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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RIPing Dorian looks to be nonsense , please don't play TAPS yet ! And when we get Erin she will be a cane .
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Quoting 1925. Grothar:


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

Lol!
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Quoting 1910. unknowncomic:
Some banding now. If it gets an ULAC, could strengthen.



new shear maps suggest that it does have it now

Quoting 1912. CaribBoy:
I see that even the NHC had difficulties to forecast Dorian. Now they are predicting dissipation, but ....


yep

Quoting 1917. CaribBoy:
COC is at 18.3N why on earth some are telling it is at 19N LOL


yeah I know the ATCF has it at 18.3N plus with microwave showing 18.1N 51W

Quoting 1918. Hurricanes305:
The CDO has warm some but new convection continues to develop on the western side where dry air is suppose to be. If this burst is deep enough Dorian may increase in size even more. As it stands at this very moment.



Once it hits 55W dry air will not be a problem as it starts to find mositure and water temperatures reach 28C. There is a good chance it continues to organize and start strengthening later today.



agreed
Member Since: June 13, 2009 Posts: 2 Comments: 12151
1925. Grothar
Quoting 1919. wunderkidcayman:

lol



I'm the one with the bigger horn. :)
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1924. LargoFl
if the track tomorrow..says no cuba hit, maybe all bets are off on him..right now the tracks are all guesswork
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Quoting 1920. LargoFl:
only way i see him going to hurricane is if he sits by the bahamas for a few days, stalled out with that strong front there OR he makes it into the gulf..sits on that warm gulfstream water for a day or two...otherwise he'll rebuild into a TS and stay that way..i really dont see him as a hurricane...but who knows..one day he's dying, the next he's rebuilding..we'll see in a few days huh


It has to slow down that forward motion, huh.
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Quoting 1918. Hurricanes305:
The CDO has warm some but new convection continues to develop on the western side where dry air is suppose to be. If this burst is deep enough Dorian may increase in size even more. As it stands at this very moment.



Once it hits 55W dry air will not be a problem as it starts to find mositure and water temperatures reach 28C. There is a good chance it continues to organize and start strengthening later today.


dat banding...
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Good Morning all, Please go and listen to Levi's tropical tidbit , he explains what's going on with Dorian , very well , it's on youtube ! And like I said yesterday , maybe a cane , by the Bahamas , on Sunday , or Monday ? I said maybe.
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1920. LargoFl
only way i see him going to hurricane is if he sits by the bahamas for a few days, stalled out with that strong front there OR he makes it into the gulf..sits on that warm gulfstream water for a day or two...otherwise he'll rebuild into a TS and stay that way..i really dont see him as a hurricane...but who knows..one day he's dying, the next he's rebuilding..we'll see in a few days huh
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Quoting 1908. canehater1:


Grab a horn...you and Gro can do a duet...

lol

Member Since: June 13, 2009 Posts: 2 Comments: 12151
The CDO has warm some but new convection continues to develop on the western side where dry air is suppose to be. If this burst is deep enough Dorian may increase in size even more. As it stands at this very moment.



Once it hits 55W dry air will not be a problem as it starts to find mositure and water temperatures reach 28C. There is a good chance it continues to organize and start strengthening later today.


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COC is at 18.3N why on earth some are telling it is at 19N LOL
Member Since: October 6, 2007 Posts: 0 Comments: 6229
Quoting 1896. unknowncomic:
I guess they think the shear will be to much for Dorian.

Looking good though.



NHC probably wants to concentrate on pre-Erin and is RIPing Dorian as a nuisance.
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Dorian is like Rocky Balboa. It takes a beating but keeps on coming at you.
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1914. Grothar
Quoting 1893. Hurricanes305:


Lol I did as well. Last night! Also I mentioned the wave by Africa may get a crayon today.


Yes, you did. If I remember, there were a number of bloggers who thought the same. I remember having a discussion on it with you. You all did a good job keeping Dorian Alive. There just aren't enough cookies to go around for you all.
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Let's see what shear has to say about Dorian..Still not optimistic..I wanted it to be like Danielle from 2010.That's all I asked!.
Member Since: August 14, 2010 Posts: 10 Comments: 17084
I see that even the NHC had difficulties to forecast Dorian. Now they are predicting dissipation, but ....

Member Since: October 6, 2007 Posts: 0 Comments: 6229
1911. hydrus
Quoting 1889. clwstmchasr:


I'm boarding up now!
I built me a concrete bunker 40 feet thick....A precautionary measure in case Dorian reaches cat-6 status...i wanna be pre-pared..:)
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Some banding now. If it gets an ULAC, could strengthen.

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Quoting 1885. Hurricane4Caster:
Although I am rather surprised at the overnight comeback of Dorian, I don't see a definitive COC. If there has been a recent ASCAT pass, it would be quite telling.

From 5:00 AM NHC discussion:

"IT SHOULD BE NOTED THAT RECENT
OSCAT DATA SUGGESTS THE POSSIBILITY THAT DORIAN NO LONGER HAS A
CLOSED CIRCULATION."


