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 413. MisterPerfect:


I don't see an "eye". more like a black-eye from the beating its taken.


Lol it has taken a good beating. I circled it in red, not sure if it is actual eye but appears to be one if you watch this area on the loop

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THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF SOUTH CENTRAL
AND SOUTHWEST ARKANSAS...NORTH CENTRAL AND NORTHWEST LOUISIANA...
SOUTHEAST OKLAHOMA...AND EAST AND NORTHEAST TEXAS.

.DAY ONE...THIS AFTERNOON AND TONIGHT...

SHOWERS AND THUNDERSTORMS WILL CONTINUE TO AFFECT THE
REGION...WITH LOCALLY HEAVY RAINFALL OF ONE TO OVER TWO INCHES
POSSIBLE AT TIMES WITH THE PASSAGE OF AN UPPER LEVEL DISTURBANCE
AND A NEARLY STATIONARY FRONTAL BOUNDARY. ENOUGH HEATING MAY
RESULT IN MODERATE INSTABILITY THIS AFTERNOON TO ALLOW
FOR THE DEVELOPMENT OF SEVERE THUNDERSTORMS...ESPECIALLY ACROSS
PARTS OF NORTHEAST TEXAS...NORTHWEST LOUISIANA...AND SOUTHWEST
ARKANSAS. DAMAGING WINDS...HAIL...AND LOCALLY HEAVY RAINFALL
WILL BE THE PRIMARY THREATS. THE SEVERE THREAT EXPECTED TO
CONTINUE THROUGH THIS EVENING AS LARGE SCALE FORCING INCREASES
ALONG AND AHEAD OF A SURFACE FRONT. THE HEAVY RAINFALL AND
POTENTIAL TRAINING OF STORMS WILL RESULT IN LOCALIZED FLOODING
OF LOW LYING...POOR DRAINAGE AREAS ESPECIALLY TONIGHT ACROSS
SOUTHWEST ARKANSAS AND NORTH CENTRAL AND NORTHWEST LOUISIANA.
Member Since: August 6, 2011 Posts: 4 Comments: 42266
I can't get over how High the pressure is in Dorian....1010 mb
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look guys if Dorian can pull up some convection later today and/or tonight then he will live if not then the chances of death is likely
Member Since: June 13, 2009 Posts: 2 Comments: 12719
Storm Relative 1km Geostationary Visible Imagery Loop

click Image for Loop

click Loop to ZOOM


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Coming into Central Pacific view.

Member Since: July 21, 2011 Posts: 83 Comments: 7167
Quoting 406. DavidHOUTX:
you can clearly see the LLCOC there. Even appears to have an eye on this loop. Convection is so poor though



Something else to note in that loop is the remarkable increase in low level cloud cover and structure. It would appear, at least to me, that this is just a cycle the storm is going through to moisten the environment around it. The collapsing convection adds moisture to the atmosphere.
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It tickles me to watch TWC coverage of Hugo because they had George Elliot. Not long after he moved to Wilmington and has been a local meteorologist here for years. Thank God for him with Bonnie in 98. He outdid the weather channel and the NHC.
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as i said yesterday until you see only blue water beneath it it ain,t dead
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Quoting 410. EyEtoEyE:
The TWC has gone the way of MTV , VH1 , and other channels , not what they were supposed to be .


TWC is like a badly done version of Discovery with some morning news thrown in. I don't even bother watching it
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Quoting 403. DavidHOUTX:
SHIPS in no mans land



2 models kill it....2 don't
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Quoting 404. calkevin77:


Those yacks they found out were great swimmers and eventually ended up finding their way to the South China Sea. Hence the correlation to the tropical update. That story was later interrupted by a bunch of commercials for laundry detergent and life insurance.
Right after TWC was sold, they initiated "A Flick and a Forecast", from 6pm-8pn CST. A tornado outbreak with lives lost occurred I believe in Upper Midwest. They got rid of it...probably on the advice of their attorneys.
The dumbing down of America at its best.
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Quoting 410. EyEtoEyE:
The TWC has gone the way of MTV , VH1 , and other channels , not what they were supposed to be .
Very well stated...As I said before ...It's "All about money" I remember the "pop up" videos when MTV first appeared
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My pawpaw is telling me TWC in the morning talks about politics!! Now what the heck!
Member Since: September 2, 2011 Posts: 0 Comments: 1149
Quoting 406. DavidHOUTX:
you can clearly see the LLCOC there. Even appears to have an eye on this loop. Convection is so poor though



I don't see an "eye". more like a black-eye from the beating its taken.
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Quoting 389. Tornado6042008X:
TWC was my favorite channel during those two hurricane seasons when I was 8 and 9 years old respectively. GOOD TIMES, GOOD TIMES.


