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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1378. Grothar
I declare a blo....



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1377. ncstorm
Quoting 1369. will40:


north of Cuba at 87 hrs


seems to be following the euro
Member Since: Posts: Comments:
Quoting 1359. TideWaterWeather:


yes they pegged it at 1010 and 1009 would mean it was strengthening... It is important to also remember that the actual strength of a storm, when discussing pressure, is relative to the surrounding environment which is at about 1018 mb.
thank you for explaining it to me.As I said I dont know enough to say a whole lot. But when I do see some things I try to ask questions to learn.Again thank you for your kindness
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1375. Patrap
Lat : 17:53:26 N Lon : 48:00:57 W

Member Since: July 3, 2005 Posts: 426 Comments: 128874
1374. Patrap
Quoting 1326. Patrap:
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 : 024500 UTC
Lat : 17:53:26 N Lon : 48:00:57 W


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


Final T# Adj T# Raw T#
2.0 2.5 3.0

Center Temp : +15.4C Cloud Region Temp : -9.8C

Scene Type : SHEAR (0.29^ 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 : 57.8 degrees

Member Since: July 3, 2005 Posts: 426 Comments: 128874
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
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1372. tj175
Quoting 1363. Patrap:
..Gimmie back, gimmee back my Bullet's




Can you located where the COC is?
Member Since: September 26, 2010 Posts: 0 Comments: 100
Dorian also needs some outflow that would support continued development of thunderstorms. Right now it has no ventilation at all.
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Quoting 1345. Patrap:
..it was touch and go for a spell there, but It's doing better his evening..



Still in critical condition doctor, what's your medical advice?
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1369. will40


north of Cuba at 87 hrs
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1368. Grothar
Quoting 1359. TideWaterWeather:


yes they pegged it at 1010 and 1009 would mean it was strengthening... It is important to also remember that the actual strength of a storm, when discussing pressure, is relative to the surrounding environment which is at about 1018 mb.





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Quoting 1364. TideWaterWeather:


I will give you a cookie if you can spell correctly
((((((((((((((((I gots a pictures))))))))))))))))



My motto for that is from the USPS.

"If it fits, it ships"
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Quoting 1361. Stormchaser121:

Wouldn't it head to the gulf then if those 2 highs are there?


It should.

87 hrs

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1365. Patrap
Member Since: July 3, 2005 Posts: 426 Comments: 128874
Quoting 1360. SPLbeater:


Something to be proud of :)

I gots a pictures I want to show here, but uploading to WU would take too long. Need it on IE with URL...


I will give you a cookie if you can spell correctly
((((((((((((((((I gots a pictures))))))))))))))))
Member Since: Posts: Comments:
1363. Patrap
..Gimmie back, gimmee back my Bullet's

Member Since: July 3, 2005 Posts: 426 Comments: 128874
Quoting 1343. cchsweatherman:


I noticed this too. Will be interesting to track this to see if this could be part of an expansion of the convection and organization taking place.


I'm hesitant to say that at this point. Dorian needs to continue this convective trend for several more hours. Furthermore, it can be difficult to locate a COC at night. First visibles will be telling to see how much Dorian has really come back overnight.
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Quoting 1350. Civicane49:
North of Puerto Rico in 57 hours.


Wouldn't it head to the gulf then if those 2 highs are there?
Member Since: September 2, 2011 Posts: 0 Comments: 1148
Quoting 1351. JLPR2:


One feels important when noticing stuff. I remember I was one the first that noticed the weak disturbance that spawned Hurricane Michael and was overjoyed when it developed. :P


Something to be proud of :)

I gots a pictures I want to show here, but uploading to WU would take too long. Need it on IE with URL...
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Quoting 1354. mrsalagranny:
Maybe I am wrong but didnt NHC say pressure was 1010 mb? This says 1009 mb so doesnt that mean its strengthening? Again I dont know a lot about this stuff.


yes they pegged it at 1010 and 1009 would mean it was strengthening... It is important to also remember that the actual strength of a storm, when discussing pressure, is relative to the surrounding environment which is at about 1018 mb.
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Quoting 1305. weatherganny:
Is Texas still protected by the high that has protected us in the past several years?TIA


It will be during the next 5 days or so starting Sunday. After that, it is anyones guess. If a tropical storm was to show up in the GOM next Thursday or Friday, things will get interesting for all of the Gulf states as well as Mexico.
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1357. Patrap
The ADT is a best guess estimate as well
Member Since: July 3, 2005 Posts: 426 Comments: 128874
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.
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1355. Patrap
Tenacious "D"



Member Since: July 3, 2005 Posts: 426 Comments: 128874
Quoting 1326. Patrap:
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 : 024500 UTC
Lat : 17:53:26 N Lon : 48:00:57 W


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


Final T# Adj T# Raw T#
2.0 2.5 3.0

Center Temp : +15.4C Cloud Region Temp : -9.8C

Scene Type : SHEAR (0.29^ 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 : 57.8 degrees

Maybe I am wrong but didnt NHC say pressure was 1010 mb? This says 1009 mb so doesnt that mean its strengthening? Again I dont know a lot about this stuff.
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1353. Patrap
Member Since: July 3, 2005 Posts: 426 Comments: 128874
1352. will40
GFS further north on oz run
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1351. JLPR2
Quoting 1344. SPLbeater:


After being named Wave SPL....it'll do anything. Lol.


One feels important when noticing stuff. I remember I was one the first that noticed the weak disturbance that spawned Hurricane Michael and was overjoyed when it developed. :P
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North of Puerto Rico in 57 hours.

