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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1428. Grothar
Quoting 1419. AllStar17:


Who knows.


The Shadow knows.
Member Since: July 17, 2009 Posts: 71 Comments: 27208
Quoting 1403. cchsweatherman:
Appears that frame by frame coming in, convection continues to gradually build and attempt to organize over Tropical Storm Dorian tonight. Started out as a few small popcorn showers/storms and now has grown into some of the deepest convection the storm has had in its entire lifetime within the past 4 hours. Will need to see the trends continue to be sure that Dorian is truly coming back to life.

Did't I say this would happen a lot of people was RIPing Dorian not me I said that this would turn around now look at the convection it's popping

Quoting 1408. yankees440:



Interesting more models are trending upward compaired to last runs
Member Since: June 13, 2009 Posts: 2 Comments: 12716
1426. Grothar
Quoting 1418. Tornado6042008X:
Even if you don't, you will live forever in all of our hearts:)


Aw, don't make me cry now. You all have fun and keep your eye on that blo...

I hear a little voice in the back that is getting very angry.

Nite everyone.
Member Since: July 17, 2009 Posts: 71 Comments: 27208
1425. Patrap
AL042013 - Tropical Storm DORIAN

AMSU Microwave 89GHz Imagery (4 km Mercator)

Member Since: July 3, 2005 Posts: 428 Comments: 129844
Quoting 1420. moonlightcowboy:


And, this is exactly why a CV system should never be underestimated, written-off. Dorian was hardly done earlier today, and Avila made the exact right call this evening.


Avila knows his stuff

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Link

Check out that circulation off of North FL/Georgia.
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I don't see any real organization in the convection with Dorian. However, if it continues to form that would help the storm to gain more organization.
Member Since: June 29, 2009 Posts: 13 Comments: 5315
1421. Grothar
Member Since: July 17, 2009 Posts: 71 Comments: 27208
Quoting 1404. Patrap:


And, this is exactly why a CV system should never be underestimated, written-off. Dorian was hardly done earlier today, and Avila made the exact right call this evening.
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Quoting 1407. Grothar:


I know. Actually it is funny. Especially with the dry air all around it. There must be another dynamic involved that we do not see. I have been watching these for many years, and this is a little unusual. Interesting to see what I wake up to. That is if I wake up.


Who knows.
Member Since: June 29, 2009 Posts: 13 Comments: 5315
Quoting 1407. Grothar:


I know. Actually it is funny. Especially with the dry air all around it. There must be another dynamic involved that we do not see. I have been watching these for many years, and this is a little unusual. Interesting to see what I wake up to. That is if I wake up.
Even if you don't, you will live forever in all of our hearts:)
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1417. Thrawst
.
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1416. Patrap
Make ready the Reconnaissance Flights...

I sense a disturbance, something I haven't felt in..

Hmmm,


Member Since: July 3, 2005 Posts: 428 Comments: 129844
Dorian "Staying Alive"?
http://www.youtube.com/watch?v=GoaQsHl5ooM&featur e=youtu.be
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Well, well. It managed to regenerate a little convection.

NHC plans on downgrading it to depression within the next advisory or two, and forecasts it to be a remnant low within 24 hours.
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Quoting 1408. yankees440:




Interesting. Most all of those models earlier showed weakening. Some now going back into the strengthening camp.
Member Since: June 29, 2009 Posts: 13 Comments: 5315
Quoting 1378. Grothar:
I declare a blo....



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Quoting 1330. stellarmagnitude:


I've been a lurker here for years and this is something I'm willing to come "out of the shadows" about. At the time that Katrina hit, I was living about halfway between Biloxi and New Orleans. I can honestly say that I was fascinated by the storm, even in the thick of it. But I can also say that I developed a very healthy respect and fear for such storms by living through that one. Sometimes I think people don't quite grasp the power and destruction of major hurricanes until they (or ones they love) go through one... myself included. There's a huge difference between watching it on TV and being there to see everything you know washed away. Not saying we should cut anyone a break. Just throwing that thought out there. ;-)
Nice first post.
Welcome.
Katrina was devastating, but as far as this blog goes, it was it's finest moment. We had lots of last minute panicked people. We saved some lives as we encouraged many to evacuate. There's been a few that have come back later to say ty and said the Dr's. blog gave them that final kick in the ass that saved them from misery or worse.
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Quoting 1403. cchsweatherman:
Appears that frame by frame coming in, convection continues to gradually build and attempt to organize over Tropical Storm Dorian tonight. Started out as a few small popcorn showers/storms and now has grown into some of the deepest convection the storm has had in its entire lifetime within the past 4 hours. Will need to see the trends continue to be sure that Dorian is truly coming back to life.


