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 1299. JLPR2:
Not a bad looking disturbance, but no spin whatsoever at the 850mb level.



Looked like a very broad spin between it and the Cape Verde's...Dry air is very much present though. Some cold cloud tops within that convection. I am naming this "Wave SPL" :D
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Quoting 1317. JLPR2:
Another detail is the bits of convection firing ahead of the system around 50-52W in the dry air ahead.



Dorian is heading that way...


Increments. Small, gradual increments mentioned earlier. Good eye, JLPR! ;)
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1326. Patrap
Tropical Storm 04L

UW-CIMSS Automated Satellite-Based
Advanced Dvorak Technique (ADT)
Version 8.1.4
Tropical Cyclone Intensity Estimation Algorithm

Current Intensity Analysis



UW - CIMSS
ADVANCED DVORAK TECHNIQUE
ADT-Version 8.1.4
Tropical Cyclone Intensity Algorithm

----- Current Analysis -----
Date : 27 JUL 2013 Time : 024500 UTC
Lat : 17:53:26 N Lon : 48:00:57 W


CI# /Pressure/ Vmax
2.8 /1009.1mb/ 41.0kt


Final T# Adj T# Raw T#
2.0 2.5 3.0

Center Temp : +15.4C Cloud Region Temp : -9.8C

Scene Type : SHEAR (0.29^ TO DG)

Positioning Method : FORECAST INTERPOLATION

Ocean Basin : ATLANTIC
Dvorak CI > MSLP Conversion Used : ATLANTIC

Tno/CI Rules : Constraint Limits : 0.5T/hour
Weakening Flag : ON
Rapid Dissipation Flag : OFF

C/K/Z MSLP Estimate Inputs :
- Average 34 knot radii : 50km
- Environmental MSLP : 1018mb

Satellite Name : MSG3
Satellite Viewing Angle : 57.8 degrees

Member Since: July 3, 2005 Posts: 420 Comments: 127543
Firing some thunderstorms. Going to need to multiply those thunderstorms by a thousand.

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New convection developing north and west of the center and not been sheared. Maybe ? We'll see.
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1322. Gearsts
50w
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1321. tj175
Quoting 1278. GTstormChaserCaleb:
I have always wondered what it is like myself, living here in the Tampa Bay area the highest I have seen was an 80 mph gust from Frances. I think sustained winds at the time were anywhere from 50-60 mph. and I was outside feeling the winds, it was really awesome man.



Wow that sounds just like what I experienced with Hurricane Katrina when it made 1st landfall here in North Miami in 2005. The winds with Katrina were very surprising since we were only suppose to get a tropical storm but she intensified as she got closer to land which like many of my friends and family said it totally caught us off guard that afternoon and those winds here easily had to be 60 to 70 mph. Then 2 months later Hurricane Wilma took the cake and was somewhat of the grand finale. Wilma's winds were the 1st where I had to literally brace up against something in order not to get blown over and those winds in my area were reported to be 85mph due to Wilma's quick forward speed and 100+ winds by the time she made it on the east coast of Florida. It was awesome man and a great adrenaline rush!!!!!!!!!
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Quoting 1308. hurricanes2018:
watch this tropical storm to!!
Yes it is. Its heading for Hawaii as a TS. Maybe some coconuts will fall off of trees.
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Dorian still looking nothing like a tropical storm on satellite. Badly needs a productive D-Max.
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1318. Patrap
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1317. JLPR2
Another detail is the bits of convection firing ahead of the system around 50-52W in the dry air ahead.



Dorian is heading that way...
Member Since: September 4, 2007 Posts: 7 Comments: 8499
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1315. Patrap
.."Hang on Dorian, Dorian Hang on"..



Member Since: July 3, 2005 Posts: 420 Comments: 127543
Latest Shear Tendency shows the wind shear is decreasing ahead of Dorian but still a lot of dry air.
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1313. JLPR2
What still surprises me the most is that this is firing up right in the middle of dry air. Dorian is indeed persistent.

Now it has something to build over for D-max, if it can recover a decent amount of convection its LLC could recover.
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Link Mesoscale Discussion
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Dorian should be hitting the higher SST at about DMAX.
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Quoting 1284. TimSoCal:
OK, I suppose since I asked the question, I should also answer. For me, it was a storm of terrifying power, that did some very terrible things.



Originally the local news in Tampa(where I was living) forecast it to come towards us there. I now almost think that would have been better than what actually happened.
That thing destroy the bridge that connect my city to the Eastern part of the country.
Member Since: October 15, 2011 Posts: 0 Comments: 4004
Dorian needs to hang on a little longer and continue to develop convection. If it can do that, we'll have a new ballgame.
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watch this tropical storm to!!
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1307. Patrap
Member Since: July 3, 2005 Posts: 420 Comments: 127543
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Is Texas still protected by the high that has protected us in the past several years?TIA
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So what's the story with the SHIPS model? It reintensifies Dorian. Old run?
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1303. beell
Anybody think that convection is moving southwest?
Indicative of an open wave?


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Conditions north of the Greater Antilles are very hostile for development for the time being. There's a large and potent upper-level trough inducing high wind shear across the region. Luckily for Dorian, this feature has begun to weaken today. Numerous shower and thunderstorms are firing inside it, releasing latent heat that warms the troposphere. In fact, when I checked the map this morning, when shear was blazing at 50kt across the eastern Caribbean in association with the feature; it's now down to 40 kt. There's an anticyclone near Dorian, but it's displaced; if the storm can fire persistent thunderstorm activity -- which it may or may not be able to do...waters are warm, but the environment consists of large scale sinking -- this would relocate over the system and ventilate it. We'll see. If the low-level center dissipates, Dorian probably won't be of much concern because surface pressures are very high and it would take a long time and be a difficult process to get a new one to develop.


