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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Quoting 1269. Bluestorm5:
I've been through 100 mph thunderstorms and I don't mind seeing them again. Wondering what it's like to be in hurricane.
I have always wondered what it is like myself, living here in the Tampa Bay area the highest I have seen was an 80 mph gust from Frances. I think sustained winds at the time were anywhere from 50-60 mph. and I was outside feeling the winds, it was really awesome man.
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1277. Patrap
Member Since: July 3, 2005 Posts: 414 Comments: 125476
I still think Dorian has a chance for redevelopment, yes its an open wave now, but conditions are looking a bit more favorable ahead for redevelopment. I wouldn't discount just yet and watch what happens.
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Quoting 1271. unknowncomic:
At first, I thought downcasting. Then again, some models call for strengthening again and these small ones are usually the RI ones. That maybe why sooner the better.

Is it still headed to the Gulf or will it turn north to the east coast and or FL?
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Quoting 1268. GTstormChaserCaleb:
I see the Ships model predicts a hurricane. Like I said, not giving up on Dorian until I don't see a cloud in the sky near him.
agreed !!
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1273. will40
Quoting 1269. Bluestorm5:
I've been through 100 mph thunderstorms and I don't mind seeing them again. Wondering what it's like to be in hurricane.


i went through Hurricane Hazel. wasnt a fine time
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Quoting 1241. Patrap:
..somewhere over the Rainbow Top..


yeap, its approaching 50 w, some convection should fire up a little.I'll tell you one thing though, this is just the start of a very busy and possible destructive hurricane season, especially the way the steerning patterns are laid out. US is most likely going to experience a hurricane this season.
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Quoting 1254. ecflweatherfan:
Avila "...HOPEFULLY SOONER"... What does he mean by that? Is he being a downcaster. Or does he mean that if it doesn't dissipate by then, the s*** is gonna hit the fan when it gets to the Bahamas?
At first, I thought downcasting. Then again, some models call for strengthening again and these small ones are usually the RI ones. That maybe why sooner the better.
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Quoting 1263. Grothar:
00Z models quite a bit higher than before.



No more flirting with Hispaniola and PR. Getting closer to southern Florida.



Shear beginning to fall in wake of Dorian. I see why the models want to increase it strength over the Bahamas If it survive.
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Quoting 1232. ncstorm:
I have been through A LOT of hurricanes..we even started naming them here on WU one night about a month ago and I didnt realize how many I have been through..I have been without power for over three weeks..stood in line for ice in 90 degree NC summer weather, had no running water, stood in line for a hot dinner, lost property and with that being said, I still enjoy the thrill of hurricanes..there is nothing wrong in wanting to experience a hurricane..just because someone wants to doesn't mean that they are wishing for death of people..some of yall need to relax..
I've been through 100 mph thunderstorms and I don't mind seeing them again. Wondering what it's like to be in hurricane.
Member Since: August 1, 2011 Posts: 23 Comments: 7449
Quoting 1220. MiamiHurricanes09:
Worth watching to see what Dorian does in about 3 days as it approaches 70W. Some intensity models show the possibility of re-intensification, and environmental conditions shouldn't be too bad in that region. Whatever energy is left could definitely pose a threat to the Bahamas, Cuba, and Florida, so we'll see.

I see the Ships model predicts a hurricane. Like I said, not giving up on Dorian until I don't see a cloud in the sky near him.
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Quoting 1205. plywoodstatenative:
To those in here who say that they would love to see a major cane. I will say this: For those of us on here that are survivors of Hurricane Andrew or Katrina, systems like that you DO NOT want to have happen all over. You say you want to see a system like that, but what if it were your friends or relatives that had to ride out that system? What then would you say, would you still desire to see one form? Remember this, do not wish for something that you can not yourself live through.


Whether or not nature conjures up a major or not is not up to any of us. Nothing wrong at all at wanting to see a major. They are amazing acts of nature. Majors happen every single year and rooting for the inevitable to form doesn't make one a bad person. If my family or friends decide to ride it out then it then becomes their bad choice. Rooting for a major will have zero impact on what happens in real time. Most watch to learn, share, and follow closely the few majors that do pose a threat. It's the height of the season when that happens. Many of us are delighted at the beauty of the storm, and pray for those in the path. The two don't have to be at odds.
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1266. docrod
Quoting 1263. Grothar:
00Z models quite a bit higher than before.



Sump'em here in the Keys mid week 4 sure
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1265. Grothar
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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.

Moonlight that was my thoughts exactly. I dont know a thang about these storms but it just looked to me as if another was forming. Could it be the llc is indeed relocating?
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1263. Grothar
00Z models quite a bit higher than before.

