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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1828. Grothar
Quoting 1822. Hurricanes305:


Hey Gro, when will all the downcasters learn not to called a system dead when its still a well define low. First it was Chantal and now this. And do I dare say its defying all the odds from last night to starting convective banding on North and South side of Dorian. Interesting day for Dorian and everybody else involved.


We will always have wishcasters and downcasters and upcasters, it is just part of life. That is why I was one of the first to say "Never write off a system too fast" (toot) (toot). :)

It really was a tough call though. Even the experts did not expect this.
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1827. LargoFl
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1826. LargoFl
thats it Lil One..you can do it...........
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Quoting 1822. Hurricanes305:


Hey Gro, when will all the downcasters learn not to called a system dead when its still a well define low. First it was Chantal and now this. And do I dare say its defying all the odds from last night to starting convective banding on North and South side of Dorian. Interesting day for Dorian and everybody else involved.


If it's anything like the last two seasons, it'll be as interesting as watching paint dry. An awful lot of storms in the general area Dorian finds itself have been persistent, yet not developed.
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Besides myself and probably Grothar (he's old enough) has anyone read the Picture of Dorian Gray? Looks can be deceiving! It lives!!!
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1823. Grothar
Quoting 1820. K8eCane:



Gro...I just gotta love you!


Hey, it's a difficult job, but I guess someone has to do it.
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Quoting 1817. Grothar:
This is my horn. My mother told me never to do this.






Hey Gro, when will all the downcasters learn not to called a system dead when its still a well define low. First it was Chantal and now this. And do I dare say its defying all the odds from last night to starting convective banding on North and South side of Dorian. Interesting day for Dorian and everybody else involved.
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1821. Grothar
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1820. K8eCane
Quoting 1817. Grothar:
This is my horn. My mother told me never to do this.







Gro...I just gotta love you!
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1819. VR46L
Quoting 1816. JrWeathermanFL:
Good Morning all!
I see Dorian has atempted to make a comeback. If he can develop a good enough moisture field, and mix out the dry air, Dorian might be hanging on a while. I don't really ever see him making hurricane status, but if it stays north of Hispanola, some ACE numbers would at least help for the season total.


Could Care less about ACE numbers . No true reflection on a season as regards to landfall or damage ..Its the fish years that have big ACE totals
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1818. barbamz

Dorian seems to be a quite determined guy.

Concerning Europe:
Estofex.org just has enlarged its alert for France. We are in for a very, very wild weekend (my region near Frankfurt will be hit tomorrow, presumably). The lead on my terrace is already hitting 100F. Of course, it's downtown, so it's not an official reading.

From Estofex:Confidence has grown that the storms which initiate during the evening across the confluence zone/cold front will produce extremely severe weather in particular damaging wind gusts. Latest NWP guidance suggests somewhat stronger synoptic scale support across the area. Given that very strong 30-35 m/s deep-layer (0-6 km) shear should overlap with abundant low-level humidity and steep lapse rates, supercell storms are forecast to develop. A threat of large or very large hail will exist with these storms. Tornadoes are also possible as the boundary layer cools. The highest threat appear to be the damaging winds that will evolve when cells transform into one or multiple bow echoes. These should move quickly to the north-northeast producing swaths of extreme winds (> 32 m/s). Overnight, the intensity of the bow echo(es) will gradually diminish.

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1817. Grothar
This is my horn. My mother told me never to do this.




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Good Morning all!
I see Dorian has atempted to make a comeback. If he can develop a good enough moisture field, and mix out the dry air, Dorian might be hanging on a while. I don't really ever see him making hurricane status, but if it stays north of Hispanola, some ACE numbers would at least help for the season total.
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1815. Grothar
Quoting 1767. VR46L:
Why do I feel I should be served Crow right now...... Defying all the conditions Well done WKC and others who said it was not completely finished




Thank you VR. I accept your apology.
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For once it has some really deep convection over the center which has lead in a miniature CDO. And the SW side that blow up of thunderstorms is about to wrap around it will act to only increase its convective size. By 55W dry no longer becomes a problem and shear will have to be monitored as to whether Dorian survives or not.
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1813. LargoFl
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1812. LargoFl
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Quoting 1801. ncstorm:
Dorian just wont give up..this might be one for the textbooks..


last night I said this would happen and I got yelled at and was beat down by others about it now look at it

Member Since: June 13, 2009 Posts: 2 Comments: 9548

Quoting 1793. TheDawnAwakening:


I see better development on the southwestern side and southern side of the cyclone.
If you look closely at 20N and 50W the low level outflow is starting to become apparent as there is upper divergence over over cyclone low level banding appearing from the southern end of the cyclone



Either way it getting structurally better organized. Would not surprise me at all if it increase to 40 kts at 11pm.