But the expertise of John Hope is really what I miss about the tropical updates and storm alerts. If he had been around for those two seasons, there would be a wealth of information that was not covered by the 2004/2005 staff. Hope was THE expert... You think the coverage was great during 2004/2005... Have you seen the footage from the coverage of Andrew?
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Given all the negative factors going against him, Dorian has been fighting. He's certainly struggling now but that was expected. The LLC is still closed and fairly vigorous and it does appear to be trying to wrap some convection again, though it's not much. Dissipation is the most likely outcome to be sure, but I won't call Dorian dead until he actually is. No need to jump the gun.
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The TWC has gone the way of MTV , VH1 , and other channels , not what they were supposed to be .
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Quoting 392. aislinnpaps:
John Hope

Born May 14, 1919
Died June 13, 2002 (aged 83)

Can you imagine the changes he saw in meteorology from the time he was born, went into meteorology, until he died?
He made Jim Cantore look like.....well, jim cantore. No one since can wipe his butt. He is the King. Tropical systems breathed a sigh of relief when John passed...knowing they could now get away with murder.
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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 : 26 JUL 2013 Time : 184500 UTC
Lat : 17:53:04 N Lon : 44:39:14 W


CI# /Pressure/ Vmax
3.0 /1006.7mb/ 45.0kt


Final T# Adj T# Raw T#
3.0 3.4 3.6

Center Temp : -51.2C Cloud Region Temp : -34.8C

Scene Type : CURVED BAND with 0.79 ARC in MD GRAY

Positioning Method : FORECAST INTERPOLATION

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

Tno/CI Rules : Constraint Limits : 0.7T/6hr
Weakening Flag : OFF
Rapid Dissipation Flag : OFF

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

Satellite Name : MSG3
Satellite Viewing Angle : 54.4 degrees



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Gotta love all the people writing off Dorian right now when Dorian currently has a very vigorous and well defined low level circulation; probably the most well defined in its entire lifetime thus far.
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you can clearly see the LLCOC there. Even appears to have an eye on this loop. Convection is so poor though

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Quoting 393. PalmBeachWeather:
I know...I tune in to see the "Tropical Update" and I see how a Siberian Yak was rescued from a river in the Cossacks of Russia


Those yacks they found out were great swimmers and eventually ended up finding their way to the South China Sea. Hence the correlation to the tropical update. That story was later interrupted by a bunch of commercials for laundry detergent and life insurance.
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SHIPS in no mans land

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Quoting 392. aislinnpaps:
John Hope

Born May 14, 1919
Died June 13, 2002 (aged 83)

Can you imagine the changes he saw in meteorology from the time he was born, went into meteorology, until he died?
I loved that man
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Satellite: African WAVE Train
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Quoting 374. calkevin77:
What is that hole in the latest sat run? A new core or just a break in the storms? This whole thing is just odd.


that my friend is the LLCOC

Quoting 380. CaribBoy:


...

interesting
Member Since: June 13, 2009 Posts: 2 Comments: 12719
just realized MJO is in downward phase which is causing sinking air. thats whats killing him plus shear. july/downward mjo vs rare cape verde storm = death. nothing rare in that
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Quoting 393. PalmBeachWeather:
I know...I tune in to see the "Tropical Update" and I see how a Siberian Yak was rescued from a river in the Cossacks of Russia

Haha...what does that have to do with weather? Come on TWC...
Member Since: September 2, 2011 Posts: 0 Comments: 1149
Quoting 384. Bluestorm5:
And they had forecasting on Dorian on a RERUN. I don't watch TWC as much as I used to do.


Lol that is so true. They have turned into Reality TV just like every other channel has already done or is in the process of doing.
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I know this may be a bit premature, but I do see a small storm cell developing on one of the curved bands to the west of the LLC of Dorian. Lets see if it holds, and causes further development along the band. Just shows that there is still some instability in the area. The cloudiness with the LLC itself appears to be consolidating a bit as well. A couple hours after sundown COULD yield some better results if it wants to survive.
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Quoting 384. Bluestorm5:
And they had forecasting on Dorian on a RERUN. I don't watch TWC as much as I used to do.



Me neither. But i sure do like to look at Carl Parker.
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Issued by The National Weather Service
Tampa Bay/Ruskin, FL
3:03 pm EDT, Fri., Jul. 26, 2013

... STRONG THUNDERSTORMS OVER THE WATERS...

AT 303 PM EDT... NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED THUNDERSTORMS... PRODUCING STRONG WINDS UP TO 33 KNOTS OVER GANDY BRIDGE... OR ABOUT 3 NM WEST OF GANDY BRIDGE... MOVING SOUTH AT 10 KNOTS.