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1262. docrod
,spit

Link

105F heat index ....
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1261. Pallis
Quoting 1164. canehater1:
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...
Smart natives don't live in those areas, and if they do have property there, they evacuate early. We don't give out this kind of information for free in a handbook because we are tired of carpetbaggers polluting our coast.
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Quoting 1251. BaltimoreBrian:
Dorian's Degenerative Death Disappoints Droves; Doubters Dismiss Dorian's Demise; Dominicans Desire Disaster Deterrence.


Woah. That's a good one. *Tear*
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For Atlantic side, Gilbert for me.



For overall, I got legendary Typhoon Tip. That cyclone broke all kind of records.



Member Since: August 1, 2011 Posts: 23 Comments: 7449
look at tropical storm Flossie!! nice looking tropical storm!
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Quoting 1226. Grothar:
I do not see a due west movement. I have seen a consistently WNW movement from the beginning. Regardless of the strength of the system and moving in tandem with the high, I still feel a slight erosion at the end of the run will keep it on a basically WNW motion. I did post 4 times today to not write this one off.

Look at the convection and retreating dry air and the steering currents.




Disclaimer: Due to the age and deteriorating health of the author of the above opinion, take it with a grain of salt. If you take too much salt you will retain water.





I will not even try to best that one. .... Well played, my friend. Well played. :)
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Dorian may have one last chance.
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I was catching up on the last 400 comments here. I love what ya'll post, it becomes fodder for a future blog post of mine. Some week in September I'll rip 10 blogs off, lol.

Keep it up!
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Avila "...HOPEFULLY SOONER"... What does he mean by that? Is he being a downcaster. Or does he mean that if it doesn't dissipate by then, the s*** is gonna hit the fan when it gets to the Bahamas?
Member Since: March 19, 2010 Posts: 0 Comments: 1146
Quoting 1232. ncstorm:
I have been through A LOT of hurricanes..we even started naming them here on WU one night about a month ago and I didnt realize how many I have been through..I have been without power for over three weeks..stood in line for ice in 90 degree NC summer weather, had no running water, stood in line for a hot dinner, lost property and with that being said, I still enjoy the thrill of hurricanes..there is nothing wrong in wanting to experience a hurricane..just because someone wants to doesn't mean that they are wishing for death of people..some of yall need to relax..
I had a tree crash on the back of my house,no power for two weeks..flooded during (some storm passing by in the 90's I think it was Flyod I keep thinking Fran) and most recently Sandy and Irene (Derecho kinda counts since we experienced hurricane force gust).But even with that I'm still in awe of these amazing storms.Just don't want them affecting populated areas.
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I've stayed for: Cilia, Rita, Frances, Ike, and Gustav. Loss of power is the worst part and the best description for a CAT 3 coming in is like a bomb going off around your house.
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Dorian's Degenerative Death Disappoints Droves; Doubters Dismiss Dorian's Demise; Dominicans Desire Disaster Deterrence.
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Quoting 1241. Patrap:
..somewhere over the Rainbow Top..



Getting interesting Pat, convection started out small and shallow but slowly but surely growing and getting colder. If this had warmer waters we might have had a hurricane two days ago.
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Quoting 1243. Patrap:
Man, Hot July night, cold as ice watermelon, Lawdy'

spit,


I'll take some of that heat if ya want Pat, only got to 83 here today, 7 below average.
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1248. docrod
Quoting 1225. moonlightcowboy:


Gotta love Avila!


Agreed Moonlight ...
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Quoting 1209. sar2401:

Hopefully it will be a good storm, especially for the Big Island, which I know needs the rain. Any idea which islands are expected to affected most? I know the last time I was on Maui, it seemed to flood from even moderate rainstorms.


Yeah, the Big Island is suffering drought during this time of year. Hopefully, Flossie will bring beneficial rain to that area by early to middle of next week when it arrives. The eastern half of the Hawaiian Islands (Big Island and Maui County) would feel its impacts first. The rest of the islands would also be affected. Hopefully, its impacts won't be too bad.
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It's impressive what a strong LLC can do for a system, even as small as Dorian. Seen it happen before, no doubt Dorian would have bit the dust(literally) by now if not for that. Warm SST's ahead could do a small system like him some good, but only if he can utilize the convection he's firing now and up until peak D-Max. Also appears that the dry air is abating some. Will be interesting to see what he's like come tomorrow morning.
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NWSNewOrleans ‏@NWSNewOrleans 2m
Watching storms near the AR/LA border. This activity will continue to work south overnight and could move into our region around 5am.
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Quoting 1237. PedleyCA:
Rained again here today. .13 by the PWS up the hill. Must not have across the river as the Airport said a Trace. So the this is the 2nd time it rained in July which is unheard of. .23 for the month. We have rain barely thanks to the monsoon flow. Take what you can get. Temps 67.7/86.4


I've counted 5 days in July with water falling from the sky here in the San Fernando Valley. It was apparently only measurable precipitation on two of those days, though.
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1243. Patrap
Man, Hot July night, cold as ice watermelon, Lawdy'

spit,
Member Since: July 3, 2005 Posts: 414 Comments: 125476
1242. Gearsts
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1241. Patrap
..somewhere over the Rainbow Top..

