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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Member Since: Posts: Comments:
Quoting 1973. SFLWeatherman:
Chance of precipitation is 60% today!!:)


What about Monday :)
Member Since: May 25, 2012 Posts: 0 Comments: 2043
Sea surface temperature increase to 28C along with moisture around 55W. So its bulking up till it gets there by 2pm. After that it has a chance of strengthening just in time for the 5pm advisory. If this happens Tropical storm watches may be issued for the Northern Hispaniola and/or SE Bahamas (Turks and Caicos) by 11pm tonight granted it survive today. Next 24-48 hours is crucial!
Member Since: May 25, 2012 Posts: 0 Comments: 2043
1975. LargoFl
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Quoting 1966. JrWeathermanFL:

Looking at that map..it has stronger shear above Dorian. Even though he will move into the reigon with higher shear, the shear above him might aid in outflow.

it is decreasing

Quoting 1967. Grothar:


That deserves a LOL and a toot!!

deserves double LOL LOL and Bah Bah

Member Since: June 13, 2009 Posts: 2 Comments: 12324
Chance of precipitation is 60% today!!:)
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Quoting 1961. Grothar:
This is a little switch from the earlier model.



That is about as tight a cluster of model tracks as you will ever see...
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1971. hu2007
wow i will say that dorian will soon be history and after dorian i dont see anymore development until at least mid aug, there will be a lot of shear in the central tropical atlantic and still a lot of mid and upper dry air mix with dust of africa in place, very hard to any system to try and develops under this condition soo this may change a little the outlook for 2013 season heart of the activity in the atlantic and by the way a interesting storm flossie heading to hawaii but same case will happen to flossie with dry air and shear too weaken the storm in central pacific
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heck the way things are looking miami officialsmight have to take down the banners that welcomed chantel & erase that and put dorian.
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Quoting 1956. Grothar:
What I find remarkable about Dorian, is that is has never once gone due west. It has maintained a continuous WNW motion the entire time.





yes it has well not 270 degrees W but just Due N of W at 274 degrees was the closest and it moving W now

Quoting 1961. Grothar:
This is a little switch from the earlier model.


Cuba/NW Carib on to GOM
Member Since: June 13, 2009 Posts: 2 Comments: 12324
1967. Grothar
Quoting 1953. canehater1:


lol actually as Gro has stated, there were others who said "Not so Fast" when we RIP'd Dorian...so maybe a
band is in order....


That deserves a LOL and a toot!!
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Quoting 1939. Hurricanes305:
Wind Shear continues to see widespread decreasing as the ULL has started to weaken. Our TS remains in 5-15kt of Shear. Satellite imagery suggest a more anticyclonic flow may start thats just premature.



Upper level remains probably the only thing will stop Dorian. We shall see how the ULL interacts with Dorian if its does favorably the Bahamas, Cuba, and Florida will be in for a very potent system. By the end of the day we should get a better understanding of whether Dorian lives or dissipate.

Looking at that map..it has stronger shear above Dorian. Even though he will move into the reigon with higher shear, the shear above him might aid in outflow.
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1965. Grothar
Quoting 1958. GeoffreyWPB:


Yes...I was popping in and out yesterday and missed a lot of other posts. Congrats to all and I'll make another batch of cookies for everyone. I'll even ask Nea if he can bake a few pies.


What a nice guy. So what do the IntelliGeoff models tell you today about Dorian?
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I'm going to a family reunion in southern M.D today so I won't be around to see Dorian's challenges like that.Besides this storm bores me.Toddles.
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I guess I got 20 more minutes... well, Dorian is little warmer as I mentioned earlier, but I guess we'll have to see what new convection it's firing up will do to the storm.
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Quoting 1950. CaribBoy:


Yeah definitely! That's crazy how weather can influence my mood XD I feel so good and healthy when something is coming my way. And depressed otherwise lol

yeah I know how you feel I use to be like that until 01 when the M storm passed here the waves where massive like waves you would only see in movies anyway my mother was outside when this this was passin us and the sea came up and took my mother dragged her up our lawn slapped her on the concrete wall surrounding our Front and sides and dragged her towards the sea luckily for her me and a few other jumped in to save her and then more so in 04 with Ivan and well that was self-explanatory what Ivan done to Cayman
Member Since: June 13, 2009 Posts: 2 Comments: 12324
1961. Grothar
This is a little switch from the earlier model.