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1349. Grothar
Quoting 1340. ecflweatherfan:
Just a question... Does anyone see/notice the appendage moving east on the farthest SW thunderstorm complex on Dorian (on the Rainbow imagery channel). Looks like it may be trying to wrap up? Seems to be very near the neighborhood of the CoC.


yes.

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Quoting 1332. Patrap:
"Yes, we see Dorian's up-tick NHC Director, we're on it"

He's busy discussing storms with me on Twitter :)
Member Since: August 1, 2011 Posts: 28 Comments: 8032
Quoting 1193. Pallis:
Really? You guys are emotionally retarded hypocrites! I want to see a category 10 destroy my entire coast because I hate Yankees,jetskis,speedboats,megayachts,tourist traps, seawalls, and condos! I do not care who I piss off, we were here before this was a state. Go home if you don't like it, and take your illegal aliens with you. I say the storm is going on relative track with the computer models and will flare up tomorrow if it reaches the quite warmer water ahead right ahead of it. Wind shear is the only enemy, and the barely visible (Ull?) below it seems to be in tandem.


WOW..well more power to you. But being in the UK, unlikely to have any hit here anyway. I may not like many others...and likely many of those with condos and mega yachts, I still have no desire to watch a cane destroy anywhere. I'm just fascinated with weather and natural events...whether rural and uninhabited or not. I just hope not. But will be fascinated as much by ones that do hit places inhabited as uninhabitated, as it's all about what the weather or any natural forces is doing for me. Not sure how it would make me a hypocrit, but to each his own idea I suppose.
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1346. DDR
Good evening
It feels good to have some much needed rain here in Trinidad,its been abnormally dry but its raining at the moment.
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1345. Patrap
..it was touch and go for a spell there, but It's doing better his evening..

Member Since: July 3, 2005 Posts: 426 Comments: 128874
Quoting 1338. JLPR2:


Yes there is a spin ahead of the convection, but it's been eaten by SAL. And I see that in the last update some vorticity has appeared right offshore closer to the disturbance.




Hmm... it might try something.


After being named Wave SPL....it'll do anything. Lol.
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Quoting 1340. ecflweatherfan:
Just a question... Does anyone see/notice the appendage moving east on the farthest SW thunderstorm complex on Dorian (on the Rainbow imagery channel). Looks like it may be trying to wrap up? Seems to be very near the neighborhood of the CoC.


I noticed this too. Will be interesting to track this to see if this could be part of an expansion of the convection and organization taking place.
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Quoting 1337. Stormchaser121:
I went through Humberto in 2007 and everyone I knew was either huddled in a corner in the fetal position going "mommy mommy" or slept through it....I however...was OUTSIDE IN THE STORM recording it! It doesn't scare me...it really doesn't I was absolutely EXCITED.


Sorta like me at the beach a month or so ago...Had a severe storm coming in, everyone was inside while I was outside. The wind would gust to like 40 or 50mph for 2-3 seconds then stop. Then it would do it again, and again...was my first real feel of a high wind event.
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Quoting beell:
Anybody think that convection is moving southwest?
Indicative of an open wave?




Looks like it's going to go back to a T-wave very soon. has the classic V shape appearing.

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Just a question... Does anyone see/notice the appendage moving east on the farthest SW thunderstorm complex on Dorian (on the Rainbow imagery channel). Looks like it may be trying to wrap up? Seems to be very near the neighborhood of the CoC.
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Seems like we may have the beginnings of a potential CDO taking shape over the circulation center of Dorian tonight. Convection with cloud tops to -70 C have popped and seem to be expanding now.
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1338. JLPR2
Quoting 1328. SPLbeater:


Looked like a very broad spin between it and the Cape Verde's...Dry air is very much present though. Some cold cloud tops within that convection. I am naming this "Wave SPL" :D


Yes there is a spin ahead of the convection, but it's been eaten by SAL. And I see that in the last update some vorticity has appeared right offshore closer to the disturbance.




Hmm... it might try something.
Member Since: Posts: Comments:
I went through Humberto in 2007 and everyone I knew was either huddled in a corner in the fetal position going "mommy mommy" or slept through it....I however...was OUTSIDE IN THE STORM recording it! It doesn't scare me...it really doesn't I was absolutely EXCITED.
Member Since: September 2, 2011 Posts: 0 Comments: 1148
Unless I'm missing something, the GFS appears to keep Dorian closed for the next day. Weak, but still closed.
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1335. help4u
can anyone post the latest gfs run
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Quoting 1329. TideWaterWeather:
Rapid Dissipation Flag : OFF


I generally find those useless.
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1332. Patrap
"Yes, we see Dorian's up-tick NHC Director, we're on it"

Member Since: July 3, 2005 Posts: 426 Comments: 128874
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Quoting 10. odinslightning:
.....aaaah tabula rasa.....hopefully today there wont be as much arguing and out and out unjustified guessing (mainly in the hopes of a large cane hitting CONUS).....

even as a cat adjuster i find it disturbing to see so many people cheering for a potent large hurricane....i worked the beachhead from pascagoula to n.o in Katrina......and trust me, not even us good cat adjusters want a huge devastating hurricane.....it is heartbreaking when they happen, so please wishcasters, quit wishing for destruction.....


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. ;-)
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Rapid Dissipation Flag : OFF
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Quoting 1299. JLPR2:
Not a bad looking disturbance, but no spin whatsoever at the 850mb level.



Looked like a very broad spin between it and the Cape Verde's...Dry air is very much present though. Some cold cloud tops within that convection. I am naming this "Wave SPL" :D
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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.