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 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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1877. LargoFl
one more day he'll be in the moisture..........
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1876. hydrus
Quoting 1827. LargoFl:
I do believe the precip amounts on that run are way overdone . It will be interesting to see how it pans out tho.
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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
Member Since: June 13, 2009 Posts: 2 Comments: 11215
Dorian's clouds already getting warmer.
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1872. ackee
THE wave leaving the coast of Africa does any of the model develop it ?
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Quoting 1852. tj175:
Can anyone pinpoint where the actual COC is?


I see it roughly at 19N 51W
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1870. barbamz
Quoting 1867. yonzabam:


Yeh, but last time I saw a shear map, there was 40 knts waiting to tear Dorian's head off.


As much as I can see, it's 30 now and predicted to lessen, at least in the next part of his way.


Member Since: October 25, 2008 Posts: 52 Comments: 5705
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
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Quoting 1860. GeoffreyWPB:
Have to admit Gro, you nailed it last night!


I nailed it last night !!!!!!
Member Since: June 13, 2009 Posts: 2 Comments: 11215
Quoting 1861. LargoFl:
if he looks like this now, imagine when he his that moisture.


Yeh, but last time I saw a shear map, there was 40 kts waiting to tear Dorian's head off.
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1866. hydrus
Quoting 1860. GeoffreyWPB:
Have to admit Gro, you nailed it last night!

What did he say? What did he say?
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1865. hydrus
Quoting 1853. PensacolaDoug:



G'mornin' back. The TH had a good run back then.
Indeed. They were considered ahead of there time even for a new wave band. Cannot say I liked all there songs, but I did dig a few.
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@1859:
Flossie..
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Quoting 1852. tj175:
Can anyone pinpoint where the actual COC is?

it under the convection and is estimated though with some skill using microwave imagery near 18.1N 51.1W moving W bound
Member Since: June 13, 2009 Posts: 2 Comments: 11215
Quoting 1856. JrWeathermanFL:
I just seen where the NHC has predicted a U.S. landfall..that'd make 2 this year.


If the ridge doesn't break down, Floridians will be living in interesting times, come September.
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1861. LargoFl
if he looks like this now, imagine when he his that moisture.
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Have to admit Gro, you nailed it last night!

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1859. tj175
Quoting 1856. JrWeathermanFL:
I just seen where the NHC has predicted a U.S. landfall..that'd make 2 this year.



Where did you see that?
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1858. pottery
Quoting ncstorm:
Dorian just wont give up..this might be one for the textbooks..



Well, it's just a small part of another Busted Forecast on my part....

I think it must be because someone has angered those Nordic entities that control these things.
'Thor', the god of Thunder comes to mind.

And I place the blame squarely on the shoulders of Grothar......

I will be seeking Retribution, Compensation and the like.
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Quoting 1847. Tropicsweatherpr:
12z Best Track.

AL, 04, 2013072712, , BEST, 0, 183N, 511W, 35, 1011, TS

same wind same pressure still a TS ok depending if current trends continue that may change and be high wind and lower pressure
Member Since: June 13, 2009 Posts: 2 Comments: 11215
I just seen where the NHC has predicted a U.S. landfall..that'd make 2 this year.
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Quoting 1854. hydrus:
The globe will warm up if one believes in revelations.



I'm in no hurry for that one!
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1854. hydrus
Quoting 1850. PensacolaDoug:




Heck of a way to run a global warm-up.
The globe will warm up if one believes in revelations.
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Quoting 1851. hydrus:
Talking Heads..I still hear them every so often..Good Morning Doug.



G'mornin' back. The TH had a good run back then.
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1852. tj175
Can anyone pinpoint where the actual COC is?
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1851. hydrus
Quoting 1843. PensacolaDoug:




Same as it ever was.
Talking Heads..I still hear them every so often..Good Morning Doug.
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Quoting 1845. hydrus:
Record low temperatures for Mid TN...Love it...


VERY UNUSUAL FORECAST FOR MID SUMMER. H5 CLOSED LOW WILL MOVE
SLOWLY EAST THRU THE GREAT LAKES THIS WEEKEND. ASSOCIATED SFC LOW
WILL HELP BRING A COLD FRONT INTO NW MIDDLE TN LATE TODAY...AND TO
THE REST OF THE AREA BY MIDNIGHT. SCATTERED SHOWERS AND MAYBE A
FEW STORMS WILL REDEVELOP THIS AFTERNOON AND EARLY EVENING AHEAD
OF THE FRONT. RAINFALL AMOUNTS WILL BE LIGHT WITH LESS THAN 1/4
INCH IN MOST AREAS.

COOLER AND DRY WEATHER ON TAP FOR SUN/MON AS SFC RIDGE SETTLES
INTO MID TN. MONDAY MORNING MIN TEMPS COULD APPROACH RECORDS.
GFS GUIDANCE IS AN OUTLIER BY FORECASTING 2 DEGREES COLDER THAN
THE RECORD AT BNA...AND A WHOPPING 5 DEGREES COLDER THAN THE
RECORD AT CSV.
SO...LEANED MORE TOWARD THE WARMER AND SEEMINGLY
MORE REASONABLE NAM/ECMWF GUIDANCE.




