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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Not too healthy in WV Loop

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Slight rotation off the east coast of Florida.
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Quoting 463. TropicalAnalystwx13:

There's very clearly a well-defined low-level center just west of 45W and just south of 18N in visible.

That photo reminds me of Chris,Last time I said as a joke but it looks more than him,and yes people I know Chris hit a wall of shear.
Member Since: October 15, 2011 Posts: 0 Comments: 4004
Dorian may not be about to become an open wave, but it's about to become a remnant low.
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Quoting 463. TropicalAnalystwx13:

There's very clearly a well-defined low-level center just west of 45W and just south of 18N in visible.

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.



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wind shear its come back from tropical storm
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Quoting 456. weatherlover94:
This could thread the middle between Cuba and Florida ...get into the gulf and pose a threat to Texas....conditions there are very favorable.

Or it could regenerate in the Bahamas, deepen rapidly into a Cat. 1 hurricane and hit south florida, and then on into the gulf
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Quoting 455. MiamiHurricanes09:
I completely disagree. This morning's ASCAT pass revealed that the southern semicircle had become poorly defined and was at the brink of opening up. That, in combination with the horrid satellite presentation exhibited on current IR imagery would lead me to believe that Dorian is, at this time, no longer a tropical cyclone.

Visible satellite imagery does not display a well-defined circulation by any means.



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
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Dorian ingested a big chunk of low PW air, which is reason number one why he is struggling right now.

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Is it me or am I seeing two LLCs?
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Issued by The National Weather Service
Tampa Bay/Ruskin, FL
3:45 pm EDT, Fri., Jul. 26, 2013

... STRONG THUNDERSTORMS WILL AFFECT CENTRAL CHARLOTTE... SOUTHEASTERN SARASOTA AND SOUTHWESTERN DESOTO COUNTIES...

NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATES THUNDERSTORMS LOCATED ALONG A LINE EXTENDING FROM 7 MILES SOUTHEAST OF MYAKKA RIVER STATE PARK TO LAKE SUZY... OR ALONG A LINE EXTENDING FROM 9 MILES NORTHWEST OF NORTH PORT TO PORT CHARLOTTE... MOVING SOUTHEAST AT 5 MPH... WILL AFFECT MYAKKA RIVER STATE PARK... LAKE SUZY... FORT OGDEN PORT CHARLOTTE... AND NORTH PORT... UNTIL 430 PM EDT.

THIS INCLUDES INTERSTATE 75 BETWEEN EXITS 161 AND 193.

GUSTY WINDS 35 TO 45 MPH WILL OCCUR. WHEN DRIVING THROUGH HEAVY RAIN... SLOW DOWN. ALWAYS LEAVE A SAFE DISTANCE BETWEEN YOU AND OTHER VEHICLES.

&&
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Well, I just got done planting a South Florida Slash Pine in my yard. It will help replace the canopy as I expect the huge, leaning Sand Pines to fall done in the next hurricane.
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Ok back to the weather , so do we have 90 L or not ?
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Quoting 455. MiamiHurricanes09:
I completely disagree. This morning's ASCAT pass revealed that the southern semicircle had become poorly defined and was at the brink of opening up. That, in combination with the horrid satellite presentation exhibited on current IR imagery would lead me to believe that Dorian is, at this time, no longer a tropical cyclone.

Visible satellite imagery does not display a well-defined circulation by any means.


There's very clearly a well-defined low-level center just west of 45W and just south of 18N in visible.

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I see no sign of Convection redevelopment.
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Belay the adult theme.

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Quoting 436. Tornado6042008X:

I really miss the tropical update intro music from the early 2000s. 

They don't even play intro music anymore. :'(


Hahaha yes I forgot about that. That music was epic. Cheesy but you knew when the tropical update came on if you were in the other room.
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Quoting 401. Patrap:
Satellite: African WAVE Train


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.Ok, It's time to put on my best blue bowling skirt and head to the Bowling Alley on Lake Worth Road...It's Friday and I'm ready to kick some redneck butt..See you all tomorrow.
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This could thread the middle between Cuba and Florida ...get into the gulf and pose a threat to Texas....conditions there are very favorable.
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Quoting 407. cchsweatherman:
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.
I completely disagree. This morning's ASCAT pass revealed that the southern semicircle had become poorly defined and was at the brink of opening up. That, in combination with the horrid satellite presentation exhibited on current IR imagery would lead me to believe that Dorian is, at this time, no longer a tropical cyclone.

