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 521. stormpetrol:


earlier



most recent = Dorian now an open wave


Yes, and should be downgraded at the 5pm advisory.
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Quoting 518. weatherlover94:
The NHC actually called for this weakening at 11:00 am....this could just be a temporary thing.
yes many days ago NHC had him dfown to a depression right where he is now BUT..he regains his strength and becomes a strong TS later on ..I for one am watching this guy closely..not minute to minute like some here are doing..comes wens if he (no Matter WHAT he looks like) is still around then watch out..because he then hqas ALL the necessary ingredients he needs to rebuild and remember folks....by then it will be august
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Quoting 488. AllStar17:


Dorian should probably be downgraded to a tropical depression at this point.


I'm in the tropics and I am depressed. Does that count?
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I suspect it will still be kept a tropical storm at 5 pm , I think Osact maybe messing up a bit.
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poof there she goes.
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522. MTWX
Quoting 495. buzzardswrath:
everyone was saying dorian was gonna be a cane and hit the bahamas then florida now everyone is saying it will die , i cant take it anymore. what will they be saying tommorow


Welcome to Hurricane Season on the WU!!

Don't worry, you'll get used to it if you hang around long enough...
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earlier



most recent = Dorian now an open wave
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Is the low lever cloud deck thickening up some or am I just imagining...
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Ok, i'm going to mention 2 names and don't flip out. Andrew struggled some in the Atlantic and yes, Katrina formed from the ragged remnants of TD10 in 2005. So Dorian may do the same. I am not saying he will be a Cat 5 monster and as a matter of fact I really think he might only make it to Cat 1 at most if he revives. But don't give up on him and the SE US should still watch him or his remnants
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The NHC actually called for this weakening at 11:00 am....this could just be a temporary thing.
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Quoting 512. buzzardswrath:
one last note.... the dry air and shear will remain high for quite some time, so any storms comming from africa will play the same games as chantal and dorian.... maybe by late september we might get something real
LOL.....One more before Bye
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Quoting 507. VR46L:
I am choking ,Cough sputter, gasp !



Wow that air is really dry. Is the next color black inside the yellow? lol
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This is not the appearance of a tropical cyclone. If we recall, the NHC even took an extra day or so before initiating advisories on 04L because the satellite presentation was lackluster. If this system had just emerged off of Africa, sure it would have a surface circulation, but it wouldn't be considered a tropical cyclone simply because the convective structure is so poor.

I'm gonna get out before more people get mad at me for being too much of a downcaster this afternoon. ;)

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Quoting 478. canehater1:
Not too healthy in WV Loop

Link


And it still has a pile of dry air to get through. I wouldn't be surprised if its a minimal TS at 40 mph with the 5 PM advisory from NHC.
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Just a few observations:
Dorian does appear to have two centers; one around 45W and 17.5-18N and the other 43W 18.5N. However, it does appear dry is retreating a bit and overall areal coverage with the storm seems to be increasing.
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well dorian may be lacking the storms and is weakening BUT..he still has a closed circulation and if he can huff and puff his way to the bahamas he'll have all the warm waters and moisture he needs to Boom himself strong again...
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Quoting 505. buzzardswrath:
I cant take anymore later all
Ok bye
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Quoting 503. CaicosRetiredSailor:
5pm forecast predictions... ?


45 mph, 1010mb, not dissipated, slightly more south track. Probably will call for dissipation within next 24 hours.
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507. VR46L
I am choking ,Cough sputter, gasp !

Member Since: March 1, 2012 Posts: 0 Comments: 6898
dont compare dorian to chris. dorian is dealing with downward motion of the mjo and rising pressures while chris got so sheared his convection moved sw in the caribbean while his naked swirl went wnw
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Quoting 464. EyEtoEyE:
Ok back to the weather , so do we have 90 L or not ?
.



No we do not have 90l yet
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5pm forecast predictions... ?
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I think Nicki Minaj should maybe be de-classified also..
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OSCAT sure not impressive


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Ryan Maue ‏@RyanMaue 9 m
Last few visible images show TS #Dorian torn apart, perhaps 2 centers, which means advisories would be discontinued at 5 pm or 11 pm.

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-
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what we dont have are planes flying into it and giving us True weather condition..all we have are sat pics..Obama..Loosen the purse strings for Noaa
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I think it will be declassified by morning then maybe Sunday it can regenerate.
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What is also interesting to note on the visible and RGB channels is that the low level cloud field around Dorian has really expanded today, although convection is pathetic, at best. IF it can get itself back together, it has a good amount of low level moisture to work with, and it could become a sizeable system, eventually.
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Quoting 481. buzzardswrath:
well based on all the models and the nhc track theres no way that dorians hits anywhere on the east coast of florida if it even stays together ( so false alarm again) i think thats 28 false alarms over the past 8 years for the east coast of florida )


For what it's worth, EFL is typically not under the gun until Aug. Aug 15th - Oct 15th to be precise. Rare for storms to hit the east coast of FL in July.
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ASCAT pass over #Dorian at 2:12pm still showed a closed circulation and winds of 40-45 knots
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491. VR46L
Quoting 485. Doppler22:
I think Dorian is going to die soon. BUT I also think he will regenerate later near the Bahamas


Umm are you saying like TD10 did nearly 8 years ago ....
Member Since: March 1, 2012 Posts: 0 Comments: 6898
Looks like Dorian is Bout to go POOF after a few last gasps
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Quoting 487. weatherlover94:


winds will be dropped to 45 at 5:00


Dorian should probably be downgraded to a tropical depression at this point.
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Quoting 483. LargoFl:
(the lil storm that could)..still got 50 mph winds


winds will be dropped to 45 at 5:00
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Quoting 485. Doppler22:
I think Dorian is going to die soon. BUT I also think he will regenerate later near the Bahamas



that was my thought too
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I think Dorian is going to die soon. BUT I also think he will regenerate later near the Bahamas
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Quoting 474. MiamiHurricanes09:
Lower-level inflow channels are poor. I wouldn't go as far to say that it's well-defined. A well-defined surface circulation would be what it was a few days when it was nearly a 55kt tropical cyclone.



Like I said, this is not the presentation of a well-defined surface circulation.





Also,outflow boundaries are spitting out.
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(the lil storm that could)..still got 50 mph winds
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Flossie also looks on its way to become a hurricane.
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Quoting 470. cchsweatherman:


You make such a statement that visible satellite imagery does not display a well-defined circulation, but then you post an infrared satellite image. Way to prove your point. lol
That's all you got out of the post? The reason I posted an infrared image was because I was eluding to the first paragraph.
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Dorian needs convection fast.
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Not too healthy in WV Loop

Link
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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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