Heck of a way to run a global warm-up.
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If Dorian dies today, it's of no concern most likely. If by chance it continues to become better organized, we could have trouble in the Bahamas. Wait and see situation.
Member Since: July 6, 2010 Posts: 113 Comments: 31554
Quoting 1844. Hurricanes305:


Getting its MLC back

yeah

best LLCOC location estimate is near 18.1N 51.1W moving W bound
Member Since: June 13, 2009 Posts: 2 Comments: 11215
12z Best Track.

AL, 04, 2013072712, , BEST, 0, 183N, 511W, 35, 1011, TS
Member Since: April 29, 2009 Posts: 75 Comments: 14073
Quoting 1843. PensacolaDoug:




Same as it ever was.

ok
Member Since: June 13, 2009 Posts: 2 Comments: 11215
1845. hydrus
Record low temperatures for Mid TN...Love it...


VERY UNUSUAL FORECAST FOR MID SUMMER. H5 CLOSED LOW WILL MOVE
SLOWLY EAST THRU THE GREAT LAKES THIS WEEKEND. ASSOCIATED SFC LOW
WILL HELP BRING A COLD FRONT INTO NW MIDDLE TN LATE TODAY...AND TO
THE REST OF THE AREA BY MIDNIGHT. SCATTERED SHOWERS AND MAYBE A
FEW STORMS WILL REDEVELOP THIS AFTERNOON AND EARLY EVENING AHEAD
OF THE FRONT. RAINFALL AMOUNTS WILL BE LIGHT WITH LESS THAN 1/4
INCH IN MOST AREAS.

COOLER AND DRY WEATHER ON TAP FOR SUN/MON AS SFC RIDGE SETTLES
INTO MID TN. MONDAY MORNING MIN TEMPS COULD APPROACH RECORDS.
GFS GUIDANCE IS AN OUTLIER BY FORECASTING 2 DEGREES COLDER THAN
THE RECORD AT BNA...AND A WHOPPING 5 DEGREES COLDER THAN THE
RECORD AT CSV.
SO...LEANED MORE TOWARD THE WARMER AND SEEMINGLY
MORE REASONABLE NAM/ECMWF GUIDANCE.
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Quoting 1838. wunderkidcayman:
Dorian looking a lot better on navy's NRL's coriolis image a definitive S patern


Getting its MLC back
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Quoting 1840. wunderkidcayman:

sorry I don't understand




Same as it ever was.
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I just heard from a very reliable source (means I just made it up)that the big or um little D storm is one stubborn sob.
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It'll be interesting to see how Dorian do today... disappointed I got work today, but it's life.
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Quoting 1837. PensacolaDoug:



SAIEW.

sorry I don't understand
Member Since: June 13, 2009 Posts: 2 Comments: 11215
Quoting 1826. LargoFl:
thats it Lil One..you can do it...........


Looks like pre-Erin plans to run in the ITCZ until about 40W or so.. splendid idea. She should have enough moisture when she decouples to survive the dead zone, and be large enough to pull thru the gauntlet. She might be exploring Dean's career path.
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Dorian looking a lot better on navy's NRL's coriolis image a definitive S patern
Member Since: June 13, 2009 Posts: 2 Comments: 11215
Quoting 1811. wunderkidcayman:

last night I said this would happen and I got yelled at and was beat down by others about it now look at it




SAIEW.
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@1819:
My point was that it'd last longer and give us something to track as long as it didn't hit land as a really bad storm..
I don't care about the ACE value either. But so far all the storms this season havn't really survived over 3-4 days. Be nice for one to survive longer
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1835. pcola57
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1834. barbamz
Quoting 1831. Grothar:


And the movie. It actually was required reading when I went to school. Wasn't it a true classic?


We in Germany had to read it in our classes of English literature too, lol.
Member Since: October 25, 2008 Posts: 52 Comments: 5705
Quoting 1809. Bluestorm5:
The worst thing about Dorian reviving? People will come running back after calling it RIP and start freaking out that it's strengthening into hurricane. I'm sure DMIN and extreme shear will finish off Dorian later today, hence the forecast of depression from NHC.




Same as it ever was....
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1832. LargoFl
Largo looks around,see's Aussie and he brews a fresh pot of Tea...enjoy my friend...
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1831. Grothar
Quoting 1824. leelee75k:
Besides myself and probably Grothar (he's old enough) has anyone read the Picture of Dorian Gray? Looks can be deceiving! It lives!!!


And the movie. It actually was required reading when I went to school. Wasn't it a true classic?
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UPPER-LEVEL
WINDS IN THE AREA WEST OF THE FORECAST POINT OF DISSIPATION ARE NOT
EXPECTED TO BE CONDUCIVE FOR RE-GENERATION AT THIS TIME.

From 5AM TWD...Somebody better tell "D"...
Member Since: September 8, 2008 Posts: 1 Comments: 1027
Quoting 1802. AussieStorm:


No worries mate! I told you about this page the other day. Maybe you didn't see it.

Here is another website you might like.


Thanks Aussie, nice link!
Heaviest thunder on this page went straight over me, just some heavy rain and almost no lightning.
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1828. Grothar
Quoting 1822. Hurricanes305:


Hey Gro, when will all the downcasters learn not to called a system dead when its still a well define low. First it was Chantal and now this. And do I dare say its defying all the odds from last night to starting convective banding on North and South side of Dorian. Interesting day for Dorian and everybody else involved.


We will always have wishcasters and downcasters and upcasters, it is just part of life. That is why I was one of the first to say "Never write off a system too fast" (toot) (toot). :)

It really was a tough call though. Even the experts did not expect this.
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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.