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Quoting 1294. sar2401:

Hmmm...go to NHC Data Archive. If you can find more than one advisory that says 280, you'll get a cookie. :-)


What is your point? I am well aware of what the NHC advisories were. NHC is not who I was referring too.
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1300. Grothar
Quoting 1297. Chicklit:

have an 8-5 job now and also finishing up the 3 courses within these 7 days if I live that long.


You're excused. :)
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1299. JLPR2
Not a bad looking disturbance, but no spin whatsoever at the 850mb level.

Member Since: September 4, 2007 Posts: 7 Comments: 8499
Quoting 1280. Chicklit:
evening folks, looks like Dorian ran into some shear...or was it dry air?



I think Both "Dry Air & Wind Shear"

Taco :o)
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Quoting 1285. Grothar:


Where have you been, Chicklit. It died and is possibly coming back.

have an 8-5 job now and also finishing up the 3 courses within these 7 days if I live that long.
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Quoting 1269. Bluestorm5:
I've been through 100 mph thunderstorms and I don't mind seeing them again. Wondering what it's like to be in hurricane.


Well, just imagine your 100 mile per hour thunderstorm lasting several hours, non stop. Debris blowing around and through your house. Not much thunder or lighting though, just the never ending howling of wind and the slamming of sideways rain at your shudders and doors. The feeling that at any moment your rood will lift off, leaving you and your family exposed to the sharks. Oh, sorry, that's Sharknado. Never mind.
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Quoting 1280. Chicklit:
evening folks, looks like Dorian ran into some shear...or was it dry air?


Think it was mostly dry air, and small core it had. The shear is out ahead of it. If it manages to make a comeback, I would think it would have a rough time with that SW shear...

Look at it ripping those cloud tops away N of Cuba and the Islands...


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1294. sar2401
Quoting TideWaterWeather:


You are correct on the WNW idea... so many have said it is moving true west, almost from the time it formed.. yet the true track over time has continued to be around 290 or so

Hmmm...go to NHC Data Archive. If you can find more than one advisory that says 280, you'll get a cookie. :-)
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1293. JLPR2
Quoting 1277. Patrap:


Not bad...

Didn't think it was going to be able to fire up some decent convection with the air so dry. it might actually recover some.
Member Since: September 4, 2007 Posts: 7 Comments: 8499
Quoting 1284. TimSoCal:
OK, I suppose since I asked the question, I should also answer. For me, it was a storm of terrifying power, that did some very terrible things.



Originally the local news in Tampa(where I was living) forecast it to come towards us there. I now almost think that would have been better than what actually happened.


Yup, Mitch was bad news.
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Quoting 1277. Patrap:

Convection brewing to the west of the LLC, looks like the shear has went slack.
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vas is das?
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Quoting 1280. Chicklit:
evening folks, looks like Dorian ran into some shear...or was it dry air?
Dry air mostly.
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Quoting 1280. Chicklit:
evening folks, looks like Dorian ran into some shear...or was it dry air?



Yeah, Chick, it did, but you're looking at the sequel now. Convection is firing again. ;) It just won't quit.
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Latest satellite imagery shows the convection very gradually building and deepening with convection now covering the circulation center of Tropical Storm Dorian for the first time since this morning. Need to see the convection expand and become more organized but this is definitely progress and a sign that Dorian is not dead yet.
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Quoting 1278. GTstormChaserCaleb:
I have always wondered what it is like myself, living here in the Tampa Bay area the highest I have seen was an 80 mph gust from Frances. I think sustained winds at the time were anywhere from 50-60 mph. and I was outside feeling the winds, it was really awesome man.


I lived in Tampa Bay during the 2004 season. Jeanne gave us (in St. Pete) a 78 mph gust. Also, the remnants of Olga in 2007 merged with a front and brought winds gusting to 78 mph on Clearwater Beach on a night in mid-December 2007. TS Gabrielle in 2001 brought gusts to 70 mph, and Hurricane Gordon 2000 gave us 50-60 mph winds.
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1285. Grothar
Quoting 1280. Chicklit:
evening folks, looks like Dorian ran into some shear...


Where have you been, Chicklit. It died and is possibly coming back.
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OK, I suppose since I asked the question, I should also answer. For me, it was a storm of terrifying power, that did some very terrible things.



Originally the local news in Tampa(where I was living) forecast it to come towards us there. I now almost think that would have been better than what actually happened.
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Quoting 1275. Stormchaser121:

Is it still headed to the Gulf or will it turn north to the east coast and or FL?
Depends on strength. So none knows at this point.
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Quoting 1277. Patrap:
very impressive squall line!!
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1281. Grothar
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evening folks, looks like Dorian ran into some shear...or was it dry air?
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So if Dorian does follow the track and get near or into the gulf, would he intensify rapidly? What is in Dorian/s path that would help him get his act together.
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Quoting 1269. Bluestorm5:
I've been through 100 mph thunderstorms and I don't mind seeing them again. Wondering what it's like to be in hurricane.
I have always wondered what it is like myself, living here in the Tampa Bay area the highest I have seen was an 80 mph gust from Frances. I think sustained winds at the time were anywhere from 50-60 mph. and I was outside feeling the winds, it was really awesome man.
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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.