MARINERS CAN EXPECT GUSTY WINDS UP TO 33 KNOTS... LOCALLY HIGHER WAVES... AND DEADLY CLOUD TO WATER LIGHTNING STRIKES. BOATERS SHOULD SEEK SAFE HARBOR IMMEDIATELY UNTIL THIS STORM PASSES.
Member Since: August 6, 2011 Posts: 4 Comments: 42266
Quoting 371. Stormchaser121:

Their "local on the 8s" isn't even local anymore have you noticed that? The channel itself isnt even all about weather ecfl is right. Its all....random stuff. They took away their theme song...changed the music on local on the 8s...anf their tropical update isnt even at 50 past the our anymore...they just get to it when they can.
I know...I tune in to see the "Tropical Update" and I see how a Siberian Yak was rescued from a river in the Cossacks of Russia
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John Hope

Born May 14, 1919
Died June 13, 2002 (aged 83)

Can you imagine the changes he saw in meteorology from the time he was born, went into meteorology, until he died?
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Quoting 384. Bluestorm5:
And they had forecasting on Dorian on a RERUN. I don't watch TWC as much as I used to do.

Neither do I. Oh and storm alert is gone. Its as if they aren't serious about weather anymore!
Member Since: September 2, 2011 Posts: 0 Comments: 1149
I see our little boy is holdings it's own. Just remember small storms and start spinning very quirky and can unwind very quickly.
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Quoting 356. Bluestorm5:
2004 and 2005 coverage were awesome to watch when I was just a kid. They were 100% on storms 24/7. They never took a break during the season.
TWC was my favorite channel during those two hurricane seasons when I was 8 and 9 years old respectively. GOOD TIMES, GOOD TIMES.
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Quoting 347. moonlightcowboy:


:) Hard to beat simplicity. That is exactly what it is, kman. No need to mince words about it. Agreed. It's been a fighter though, and I would not rule out a comeback yet.


I wouldn't rule it out either but it's very hostile where Dorian is now.
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It was fun checking in, but I'll be back later.
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Member Since: August 6, 2011 Posts: 4 Comments: 42266
The 11PM advisory should be the last on Dorian as it definitely looks to be degenerating into a wave. Probably not there yet so it will likely be maintained a TS at 5PM but the circulation is clearly collapsing so I can't see it hanging past 11PM, maybe 5AM at the latest. Convection has also fallen apart, which will only get worse as D-Min approaches.

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Quoting 371. Stormchaser121:

Their "local on the 8s" isn't even local anymore have you noticed that? The channel itself isnt even all about weather ecfl is right. Its all....random stuff. They took away their theme song...changed the music on local on the 8s...anf their tropical update isnt even at 50 past the our anymore...they just get to it when they can.
And they had forecasting on Dorian on a RERUN. I don't watch TWC as much as I used to do.
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I say 45 at 5:00.....40 at 11:00 with track reduced to 3 days and showing dorian dissipated by day 2
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382. MTWX
Quoting 363. washingtonian115:
???? o_0?.I wasn't directing anything at you..Sure we have the storms..but we don't have the conditions.:)


Oh... Then sorry Wash... thought you were talking about post 314. I was just pointing out the similarities in positioning of TD 8/Potent Wave/TD 10 in 05 with TS Dorian/Potent Wave behind him currently.
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Quoting 287. DataNerd:


Reason why I don't think this will happen:

1. Dry air. Still alot of dry air in the way.

2. The westerly flow does not really relax until a significant distance away from the Island, and that is likely whats killing it now more than anything.

3. Upper level shear is going to be like 50Kt or greater. A hurricane could survive that not a naked low pressure system.

The things you mentioned will finish off Dorian. Do not let the others kid you about a miraculous regeneration in 3 or 4 days. It will suffer the same fate as Chantel. This is not a favorable time for storms in this area climatologically speaking. Next week and the following week the MJO will be in place and conditions overall in the tropics will become more favorable for development.
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...
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Quoting 359. CybrTeddy:


That's not climatology though. The fact that Dorian exists defies climatology.
But for defying it,it is now paying for it.lolol.Ahh I crack my self up sometimes..
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Quoting 331. Patrap:


John Hope

Born May 14, 1919
Pennsylvania
Died June 13, 2002 (aged 83)
Macon, Georgia
United States
Fields Meteorologist


Source of Camille name on hurricane naming list

In 1969, Hope's daughter graduated from high school, so he added her name to the list of names to be used for hurricanes that year (at that time, there was no organized list of assigned names to be used, the only requirements were that the names had to be female – male names were not used at that time – in alphabetical order, and not otherwise retired).

He had no way of knowing at the time that the storm that would take his daughter's name – Camille – would become one of the most powerful and destructive hurricanes to ever hit the United States when it slammed into Mississippi as a Category five hurricane. His daughter Camille is married to former U.S. Representative Jim Marshall of Georgia.



Thanks that was very interesting. My Dad was a ham radio operator back in the day and helped out during Camille.
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