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Wow look at that LGEM up to cat 1 hurricane
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Quoting 1953. canehater1:


lol actually as Gro has stated, there were others who said "Not so Fast" when we RIP'd Dorian...so maybe a
band is in order....


Turn it up to 11 to let everyone know loud and clear... don't jump the gun on killing storms!
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Quoting 1902. Grothar:


In all fairness, wunderkidcayman also said it would come back. What I think Geoffrey is referring to, is that since yesterday afternoon, I wrote a number of times not to write Dorian off, and last night gave a detailed description why. Although there were a number of bloggers who also saw signs of it coming back strong and they all wrote it and it is still there to read. Some bloggers gave very good reasons, like Caleb and Allstar and Kori and Patrap, Hurrican305 and a number of others, probably too many to count. They all did a good job with Dorian.


Yes...I was popping in and out yesterday and missed a lot of other posts. Congrats to all and I'll make another batch of cookies for everyone. I'll even ask Nea if he can bake a few pies.
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1957. LargoFl
canadian GEM at 222 hours..one more gets pulled northward..
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1956. Grothar
What I find remarkable about Dorian, is that is has never once gone due west. It has maintained a continuous WNW motion the entire time.




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1954. will40
000
NOUS42 KNHC 261503
REPRPD
WEATHER RECONNAISSANCE FLIGHTS
CARCAH, NATIONAL HURRICANE CENTER, MIAMI, FL.
1100 AM EDT FRI 26 JULY 2013
SUBJECT: TROPICAL CYCLONE PLAN OF THE DAY (TCPOD)
VALID 27/1100Z TO 28/1100Z JULY 2013
TCPOD NUMBER.....13-056

I. ATLANTIC REQUIREMENTS
1. NEGATIVE RECONNAISSANCE REQUIREMENTS.
3. SUCCEEDING DAY OUTLOOK: BEGIN 6-HRLY FIXES FOR TROPICAL
STORM DORIAN AT 28/1730Z NEAR 19.5N 60.0W.

II. PACIFIC REQUIREMENTS...........................(NO CHNAGES)
1. NEGATIVE RECONNAISSANCE REQUIREMENTS.
2. OUTLOOK FOR SUCCEEDING DAY.....NEGATIVE.

$$
JWP
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Quoting 1925. Grothar:


I'm the one with the bigger horn. :)


lol actually as Gro has stated, there were others who said "Not so Fast" when we RIP'd Dorian...so maybe a
band is in order....
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Quoting 1945. ryang:
Is RECON scheduled to fly into Dorian? That would give us a much better idea of what's going on.

yep yes it is

Quoting 1946. washingtonian115:
Pick your one..

lol

Member Since: June 13, 2009 Posts: 2 Comments: 12324
1951. LargoFl
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Quoting 1935. wunderkidcayman:

yep


Nah I say TS into Cuba the into the NW Carib then into the GOM

with that high moving SW and with it orientating its self SW-NE it could shift Dorian WSW for a bit and then continue W Cariboy would be very very happy


Yeah definitely! That's crazy how weather can influence my mood XD I feel so good and healthy when something is coming my way. And depressed otherwise lol
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never knew bloggers could "keep a system alive"

LOL

I don't mind Dorian hanging around, for entertainment purposes for this community...however, I would rather the storm stay at its current intensity or even lower and just be a rain event as it nears Florida. So call me a downcaster. I'll take that badge.



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Quoting 1939. Hurricanes305:
Wind Shear continues to see widespread decreasing as the ULL has started to weaken. Our TS remains in 5-15kt of Shear. Satellite imagery suggest a more anticyclonic flow may start thats just premature.



Upper level remains probably the only thing will stop Dorian. We shall see how the ULL interacts with Dorian if its does favorably the Bahamas, Cuba, and Florida will be in for a very potent system. By the end of the day we should get a better understanding of whether Dorian lives or dissipate.

well with shear decreasing all in the Caribbean and for some parts N of the Caribbean it would kinda be better off for Dorian to go Caribbean Dorian has that anticyclonic flow on it now so it just need to continue pushing forward and continue fighting for her right to be on the books