I'm not plus that Oscat was very late last night and convection had formed late last night going into this morning plus Oscat is not so High Res so it could miss seeing that if the S side was weak which I believe it was

Quoting 1887. Hurricanes305:


By the time it gets to 55W it will find a moister environment. Looking at the SW side and noticing the a good piece of energy about to wrap up with CDO. So it may strengthen later today.

that piece has the highest cloud tops now when that wraps in it should bring the cloud tops back to the CDO
Member Since: June 13, 2009 Posts: 2 Comments: 12151
Quoting 1868. wunderkidcayman:

I nailed it last night !!!!!!


Grab a horn...you and Gro can do a duet...
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Katrina strengthening into hurricane in Bahamas is a good comparison so I wound't jump on people for suggesting this. TD 10 died out in MDR where Dorian is, only to be revived as TD 12 in Bahamas which rapidly strengthen into a hurricane. However, I'm only talking pre-Florida Katrina and not the monster post-Florida Katrina was.
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Quoting 1899. RascalNag:


Been watching it; it seems to have the polar opposite personality as Dorian. Right now, the shear is just below 20 knots in his vicinity, he's surrounded by moisture and the water is warm enough to keep things going. Yet he hasn't really taken advantage of it yet. I suppose it'll have to wait until that shear immediately to his north pulls away some more, as it has been. It's a race against time for this guy, though.

There's also a surface low East of florida now, though it's less vigorous at the moment.


Centralized convection is hard to come by as wind shear is out of the west, but low level convergence is developing quickly as seen with low level cumulus clouds developing around the surface low. Lets see if it can develop some convection ver it's center.
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Quoting 1891. wunderkidcayman:
brDorian



so you saying that a LLCOC that moving W along 18N just suddenly moved N by one degree plus microwave data from just about 30mins to an hour ago does show it at 18N 51W




I didn't say it moved that much, I said "I See" it there. I didn't measure with satellites and a bunch of tools, just my bad morning vision which I'm sticking with ;)and to be 100% accurate with my visual I'll give you 18.7N not exact 19 N :)
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Quoting 1900. Hurricanes305:


Andrew actually when under a ULL and rapidly strengthen. But a better comparison would be a Katrina if the situation was to happen.
TD 10 became TD 12 which became Katrina. I think you got TD 10 for the wrong storm.
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Now the street names are painted on the streets so after the storm when all the sigs are gone emergency services can still locate.
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1902. Grothar
Quoting 1866. hydrus:
What did he say? What did he say?


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.
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1901. WWPR
Quoting 1893. Hurricanes305:


Lol I did as well. Last night! Also I mentioned the wave by Africa may get a crayon today.


Ok, 305, you get a cookie too...LOL
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Quoting 1890. Bluestorm5:
Katrina?


Andrew actually when under a ULL and rapidly strengthen. But a better comparison would be a Katrina if the situation was to happen.
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Quoting 1883. TheDawnAwakening:
Surface low east of NC shows intensification of low level cloud mass, low level banding trying to get going, and develop, low is closed, but lacks any deep convection, wind shear is decreasing and could become something as it nears SNE


Been watching it; it seems to have the polar opposite personality as Dorian. Right now, the shear is just below 20 knots in his vicinity, he's surrounded by moisture and the water is warm enough to keep things going. Yet he hasn't really taken advantage of it yet. I suppose it'll have to wait until that shear immediately to his north pulls away some more, as it has been. It's a race against time for this guy, though.

There's also a surface low East of florida now, though it's less vigorous at the moment.
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Again, Dorian is getting warmer so we'll see if she come back from this again. I am still doubtful, but if he hang in there until Bahamas... we'll have trouble quickly.

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1897. hydrus
Quoting 1881. LargoFl:
I was in a met blog lil while ago, they cannot believe nor explain how he can look that good in all that dry air..we do need a plane in there huh
My theory is that the storm is has been lynching just enough heat energy to maintain itself. There has not been much warm water fot it to feed on, but the system is cruising at a quick enough pace to take advantage of what is available. Water is warmer where Dorian is now.



HWRF..

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Quoting 1869. LargoFl:
000
WTNT24 KNHC 270832
TCMAT4

TROPICAL STORM DORIAN FORECAST/ADVISORY NUMBER 13
NWS NATIONAL HURRICANE CENTER MIAMI FL AL042013
0900 UTC SAT JUL 27 2013

THERE ARE NO COASTAL WATCHES OR WARNINGS IN EFFECT.

INTERESTS IN THE NORTHERN LEEWARD ISLANDS SHOULD MONITOR THE
PROGRESS OF DORIAN.

TROPICAL STORM CENTER LOCATED NEAR 18.2N 50.0W AT 27/0900Z
POSITION ACCURATE WITHIN 30 NM

PRESENT MOVEMENT TOWARD THE WEST OR 275 DEGREES AT 19 KT

ESTIMATED MINIMUM CENTRAL PRESSURE 1011 MB
MAX SUSTAINED WINDS 35 KT WITH GUSTS TO 45 KT.
34 KT....... 50NE 0SE 0SW 50NW.
12 FT SEAS.. 60NE 0SE 0SW 45NW.
WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL
MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT.

