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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Dorian has a very nice circulation that wont quit wouldn't count it out especially when it reaches closer to the islands
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Quoting 902. nrtiwlnvragn:


I don't see a whole lot there


It looks like a wishbone to me right now. He sure has some morphing abilities, eh? AHA! I know what Dorian is. Based on his Horsehead Nebula appearance earlier, he's Ophidian (bonus points to those who don't have to google this to get the reference)
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I think the convection firing now is a little too late but it might keep it tropical for another day
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Dorian was a waste of brain cells...
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Quoting 919. Birthmark:

Dude, it's gone. Let it go. :)

NOOOOOOOOOOOOOOOOOO!!!!
JK :)
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Quoting 916. ecflweatherfan:


Every fire begins with a spark.

yep

Quoting 919. Birthmark:

Dude, it's gone. Let it go. :)

not done till there is nothing left
Member Since: June 13, 2009 Posts: 2 Comments: 11215
Meanwhile, Flossie seems extremely healthy.

Member Since: July 9, 2013 Posts: 0 Comments: 612
.."Dorian ya O-kay, yer O-kay now"...

Member Since: July 3, 2005 Posts: 421 Comments: 127664
Quoting 912. EyEtoEyE:
Don't kill Dorian yet, give him a shot!

Dude, it's gone. Let it go. :)
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Member Since: July 3, 2005 Posts: 421 Comments: 127664
Quoting 908. TimSoCal:
He is moving into warmer waters. It's possible(however unlikely) that this is the beginning of a burst of convection large enough to restore some of his health on the way to DMAX.


Every fire begins with a spark.
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Quoting 881. Seflhurricane:
interesting thing that all of the tropical systems so far have had a lot of issues from lack of convection to having a hard time maintaining there LLC, very interesting ???


Same story in 2012.....
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Quoting 907. PensacolaDoug:
That is how it was widely reported in the press back then. The press tends to sensationalize things.

That's what makes them their money, apparently.

And that's why I don't pay much attention to the news business.
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Pat, we need those nightvision goggles! ;)
Member Since: July 9, 2006 Posts: 184 Comments: 29594
Don't kill Dorian yet, give him a shot!
Member Since: July 6, 2013 Posts: 0 Comments: 460
winds will probably drop to 40 at 11:00..im waiting to see what the 0z models say
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Quoting 909. dfwstormwatch:
can someone give me a link explaining Diurnal Minimum and Maximum to me?


Here you go. Very short but to the point. Link
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can someone give me a link explaining Diurnal Minimum and Maximum to me?
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He is moving into warmer waters. It's possible(however unlikely) that this is the beginning of a burst of convection large enough to restore some of his health on the way to DMAX.
Member Since: July 9, 2013 Posts: 0 Comments: 612
That is how it was widely reported in the press back then. The press tends to sensationalize things.
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Quoting 896. dfwstormwatch:
not so sure about that now that convection is re-firing.



And it's popping on the north and west sides, so that may be a good indicator that the airmass is moistening sufficiently immediately ahead of it.
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I leave for a day and this happens lol. Complete opposite of what happened the last time I left haha.

Anyways, Dorian still has some spin. If she can fire convection during D-MAX and go far enough over DR....
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It even appears that some convection is trying to fire in that dry air
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Quoting 896. dfwstormwatch:
not so sure about that now that convection is re-firing.



I don't see a whole lot there


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Quoting 887. PensacolaDoug:



My memory works fine. I know what was being said back then. I'm not publishing peer-reviewed papers and I'm not going to bother trying to source everything I say in this forum. I don't have time for it. So ignore me if you like, that's what the feature is for.

I have no reason to ignore you. I do have a reason to point out your errors wrt AGW, though.

The fact is that *one* climate scientist is quoted as saying, "At this rate, the Arctic Ocean could be nearly ice-free at the end of summer by 2012, much faster than previous predictions." That was said in December of 2007. And he was right.

Even a quick look at the rate of Arctic Sea Ice decline at that time shows that had the rate of melt observed in 2007 continued, the Arctic would have been ice-free in 2012. Fortunately, that rate didn't continue.

So, Zwally's comment wasn't a prediction. It was a simple mathematical projection based on the rate of melt for one year. He qualified his statement clearly and his math was correct. He was making more of a point about the phenomenal rate of melt in 2007 than any real projection or prediction of an ice-free Arctic.

Of course, that reality hasn't stopped some from incorrectly claiming that he predicted an ice-free Arctic by 2012.



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Well said MLC...hope all is well.
I agree...if I'm the ref I'm not throwing up my arms on this fight just yet. There's something there that had the attention of the models for awhile. If it is called an open wave by NHC it's a pretty well concentrated one, with still a well-defined center of some sort.
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Quoting 892. moonlightcowboy:


Yeah, we've seen many systems stave off dry air, even muster their way through some shearing, but on top of those issues has been the fast, low-level easterly flow with both Chantal and Dorian. Were that not a factor, both systems may have (may still) endure the dry air and shearing which is forecast to relax in front of Dorian.

It may be down, down really low, nearly out, but it's not out yet. A low-level circulation is going to be less subject to shearing anyways. Trouble it has now is the fast forward speed and dry air. If it can get any sfc lift going at all, it may be just enough for it to maintain until conditions become more favorable. And, with the warmer waters it's traversing, may well see some new convection firing through d-max. I, for one, am not counting it out/done just yet.