Coming into real good view as well, if you look near 47W and 48W, a couple real nice curved low cloud bands. Dorian may be taking a stand!
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Quoting 1398. Camille33:

You don't know anything,in fact I am ignoring you now.


ok bye!
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1407. Grothar
Quoting 1383. TideWaterWeather:


Funny thing is, that's some of the coldest cloud tops he has had yet..


I know. Actually it is funny. Especially with the dry air all around it. There must be another dynamic involved that we do not see. I have been watching these for many years, and this is a little unusual. Interesting to see what I wake up to. That is if I wake up.
Member Since: July 17, 2009 Posts: 71 Comments: 27208
1406. Patrap
Member Since: July 3, 2005 Posts: 428 Comments: 129844
Quoting 1366. Civicane49:


It should.

87 hrs


Ill keep an eye on that.
Member Since: September 2, 2011 Posts: 0 Comments: 1149
1404. Patrap
Member Since: July 3, 2005 Posts: 428 Comments: 129844
Appears that frame by frame coming in, convection continues to gradually build and attempt to organize over Tropical Storm Dorian tonight. Started out as a few small popcorn showers/storms and now has grown into some of the deepest convection the storm has had in its entire lifetime within the past 4 hours. Will need to see the trends continue to be sure that Dorian is truly coming back to life.
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Quoting 1352. will40:
GFS further north on oz run


GFS always changes its mind.. like a roller caster.
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Well, that deescalated quickly.
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1400. Patrap
Please don't quote someone using Foul language.

Thank you.

Click the " ! " to report it.
Member Since: July 3, 2005 Posts: 428 Comments: 129844
Quoting 1387. JLPR2:
I'm no expert but I'm gonna guess that the warmer water are now making a better contrast to the falling air temperatures rendering D-max more efficient or beneficial for Dorian?

Any experts out there care to confirm, explain or deny? :)


For sure, warmer waters with the cooling air temps leading up to DMAX will increase lapse rates and give him chance to rebuild the MLC. Shear has lessened and the dry environment is being overcome now. He needs to develop an outflow channel to keep the party going...
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Quoting 1395. RufusBaker:
Dorian is crap.. probably a sign of whats to come. dying storms the rest of the season

You don't know anything,in fact I am ignoring you now.
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Quoting 1391. AussieStorm:
smartinWNTV Susie Martin Wx 13m
One heck of a #storm at the #Albuquerque Zoo! Microburst with 90 mph winds! Video here: http://instagram.com/p/cQMibYOFed/# pic.twitter.com/P5FA7LrpCy



Yikes!
Member Since: June 29, 2009 Posts: 13 Comments: 5315
Quoting 1356. Relix:
Dorian fighting back. Let's see what morning brings. Hopefully some miracle WSW movement and a decent CDO over the LLC so CaribBoy gets water.


Hopefully something GOOD will occur. We got 0.1 inch tonight.. we need 100 times more XD
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1394. Patrap
We Didn't Start The Fire

Member Since: July 3, 2005 Posts: 428 Comments: 129844
Quoting 1387. JLPR2:
I'm no expert but I'm gonna guess that the warmer water are now making a better contrast to the falling air temperatures rendering D-max more efficient or beneficial for Dorian?

Any experts out there care to confirm, explain or deny? :)


I will admit that I am not an expert, but I will agree with your assessment of the situation.
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Quoting 1387. JLPR2:
I'm no expert but I'm gonna guess that the warmer water are now making a better contrast to the falling air temperatures rendering D-max more efficient or beneficial for Dorian?