REPEAT...CENTER LOCATED NEAR 18.2N 50.0W AT 27/0900Z
AT 27/0600Z CENTER WAS LOCATED NEAR 18.0N 49.0W

FORECAST VALID 27/1800Z 18.6N 53.1W
MAX WIND 30 KT...GUSTS 40 KT.

FORECAST VALID 28/0600Z 19.2N 57.2W
MAX WIND 30 KT...GUSTS 40 KT.

FORECAST VALID 28/1800Z 19.8N 61.1W...POST-TROP/REMNT LOW
MAX WIND 25 KT...GUSTS 35 KT.

FORECAST VALID 29/0600Z 20.4N 64.7W...POST-TROP/REMNT LOW
MAX WIND 25 KT...GUSTS 35 KT.

FORECAST VALID 30/0600Z 21.0N 71.5W...POST-TROP/REMNT LOW
MAX WIND 25 KT...GUSTS 35 KT.

EXTENDED OUTLOOK. NOTE...ERRORS FOR TRACK HAVE AVERAGED NEAR 150 NM
ON DAY 4 AND 200 NM ON DAY 5...AND FOR INTENSITY NEAR 15 KT EACH DAY

OUTLOOK VALID 31/0600Z...DISSIPATED

REQUEST FOR 3 HOURLY SHIP REPORTS WITHIN 300 MILES OF 18.2N 50.0W

NEXT ADVISORY AT 27/1500Z

$$
FORECASTER BEVEN
I guess they think the shear will be to much for Dorian.

Looking good though.

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Quoting 1875. clwstmchasr:


The man of reason did call it. However, there is still a lot of dry air and shear so let's not start the Andrew comparisons again.



Andrew... NO,NO, NO, no, no, no and heck NO Been there, done that, NO
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Quoting 1880. WWPR:


Yes you did.


Lol I did as well. Last night! Also I mentioned the wave by Africa may get a crayon today.
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1892. VR46L
Quoting 1864. JrWeathermanFL:
@1859:
Flossie..


LOL!

Very Good !


But very true !!
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Quoting 1870. barbamz:


As much as I can see, it's 30 now and predicted to lessen.



Interesting to see that the upper ridge has now moved over Dorian

Quoting 1871. leelee75k:


I see it roughly at 19N 51W


so you saying that a LLCOC that moving W along 18N just suddenly moved N by one degree plus microwave data from just about 30mins to an hour ago does show it at 18N 51W

Quoting 1872. ackee:
THE wave leaving the coast of Africa does any of the model develop it ?

nope but sometimes it still develops without the models

Quoting 1873. Bluestorm5:
Dorian's clouds already getting warmer.

yeah just temporary plus its starting to pop on the W side now
Member Since: June 13, 2009 Posts: 2 Comments: 12151
Quoting 1878. Hurricanes305:


I can't help but have a feeling it may try to merge with the ULL. As its the upper low is weakening. Something similar to when TD10 did that lead to Andrew. But its very unlike to do so.
Katrina?
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1888. LargoFl
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Quoting 1874. wunderkidcayman:
looking at water vapor floater the air is now Moist N and NW and almost W of Dorian the S side still has dry air but is thinning out


By the time it gets to 55W it will find a moister environment. Looking at the SW side and noticing the a good piece of energy about to wrap up with CDO. So it may strengthen later today.
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Low even convergence is developing around the surface low, especially southeast of center.
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Although I am rather surprised at the overnight comeback of Dorian, I don't see a definitive COC. If there has been a recent ASCAT pass, it would be quite telling.

From 5:00 AM NHC discussion:

"IT SHOULD BE NOTED THAT RECENT
OSCAT DATA SUGGESTS THE POSSIBILITY THAT DORIAN NO LONGER HAS A
CLOSED CIRCULATION."
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1884. Grothar
Quoting 1834. barbamz:


We in Germany had to read it in our classes of English literature too, lol.


I just remembered the first time I read it was in
German.
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Surface low east of NC shows intensification of low level cloud mass, low level banding trying to get going, and develop, low is closed, but lacks any deep convection, wind shear is decreasing and could become something as it nears SNE
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1882. hydrus
Quoting 1875. clwstmchasr:


The man of reason did call it. However, there is still a lot of dry air and shear so let's not start the Andrew comparisons again.
Dorian will be a 5-- roughly 3000 miles in diameter, with sustained winds of 225, gusts 270....grab da shuddahs..:)
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1881. LargoFl
Quoting 1876. hydrus:
I do believe the precip amounts on that run are way overdone . It will be interesting to see how it pans out tho.
I was in a met blog lil while ago, they cannot believe nor explain how he can look that good in all that dry air..we do need a plane in there huh
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1880. WWPR
Quoting 1868. wunderkidcayman:

I nailed it last night !!!!!!


Yes you did.
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Quoting 1849. TropicalAnalystwx13:
If Dorian dies today, it's of no concern most likely. It by chance it continues to become better organized, we could have trouble in the Bahamas. Wait and see situation.


I can't help but have a feeling it may try to merge with the ULL. As its the upper low is weakening. Something similar to when TD10 did that lead to Andrew. But its very unlike to do so.
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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.