Member Since: July 3, 2005 Posts: 414 Comments: 125476
1240. sar2401
Quoting moonlightcowboy:


Gotta love Avila!

I'm almost certain he wrote one of the last ones on the Greek name storms in 2005. You could almost feel that he wanted to reach right through the computer and choke someone or something. :-)
Member Since: October 2, 2004 Posts: 0 Comments: 9656
Quoting 1226. Grothar:
I do not see a due west movement. I have seen a consistently WNW movement from the beginning. Regardless of the strength of the system and moving in tandem with the high, I still feel a slight erosion at the end of the run will keep it on a basically WNW motion. I did post 4 times today to not write this one off.

Look at the convection and retreating dry air and the steering currents.




Disclaimer: Due to the age and deteriorating health of the author of the above opinion, take it with a grain of salt. If you take too much salt you will retain water.





You are correct on the WNW idea... so many have said it is moving true west, almost from the time it formed.. yet the true track over time has continued to be around 290 or so
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Good Night all ! We shall see what he does , doesn't do ! You all have a good night .
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Rained again here today. .13 by the PWS up the hill. Must not have across the river as the Airport said a Trace. So the this is the 2nd time it rained in July which is unheard of. .23 for the month. We have rain barely thanks to the monsoon flow. Take what you can get. Temps 67.7/86.4
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1236. docrod
OR HOPEFULLY SOONER.

FORECASTER AVILA

Dr. Avila is perhaps the most colorful and insightful forecaster at the NHC.
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Quoting 1220. MiamiHurricanes09:
Worth watching to see what Dorian does in about 3 days as it approaches 70W. Some intensity models show the possibility of re-intensification, and environmental conditions shouldn't be too bad in that region. Whatever energy is left could definitely pose a threat to the Bahamas, Cuba, and Florida, so we'll see.



Very interesting although the storm is barely alive some model still that it up to cane status north of the Bahamas. what happened to TUTT? maybe its weaker and moving north out the way. Always have to watch them even when they are on there last leg. Gotta love hurricane season.
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now the thing is if Dorian increases enough convection tonight he may be able to restrengthen his Low and mid level core eventually strengthening the storm its self and staying alive longer that what I have to say for the next 3-4 hours I am out for an hour or two I'll be back soon
Member Since: June 13, 2009 Posts: 2 Comments: 9548
Shoot I could have had a 13 word alliterative sentence. Sigh......
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1232. ncstorm
I have been through A LOT of hurricanes..we even started naming them here on WU one night about a month ago and I didnt realize how many I have been through..I have been without power for over three weeks..stood in line for ice in 90 degree NC summer weather, had no running water, stood in line for a hot dinner, lost property and with that being said, I still enjoy the thrill of hurricanes..there is nothing wrong in wanting to experience a hurricane..just because someone wants to doesn't mean that they are wishing for death of people..some of yall need to relax..
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1231. sar2401
Quoting Grothar:
I do not see a due west movement. I have seen a consistently WNW movement from the beginning. Regardless of the strength of the system and moving in tandem with the high, I still feel a slight erosion at the end of the run will keep it on a basically WNW motion. I did post 4 times today to not write this one off.

Look at the convection and retreating dry air and the steering currents.




Disclaimer: Due to the age and deteriorating health of the author of the above opinion, take it with a grain of salt. If you take too much salt you will retain water.




LOL, Gro. I took that one with an entire deer lick and it still didn't go down quite right. :-)
Member Since: October 2, 2004 Posts: 0 Comments: 9656
Hurricane Dorian, but maybe in 2019....
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1229. 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 : 021500 UTC
Lat : 18:03:29 N Lon : 47:32:57 W


CI# /Pressure/ Vmax
3.0 /1006.6mb/ 45.0kt


Final T# Adj T# Raw T#
2.0 2.0 1.5

Center Temp : +15.4C Cloud Region Temp : -5.2C

Scene Type : SHEAR (>1.25^ TO DG)

Positioning Method : FORECAST INTERPOLATION

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

Tno/CI Rules : Constraint Limits : 1.0T/6hr
Weakening Flag : ON
Rapid Dissipation Flag : FLAG

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

Satellite Name : MSG3
Satellite Viewing Angle : 57.3 degrees


Member Since: July 3, 2005 Posts: 414 Comments: 125476
1228. Relix
I am calling NEXT for us at PR. Hopefully no good waves until later.
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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.