Any experts out there care to confirm, explain or deny? :)

that seems about right
Member Since: June 13, 2009 Posts: 2 Comments: 12716
smartinWNTV Susie Martin Wx 13m
One heck of a #storm at the #Albuquerque Zoo! Microburst with 90 mph winds! Video here: http://instagram.com/p/cQMibYOFed/# pic.twitter.com/P5FA7LrpCy

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Well it wouldnt let me post the image...but here it is!
Link
:D
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1389. Patrap
..the flue is open, the Fire is Growing.
Member Since: July 3, 2005 Posts: 428 Comments: 129844
1388. Patrap
Bike Duke NOLA 7 revives Dorian,

film at 11,

..wait, it is past 11 CDT.

Member Since: July 3, 2005 Posts: 428 Comments: 129844
1387. JLPR2
I'm no expert but I'm gonna guess that the warmer waters are now making a better contrast to the falling air temperatures rendering D-max more efficient or beneficial for Dorian?

Any experts out there care to confirm, explain or deny? :)
Member Since: September 4, 2007 Posts: 7 Comments: 8747
Quoting 1378. Grothar:
I declare a blo....



LOL.
Member Since: October 15, 2011 Posts: 0 Comments: 4467
Quoting 1315. Patrap:
.."Hang on Dorian, Dorian Hang on"..







Sloopy?
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1384. tj175
Quoting 1373. DavidHOUTX:
I wish I was sitting inside the NHC right now. I would love to hear the rumbling and internal discussion about Dorian. Storms starting to fire over the LLC. It would be a treat to say the least


Yeah I would love to be a fly on the wall. Maybe since the NHC is like 10 minutes from my house if I knocked on the door of the facility would they let me in, lol?
Member Since: September 26, 2010 Posts: 0 Comments: 103
Quoting 1378. Grothar:
I declare a blo....





Funny thing is, that's some of the coldest cloud tops he has had yet..
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1382. Patrap
updated

Tropical Storm 04L

UW-CIMSS Automated Satellite-Based
Advanced Dvorak Technique (ADT)
Version 8.1.4
Tropical Cyclone Intensity Estimation Algorithm

Current Intensity Analysis



UW - CIMSS
ADVANCED DVORAK TECHNIQUE
ADT-Version 8.1.4
Tropical Cyclone Intensity Algorithm

----- Current Analysis -----
Date : 27 JUL 2013 Time : 031500 UTC

Lat : 17:54:34 N Lon : 48:11:02 W


CI# /Pressure/ Vmax
2.8 /1009.1mb/ 41.0kt


Final T# Adj T# Raw T#
2.1 2.5 3.2

Center Temp : +16.1C Cloud Region Temp : -16.2C

Scene Type : SHEAR (0.15^ TO DG)

Positioning Method : FORECAST INTERPOLATION

Ocean Basin : ATLANTIC
Dvorak CI > MSLP Conversion Used : ATLANTIC

Tno/CI Rules : Constraint Limits : 0.5T/hour
Weakening Flag : ON
Rapid Dissipation Flag : OFF

C/K/Z MSLP Estimate Inputs :
- Average 34 knot radii : 50km
- Environmental MSLP : 1018mb

Satellite Name : MSG3
Satellite Viewing Angle : 57.9 degrees

Member Since: July 3, 2005 Posts: 428 Comments: 129844
Quoting 1372. tj175:



Can you located where the COC is?

Pretty close to being fully under the deepest convection. Now if Dorian could sustain, organize, and develop more convection while keeping it's llc together then it has a chance. If it fully loses it's llc then I don't think it would be able to come back. There sure have been a lot of "ifs" with Dorian.
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Quoting 1377. ncstorm:


seems to be following the euro

Was going to say the same, the euro has been consistent with an approach to Miami..
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well will you look at that looks like Dorian is back in the game I told you guys that Dorian needed the D-Max I was right by the time this hits peak D-Max it should look a lot better
Member Since: June 13, 2009 Posts: 2 Comments: 12716
1378. Grothar
I declare a blo....



Member Since: July 17, 2009 Posts: 71 Comments: 27208

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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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