Member Since: June 13, 2009 Posts: 2 Comments: 12324
1947. LargoFl
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Quoting 1943. Bluestorm5:
Honestly about Dorian's future? We all got no idea. Experts don't have idea. Clever bloggers on here got no idea. Us teens (TA13, me, few others) got no idea. We just got to see how Dorian do against DMIN before even debating about anything else.
Pick your one..
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1945. ryang
Is RECON scheduled to fly into Dorian? That would give us a much better idea of what's going on.
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I'm out for work. Peace out.
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Honestly about Dorian's future? We all got no idea. Experts don't have idea. Clever bloggers on here got no idea. Us teens (TA13, me, few others) got no idea. We just got to see how Dorian do against DMIN before even debating about anything else.
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1942. Grothar
Quoting 1937. hydrus:
I said yesterday that Dorian would have some decent convective bursts due to warmer water, but if the shear forecast near the Antilles does not ease up, Dorian will have more trouble. By the way, I never write any spin off until it has vanished completely..Storms can form in a flash.


hydrus, How could I have forgotten you. Yes, I can almost remember the time you wrote that. Sorry for not including you. I have to give one of my cookies back now.
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1941. LargoFl
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1940. LargoFl
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Wind Shear continues to see widespread decreasing as the ULL has started to weaken. Our TS remains in 5-15kt of Shear. Satellite imagery suggest a more anticyclonic flow may start thats just premature.



Upper level remains probably the only thing will stop Dorian. We shall see how the ULL interacts with Dorian if its does favorably the Bahamas, Cuba, and Florida will be in for a very potent system. By the end of the day we should get a better understanding of whether Dorian lives or dissipate.
Member Since: May 25, 2012 Posts: 0 Comments: 2043
1938. LargoFl
Taken from another blog.....................Tropical Storm Dorian continues to struggle against a dry air environment and wind shear. However, the latest enhanced satellite images suggest the system is still alive and in fairly good shape considering the odds.
Mid level outflow, feather like cirrus fingers, are present from NNW through to the ENE sectors.
The green denotes the highest cloud tops, most are centrally located. Although they aren't clustered.
The darker shades of white, grey and black are areas of moist air, what's left of it.
It appears that Dorian is still somewhat encased in a small pocket of moisture.
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1937. hydrus
Quoting 1902. Grothar:


In all fairness, wunderkidcayman also said it would come back. What I think Geoffrey is referring to, is that since yesterday afternoon, I wrote a number of times not to write Dorian off, and last night gave a detailed description why. Although there were a number of bloggers who also saw signs of it coming back strong and they all wrote it and it is still there to read. Some bloggers gave very good reasons, like Caleb and Allstar and Kori and Patrap, Hurrican305 and a number of others, probably too many to count. They all did a good job with Dorian.
I said yesterday that Dorian would have some decent convective bursts due to warmer water, but if the shear forecast near the Antilles does not ease up, Dorian will have more trouble. By the way, I never write any spin off until it has vanished completely..Storms can form in a flash.
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The gulf has been very favorable all month long..We just haven't had a significant disturbance in there to take advantage of these conditions.
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Quoting 1928. EyEtoEyE:
RIPing Dorian looks to be nonsense , please don't play TAPS yet ! And when we get Erin she will be a cane .

yep

Quoting 1930. HimacaneBrees:
If Dorian somehow survives and shoots the gap between Florida and Cuba then gets into the Gulf. Could it be serious trouble for the Gulf Coast? Are the conditions favorable in the Gulf ATM? I know the water is warm.

Nah I say TS into Cuba the into the NW Carib then into the GOM

with that high moving SW and with it orientating its self SW-NE it could shift Dorian WSW for a bit and then continue W Cariboy would be very very happy
Member Since: June 13, 2009 Posts: 2 Comments: 12324
1934. LargoFl
gulf waters are warm and moist and the winds are light....
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1933. LargoFl
Quoting 1923. GeoffreyWPB:


It has to slow down that forward motion, huh.
yes it was his forward speed that ruined him, he outran his moisture but if he can slow down some when he hits that good moisture tomorrow..the tracks may change
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1932. Grothar
Quoting 1923. GeoffreyWPB:


It has to slow down that forward motion, huh.


I don't think it will make it through the day. :)
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Quoting 1925. Grothar:


I'm the one with the bigger horn. :)

lol
Member Since: June 13, 2009 Posts: 2 Comments: 12324
If Dorian somehow survives and shoots the gap between Florida and Cuba then gets into the Gulf. Could it be serious trouble for the Gulf Coast? Are the conditions favorable in the Gulf ATM? I know the water is warm.
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1929. LargoFl
remember a few days ago, the models says he dies on those huge mountains, today they say no..now its Cuba...she hasnt sung yet folks
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RIPing Dorian looks to be nonsense , please don't play TAPS yet ! And when we get Erin she will be a cane .
Member Since: July 6, 2013 Posts: 0 Comments: 460

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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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