Visible satellite imagery does not display a well-defined circulation by any means.

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Ok , Thank you , Patrap.
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Quoting 444. PalmBeachWeather:
Not that rauchy sex is all bad EyE
The sex on needs to be tasteful , not raunchy! If at all.
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Quoting 440. weatherlover94:
Link

8 models keep Dorian alive.....5 kill it.....what model shows it going up 80 mph ? is that the ships ?


The SHIPS and the LGEM both strengthen it to 50kts by the end of their forecast periods, they've been the most consistent models so far with intensity.
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While the topic of TWC being terrible is still relevant, does anyone get Weatherscan?

Weatherscan wiki

Good for local weather, but that's all it really is. Even so, that a huge step up from the current TWC.
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Quoting 438. ncstorm:


WOW..I didnt know he was with TWC..


yep. there are some archived videos of Hugo coverage someplace, either here or there lol. Sorry but I don't remember where they are
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Please put some cloud clothes on Dorian , please hurry!
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Quoting 433. buzzardswrath:
so now florida wont get anything from dorian right?


I would not count on that. There is still uncertainty in both the track and the intensity forecasts for Dorian. It would be good to continue to keep an eye on it. It is always better to be prepared rather than caught off guard by a "dead" storm.
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Quoting 437. EyEtoEyE:
. Greedy people , music videos , now we have , is raunchy sex on MTV , and who cares politics on the TWC , what a bunch of GARBAGE ! They need to get back as they were , music videos , and the weather period .
Not that rauchy sex is all bad EyE
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Quoting 422. cchsweatherman:


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.


Also note, when you view the satellite loops in Rainbow enhancement, even though cloud tops have been warming this afternoon (typical as we are in Dmin), you can see the MLC (or what appears to be) actually closer to the LLC as opposed to what it was 12 hours ago or so. Then, it lagged nearly 150 miles behind (to the east) of the LLC, now it about 60 miles or so... A sign that the shear is decreasing a tad.
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Quoting 420. will40:
as i said yesterday until you see only blue water beneath it it ain,t dead


LOL!! exactly!!
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Quoting 429. Stoopid1:


Given the higher pressures in the area, it probably works out. I remember Hurricane Danny(can't think of the year, 2003 maybe?) was 1000mb but was a 75mph cat 1.
Dorian's pressure is so high..he actually brings good weather, low winds, and no rain to whoever he visits. What a guy.
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8 models keep Dorian alive.....5 kill it.....what model shows it going up 80 mph ? is that the ships ?
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Quoting 433. buzzardswrath:
so now florida wont get anything from dorian right?


Too soon to tell, too many factors still in play.
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Quoting 421. K8eCane:
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.


WOW..I didnt know he was with TWC..
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Quoting 416. PalmBeachWeather:
Very well stated...As I said before ...It's "All about money" I remember the "pop up" videos when MTV first appeared
. Greedy people , music videos , now we have , is raunchy sex on MTV , and who cares politics on the TWC , what a bunch of GARBAGE ! They need to get back as they were , music videos , and the weather period .
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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
I really miss the tropical update intro music from the early 2000s. 

They don't even play intro music anymore. :'(
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Quoting 430. stoormfury:
THE NAKED SWIRL OF DORIAN APPEARS TO BE TRACKING SOUTHWEST.


280 degrees west.
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Quoting 430. stoormfury:
THE NAKED SWIRL OF DORIAN APPEARS TO BE TRACKING SOUTHWEST.
"Turn out the lights, the party's over" -Dandy Don Meredith
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If it could get some convection to wrap around it, it would be fine.
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THE NAKED SWIRL OF DORIAN APPEARS TO BE TRACKING SOUTHWEST.
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Quoting 426. weatherlover94:
I can't get over how High the pressure is in Dorian....1010 mb


Given the higher pressures in the area, it probably works out. I remember Hurricane Danny(can't think of the year, 2003 maybe?) was 1000mb but was a 75mph cat 1.
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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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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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