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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There is such a hunger for a Major Hurricane(myself included.) My prediction is this site will crash when we get the first one this season. I want to see a bona-fide cape verde beautiful Major Hurricane. Oh, and please dont respond with, "oh Scott, you want death and destruction." 99.9999% of you who say that, want to see a major too.
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Igor was under a lot of dry air as well, but managed to miss out on Category 5 by 1 mph. Dry air will always be there most of the times, but it depends on how storms protect itself from them how strong they get. Dorian was feeding on ITCZ and didn't have lot of protection build up.

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Quoting 957. RTSplayer:
Wow. Dying hard.

There's only one broken squall line remaining for Dorian. It could get declassified.


Before that it had nothing completely
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Looking at water vapor super dry air right over Dorian .
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guys stop RIPing it yeah most of the convection is gone but thats mainly due to D-Min among other little less minor things but it has finished with D-Min and now starting to build on D-max
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Quoting 970. Seflhurricane:
despite everything against it dorian has held its LLC rather good , the issue has been convection if it can fire some good convection overnight it has a chance if not there appears to be another opportunity north of Hispaniola. Remember Katrina TD#10


They won't declassify it no matter how naked it is if it remains closed
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With the budget being tight, Hurricane Hunters shouldn't bother making the trip to St. Croix...
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Quoting 913. moonlightcowboy:


Pat, we need those nightvision goggles! ;)





them make it look no better either
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Quoting 964. wunderkidcayman:
Dorian is not doing too terribly bad for a dead storm. still got its LLCOC its now developing convection on the NW N and NE side of that LLCOC. I think NHC might keep Dorian as a TS, but down it to a 40mph TS at 11pm and depending on how well Dorian does overnight they just may keep it tomorrow. yeah Dorian has a lot of work to do but Dorian is a fighter, he could make it, he just know he'll have to work very hard and build the convection. with D-min decreasing and D-max increasing it should hep him out.
despite everything against it dorian has held its LLC rather good , the issue has been convection if it can fire some good convection overnight it has a chance if not there appears to be another opportunity north of Hispaniola. Remember Katrina TD#10
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Quoting 944. Gearsts:
Dorian is aproaching the magic 50w line


Tropics do funny things sometimes. Dorian may be getting some slight help from the upper trough to its northwest dampening the dry air from the moisture created from the latent heat. Albeit gradual.
Member Since: July 9, 2006 Posts: 184 Comments: 29594
Quoting 959. Patrap:
INVEST Spirit


Satellite: African WAVE TRAK


That animasjon shows quite a nice wave train heading towards the atlantic!
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966. beell
Quoting 954. nrtiwlnvragn:


Think reference was to member in #812


Oh, geez.

Public apology to sar for inappropriate and misguided comment.

** beell slinks off to change his feet **

Thanks, nrt!
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Quoting 963. Seflhurricane:
very strong likelihood dorian will be downgraded to a remnant low


Yes it could.
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Dorian is not doing too terribly bad for a dead storm. still got its LLCOC its now developing convection on the NW N and NE side of that LLCOC. I think NHC might keep Dorian as a TS, but down it to a 40mph TS at 11pm and depending on how well Dorian does overnight they just may keep it tomorrow. yeah Dorian has a lot of work to do but Dorian is a fighter, he could make it, he just know he'll have to work very hard and build the convection. with D-min decreasing and D-max increasing it should hep him out.
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very strong likelihood dorian will be downgraded to a remnant low
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Dorian will soon be Open Wave Dorian if this keeps up.
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We could always watch The Weather Channel's coverage of Superstorm Sandy, right? I mean, this one user does have many, many hours of it. :)

Member Since: July 6, 2010 Posts: 113 Comments: 31519
INVEST Spirit


Satellite: African WAVE TRAK
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Quoting 951. buzzardswrath:
goodnite ill check back in a few weeks


Don'
Quoting 949. Gearsts:
I see the trolls are out!


And to those trolls, I say... Don't let the door hit ya where the good lord split ya.
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Wow. Dying hard.

There's only one broken squall line remaining for Dorian. It could get declassified.

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Quoting 945. HurricaneAndre:
The wave that exited Africa,yes,and a Category 3,So we can have 5-1-1


We are going to have a traffic information line?

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Quoting 950. captainmark:
And your point is???


Ignore is there for a reason. I am surprised the handle still exist to be honest..
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Quoting 938. beell:


**cough*** open mouth change foot ***cough**

:)



Think reference was to member in #812
Member Since: September 23, 2005 Posts: 14 Comments: 10901
Quoting 857. Patrap:
Published on Oct 2, 2012

Defense contractor Northrop Grumman is partnering with NASA to turn former Global Hawk spy drones into hurricane hunters.





Snowden would be proud.

There is a reason for the high shear in the Atlantic - its called July.
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Quoting 918. buzzardswrath:
omg i cant take it anymore ..... its done, its over its dead, its been over for days now....never was gonna amount to much just like chantal
And your point is???
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I see the trolls are out!
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Quoting 920. Patrap:
.."Dorian ya O-kay, yer O-kay now"...

"you been hit by-you been hit by-a smooth smack of dry air."
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TXNT28 KNES 270003
TCSNTL

A. 04L (DORIAN)

B. 26/2345Z

C. 17.9N

D. 47.1W

E. THREE/GOES-E

F. TOO WEAK

G. IR/EIR/SWIR

H. REMARKS...THIS SYSTEM IS TOO WEAK TO CLASSIFY DUE TO BANDING LESS
THAN .2

I. ADDL POSITIONS

NIL


...SCHWARTZ


Link
Member Since: September 23, 2005 Posts: 14 Comments: 10901
Quoting 940. EyEtoEyE:
So where's Erin go to form , and is she going to our first hurricane ? And how strong ?
The wave that exited Africa,yes,and a Category 3,So we can have 5-1-1
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Dorian is aproaching the magic 50w line
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Keep in mind the possibility of Dorian regenerating later near Bahamas or Cuba.
Member Since: February 13, 2012 Posts: 11 Comments: 3659
Quoting 935. HurricaneAndre:
Looks like August and September is fairly busy.



Well since those are peak months, you could say that every year and be right.
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The convection firing along the outer periphery of Dorian's circulation is extremely shallow, and shows no signs of organization.

Dorian's probably done. I know everyone's thirsty for hurricanes, but there will be ample opportunity in the future. And the ones we do get will be much bigger than this.
Member Since: March 7, 2007 Posts: 557 Comments: 19977
So where's Erin go to form , and is she going to be our first hurricane ? And how strong ?
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Quoting 936. buzzardswrath:
i never believe what the models say never
Sometimes,storms can develop without the models.
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938. beell
Quoting 860. sar2401:

***cough***check join date***cough***ignore***cough***


**cough*** open mouth change foot ***cough**

:)

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Quoting 930. HurricaneAndre:
Was that in the CFS MODEL.
Looks like August and September is fairly busy.
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thanks
Quoting 910. DavidHOUTX:


Here you go. Very short but to the point. Link

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933. xcool
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Quoting 923. buzzardswrath:
next crazy storm on august 24th
Was that in the CFS MODEL.
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Quoting 923. buzzardswrath:
next crazy storm on august 24th

Crazy or Storm?
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Dorian has a very nice circulation that wont quit wouldn't count it out especially when it reaches closer to the islands
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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.