: I will say though, that I think there's a time and place to admit you want to see a hurricane. It's all in the presentation. That's why JFV has garnered such a bad reputation around these parts, because he lacks compassion. For example, I would never admit to wanting a hurricane while one is bearing down somewhere. That's disrespectful. I would admit to wanting one during quiet and solitude like now though, and if that doesn't sit well with some people, so be it.

That's the reason I asked you to clarify your statement. I never labeled you as anything. I posed a question and asked you to clarify your position. Your answer is you want to see a hurricane and all your friends know you want to see a major hurricane. If your friends know you want to see a major hurricane, but somewhere out to sea, where there won't be death and destruction, why would that make them hate you? It's not that most people want destruction, but some people don't care if there's destruction, as long as their wish is fulfilled. When you're young and you have no family (as in wife and kids) or property that can be destroyed, you tend to have a different view of hurricanes. I don't even know where you live and if your own life is at risk. You may not realize it, or may not even care, but it's disrespectful to ever want to see a hurricane that will cause destruction and death. They will do so, regardless of what you or I want, but that's still different than wanting to see one that will cause death and destruction. I would gladly take an entire season of really ugly cat 1's out to sea than ever see another Katrina. It's not how beautiful she looked from above, which is undeniable. It's how ugly she looked at ground level. Unless your were there, you can never understand.

Member Since: October 2, 2004 Posts: 0 Comments: 17447
Very interesting to note that the deepest convection building thus far is in a cluster to the southwest of the circulation center. If this were indeed a sheared environment, it would not be building in this portion of the circulation center.
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Quoting 1129. Patrap:


Sinus rhythm once again! :)
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I'm seeing references between Andrew and Dorian. It's a good idea to do that though, don't count out any storm, no matter how weak it is. Like Andrew, it was ready to die but turned into the worst storm in history at the time.
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Dorian needs to take his antidepressant medication.
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Quoting 1130. stormpetrol:


Convection west of the center where I estimate to be around 17.9/18N/47.5W, shielding the core.
Based on continuity, I'd expect that's about exactly where the NHC will put it at 11.
Member Since: August 3, 2005 Posts: 10 Comments: 5688
I'm no downcaster, but it's dying. I don't see this surviving the next 36 hours, but I do see the risk for this storm to redevelops in Bahamas. For now, Dorian is out of game.
Member Since: August 1, 2011 Posts: 28 Comments: 8075
Quoting 1126. moonlightcowboy:


There's a fascination about them, sure. Last year, Isaac came through here, not even a Cat 1, just a TS and it was miserable. Lingering rain and wind, flooding. Work stops, traffic stops, etc. Twas not fun at all.
Note: "abandon island"..Definition..

a·ban·don
/əˈbandən/
Verb
Give up completely (a course of action, a practice, or a way of thinking).
Noun
Complete lack of inhibition or restraint.
Synonyms
verb. leave - relinquish - forsake - quit - give up - desert
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I swear this storm never ceases to surprise me.

We may get a remnant low downgrade anyway at 11 but even so the center is firing new convection again, if looks count it may be a large burst about to start here.

Should that happen its not too much of a stretch for the low to close back off again.


Annoying little bugger.





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Quoting 1105. KoritheMan:

Agreed. I'm not one of those people, though. :)

Despite what a lot of people think, the desire for a major hurricane and compassion are not mutually exclusive.


Well, I really do like to see ridiculously strong storms, just over water, not on land.

The strong ones are like a work of art or something. Just awesome in so many ways.

What I might say, carefully, is that sometimes even the destruction is an awe inspiring display of nature, but it makes you sad when you think about the individuals harm or losses. Yet there is something incredibly thrilling about going out and feeling 80mph or more wind and rain in your face and leaning into it or holding on to something.

I remember those videos from those guys riding out Charley eye wall, or Geraldo in Galveston with the anemometer showing 140mph gusts in Ike.

It is a shame the damage and loss of life that's caused. I guess it would be great if we could somehow build and plan in a way that no lives were ever lost and no major damage ever done to homes and such, but it doesn't look like that day is near.

We can do without landfalls above cat 1, even though I admit it is thrilling to stand and feel the wind and rain of an Andrew or Gustav, or even Katrina, but it's also gut wrenching the amount of harm it was. I feared the loss of life from Katrina, and my fears were well founded, but I guess I didn't actually "expect" the loss of life to be that bad, yet it was literally a perfect storm over water.

I guess there's no "PC" way to frame the concept of how nature can be both beautiful and horrible at the same time.
Member Since: January 25, 2012 Posts: 33 Comments: 1520
Quoting 1119. ncstorm:
convection starting to fire again..oh downcasters??

They will be out in FULL FORCE tonight.
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Convection west of the center where I estimate to be around 17.9/18N/47.5W, shielding the core.
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1129. Patrap
Member Since: July 3, 2005 Posts: 428 Comments: 129902
1128. will40
1124. Civicane49


i imagine you get very few of those right?
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