Like I said last night that SST was always Dorian biggest problem. Now that is taken care of dry air and the ULL is its next obstacle and its starting to find moisture around 55-60W
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The worst thing about Dorian reviving? People will come running back after calling it RIP and start freaking out that it's strengthening into hurricane. I'm sure DMIN and extreme shear will finish off Dorian later today, hence the forecast of depression from NHC.
Member Since: August 1, 2011 Posts: 23 Comments: 7451
1808. LargoFl
yes its just about August in a few days and we must also be alert for home grown storms too huh
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He is done... Everyone back to their regularly scheduled lives...
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1806. LargoFl
Quoting 1803. PensacolaDoug:
This may be Dorians last gasp. I truly thought it was toast yesterday. I still think it probably is, but I'm not as sure as I was. Good morning all. Largo! Hit me!
here ya go..freshly brewed..im on my second cup lol
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06Z cyclone models







Member Since: September 23, 2005 Posts: 13 Comments: 10458
Quoting 1801. ncstorm:
Dorian just wont give up..this might be one for the textbooks..



Just looked, wondered *what* is *that* round blob? I can't believe - actually I can believe - it's Dorian!
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This may be Dorians last gasp. I truly thought it was toast yesterday. I still think it probably is, but I'm not as sure as I was. Good morning all. Largo! Hit me!
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Quoting barbamz:


Wow, European weather! Thanks for posting, Aussi.


No worries mate! I told you about this page the other day. Maybe you didn't see it.

Here is another website you might like.
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1801. ncstorm
Dorian just wont give up..this might be one for the textbooks..

Member Since: August 19, 2006 Posts: 13 Comments: 13436
Looks like Dorian is making a play for a comeback


still weak
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Quoting 1780. VR46L:



I will have it stewed in Red wine Jus

Doing Ok !:)

I believe folk should be given credit where its due and you took a beating yesterday and you were right !

yeah and you know that beating really hurts

Quoting 1786. Hurricanes305:
I'm beginning to see hints at some low level banding starting to to get going on the northern side from the SE where the moisture is available. Also note the new convective activity from the SE. Dorian maybe trying to get its MLC back which if it does we will likely see a 45 kt TS by 5pm.

yeah its doing good however it need to work on those bandings on the S side banding has already started forming on the W side
Member Since: June 13, 2009 Posts: 2 Comments: 9548
Quoting 1789. RascalNag:


That's two lows going up the East Coast now. This one might have a better shot than the one already up by NC.


I agree the southern lo, near Florida has a much better chance of developing into a tropical storm than the one up near NC.
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1797. WWPR
Quoting 1784. GatorWX:


I don't think so. I think it's a bit more of a concern, perhaps mixed with anticipation.


GatorWX, If you put it that way, it's understandable..but someone yesterday posted tha he was bored because all he was getting to see was a weak TD that might have been going into the Islands...If watching a strong TS or Cane hit a country is entertainment then that's very wrong.
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1796. barbamz
Quoting 1783. AussieStorm:


Stormhunter-NL
Here is photo of the massive shelf cloud coming in from Veldhoven NL! Awesome shot by Kevin van Berlo!

Coming in from Bilzen, Belgium!
Huge shelf cloud!



Amazing shot of a supercell near Loiret in Southwestern France from last night! Wow!



Wow, European weather! Thanks for posting, Aussi.
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Once Dorian get pass 55W if it shear conditions become favorable dry air wont longer be a issue. I'm interested to see if it interacts with the ULL positively thus stack up over cyclone to the upper atmosphere very similar situation to Hurricane Andrew. Sorry for the comparisons. Its just something to keep in mind! 
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Quoting 1786. Hurricanes305:
I'm beginning to see hints at some low level banding starting to to get going on the northern side from the SE where the moisture is available. Also note the new convective activity from the SE. Dorian maybe trying to get its MLC back which if it does we will likely see a 45 kt TS by 5pm.