I agree completely moonlight. I think if we do see some convection start firing soon through d-max we may have something that will really open some eyes as Dorian hits the warmer SST's. I haven't counted him out at all even though the dry air is still there and the LLC is booking it westward. The odds are against him as of now but we all know that can change rapidly. Especially with smaller storms like Dorian. It is these kind of things that keep me (as well as others on this blog) tracking these things during our lifetimes!
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Member Since: July 3, 2005 Posts: 421 Comments: 127664
Quoting 812. L1990:
very disappointed with these pathetic trop
l systems this year






Well we are really not quite in the heart of hurricane season yet and also.... it only takes one to cause major problems



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not so sure about that now that convection is re-firing.
Quoting 895. nrtiwlnvragn:
Avila is going to need to be creative to keep advisories past 11.

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Avila is going to need to be creative to keep advisories past 11.
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hey guys convection is starting to pop what did I tell all of you guys I said Dorian had hit the peak of D-Min and after this convection should start popping I was right its now starting to pop
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Quoting 888. TimSoCal:


Is that convection trying to fire?


Yeppers! Certainly is... Dorian is trying to hang in there!
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Quoting 884. DavidHOUTX:


It's still just July. If this same thing was happening one month from now then that would be very interesting!


Yeah, we've seen many systems stave off dry air, even muster their way through some shearing, but on top of those issues has been the fast, low-level easterly flow with both Chantal and Dorian. Were that not a factor, both systems may have (may still) endure the dry air and shearing which is forecast to relax in front of Dorian.

It may be down, down really low, nearly out, but it's not out yet. A low-level circulation is going to be less subject to shearing anyways. Trouble it has now is the fast forward speed and dry air. If it can get any sfc lift going at all, it may be just enough for it to maintain until conditions become more favorable. And, with the warmer waters it's traversing, may well see some new convection firing through d-max. I, for one, am not counting it out/done just yet.
Member Since: July 9, 2006 Posts: 184 Comments: 29594
891. JLPR2
Quoting 885. stormpetrol:


LLC easy to be seen , look like some convection might want to fire around it.


I'll be more interested if those grow or multiply.

For the moment I'll give Dorian's three little showers a Meh...

Well I'm out, I'll be back later to see what's happening with it.
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Quoting 880. Patrap:
TS Dorian is in Stealth Low Mode Charlie..gliding along for Fairer Seas.






Hold on! Link
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Quoting 880. Patrap:
TS Dorian is in Stealth Low Mode Charlie..gliding along for Fairer Seas.





Is that convection trying to fire?
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Quoting 841. Birthmark:

You've made a claim. Can you back it up with evidence or not? If it's simply your opinion, then no evidence is needed, but evidence *is* needed if you are claiming that your statement is factual.



My memory works fine. I know what was being said back then. I'm not publishing peer-reviewed papers and I'm not going to bother trying to source everything I say in this forum. I don't have time for it. So ignore me if you like, that's what the feature is for.
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Quoting 880. Patrap:
TS Dorian is in Stealth Low Mode Charlie..gliding along for Fairer Seas.






How amazing would it be if the LLC started firing again! That would be super stealth mode if I have ever seen one. Of course as far as a tropical system is concerned!
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LLC easy to be seen , look like some convection might want to fire around it.
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Quoting 881. Seflhurricane:
interesting thing that all of the tropical systems so far have had a lot of issues from lack of convection to having a hard time maintaining there LLC, very interesting ???


It's still just July. If this same thing was happening one month from now then that would be very interesting!
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Quoting 875. TropicalAnalystwx13:
It's only a matter of time before we inflate the number with a few mid-latitude tropical storms/hurricanes. Check out the sea surface temperature anomalies up there!



(Image is nearly a week old)


here is current todays sst's

REM





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Quoting 876. Patrap:


For those thinking what is off the SE CONUS will develop... Not as long as it is attached to a front.
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interesting thing that all of the tropical systems so far have had a lot of issues from lack of convection to having a hard time maintaining there LLC, very interesting ???
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TS Dorian is in Stealth Low Mode Charlie..gliding along for Fairer Seas.




Member Since: July 3, 2005 Posts: 421 Comments: 127664
Quoting 867. cchsweatherman:
I would imagine that the National Hurricane Center will likely downgrade Dorian to a tropical depression at the least at the next advisory given the absence of any convection at the moment. Unless Dorian can fire some decent convection within the next 6 to 12 hours, this tropical storm may be finished. Not writing it off completely yet and am interested in the new popcorn showers/thunderstorms just now firing in a curved band around the circulation.
I Agree with you but I am not seeing any intentions of dorian to fire any convection soon so I really don't see any reason for the NHC to downgrade to a tropical wave at 11 pm
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Quoting 867. cchsweatherman:
I would imagine that the National Hurricane Center will likely downgrade Dorian to a tropical depression at the least at the next advisory given the absence of any convection at the moment. Unless Dorian can fire some decent convection within the next 6 to 12 hours, this tropical storm may be finished. Not writing it off completely yet and am interested in the next popcorn showers/thunderstorms just now firing in a curved band around the circulation.


I am too. There was one that popped earlier in one of the curved bands that didn't last but 15 mins before it collapsed. These are going on 45 mins (not saying much, but progress nonetheless). And now the area of dry air is diminishing, so these very well could, COULD, sustain themselves for a bit... Now that we are coming out of Dmin.
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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.