I see better development on the southwestern side and southern side of the cyclone.
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Quoting 1779. TheDawnAwakening:
Newly developing surface low east of central Florida.


That's two lows going up the East Coast now. This one might have a better shot than the one already up by NC.
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Erin has to wait a while longer as TWO has no mention of areas of interest.


TROPICAL WEATHER OUTLOOK
NWS NATIONAL HURRICANE CENTER MIAMI FL
800 AM EDT SAT JUL 27 2013

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

THE NATIONAL HURRICANE CENTER IS ISSUING ADVISORIES ON WEAKENING
TROPICAL STORM DORIAN...LOCATED OVER THE TROPICAL ATLANTIC WELL TO
THE EAST OF THE NORTHERN LEEWARD ISLANDS.

ELSEWHERE...TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE
NEXT 48 HOURS.

$$
FORECASTER PASCH
NNNN
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Quoting 1773. Hurricanes305:




Hey WKC remember last I said that wave could be a playmaker starting today the upper level winds are very light now that Dorian is paving away if it stays within the ITCZ it may be a problem for folks in the Caribbean.

sure seems that way

Quoting 1774. TheDawnAwakening:



I don't feel I'm being wishful at all, just as it was dying earlier yesterday, today it's on the uptick, and could become a strong tropical storm again enforce nearing the Lesser Antilles, shear is dropping and SSTs are increasing, so he has what he needs for an increase in moisture and convection as well as organization, and when he does organize further, it will be able to feel the upper level steering flow and begin to feel the weakness in the ridge and move more WNW instead of west.

that weakness is closing off

Quoting 1776. nrtiwlnvragn:


Ya, I feel I deserve this look after thinking last night he was done.


yeah lol but no yeah we all have our moments Dorian this morning is my moment
Member Since: June 13, 2009 Posts: 2 Comments: 9548
I'm beginning to see hints at some low level banding starting to to get going on the northern side from the SE where the moisture is available. Also note the new convective activity from the SE. Dorian maybe trying to get its MLC back which if it does we will likely see a 45 kt TS by 5pm.
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How is it wishful inking when it's certainly possible as is it heading straight for Cuba? I'm not a wish faster, I'm forecasting from what I see out there in the weather world. Not wish casting.
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1784. GatorWX
Quoting 1770. JoeBarnickel:
It seems to me that people posting on here are EAGER for this TS to be a Hurricane. Really?


I don't think so. I think it's a bit more of a concern, perhaps mixed with anticipation.
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Stormhunter-NL
Here is photo of the massive shelf cloud coming in from Veldhoven NL! Awesome shot by Kevin van Berlo!

Coming in from Bilzen, Belgium!
Huge shelf cloud!



Amazing shot of a supercell near Loiret in Southwestern France from last night! Wow!

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Quoting 1770. JoeBarnickel:
It seems to me that people posting on here are EAGER for this TS to be a Hurricane. Really?
Unless you believe in the power of manifestation (which I do not) then there is nothing to lose or to be gained from someone's wishful thinking. Let them wishcast, it certainly won't magically make it so.

I thought for sure I'd wake up to Dorian's dissipation. Talk about surprised when I looked at the floater :)
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Quoting 1770. JoeBarnickel:
It seems to me that people posting on here are EAGER for this TS to be a Hurricane. Really?
We need to get a life.
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1780. VR46L
Quoting 1771. wunderkidcayman:

why thank you how would you like your crow cooked lol
anyway how are you mate



I will have it stewed in Red wine Jus

Doing Ok !:)

I believe folk should be given credit where its due and you took a beating yesterday and you were right !
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Newly developing surface low east of central Florida.
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1778. GatorWX
I wonder what caused that energy to stay so focused and isolated. Could it be the way the air is moving at different levels? It's FL low level moisture is pretty abundant and don't know effect moisture content would have have at those levels necessarily. I've seen quite a few "trailing" storms this summer.

Oops,

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About JeffMasters

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