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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However, if it somehow miraculously is able to survive the trek until about 60W without degenerating into an open wave, I do believe that it could potentially pose a threat to Cuba, the Bahamas, and Florida. But once again, that's a big IF.
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Quoting 186. opal92nwf:

Once it conquers that tongue of dry air, it should do a little better.

Then it will face the stationary ULL north of Puerto Rico which will finish it off if it isn't already dissipated by then.
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Quoting HurricaneHunterJoe:
Why does it do that triplicate thingee sometimes?

I think you mean tripole. Check out this UK Met page for a pretty good explanation of the tripole.

Edit: Never mind, you mean the triplicate quotes. No idea, just weird software here.
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Satellite IR loops reveal that the convective structure, organization, and intensity associated with Dorian have become less organized over the past few hours. At this point, I do believe that degeneration into an open wave is more likely than not unless it's able to start firing at least some sort of convection in the very near future. The fact that it completely lacks cloud tops colder than -60˚C is bad news for the cyclone, let alone any sort of banding features. At this point in time I'd assess the intensity no higher than 35kts.

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Quoting 217. moonlightcowboy:


Dorian may track slightly more west/northwest for a time, but expect a shift more due west as the Atlantic high pushes southwest. The trough has also split and is lifting out, closing the escape route out to sea. If it can retain some organization despite the dry air, moderate shearing, Dorian will be a land-falling system for the islands, possibly FL, and quite possibly even becoming a GoM system.

Low level convergence has improved and outflow is looks good as it tries to get sfc lift going and growing its system.

This is now less about track and more about intensity. Will it make a comeback? Warmer waters ahead, and so far has managed to still fight the dry air similarly to Chantal. Forward speed is still a bit too quick though. So, we shall see. Next 12-36 hours will be critical to any longevity as it approaches the northern Antilles.

agreed
Member Since: June 13, 2009 Posts: 2 Comments: 11235
Quoting 204. washingtonian115:
Dorian is pathetic..And if it hits Hispaniola you can forget about it.


Whether it dies or not though, Hispanola does NOT need another tropical storm.
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Quoting 173. SPLbeater:
I aint one to change my view with the wind, but looking at satellite loops...I cant find a good circulation. To me...

Dorian looks dead.


The closest thing I can see to a circulation would be near 17.9N 44.7W.
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TROPICAL STORM FLOSSIE
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Quoting 208. sar2401:

Yeah, these small storms are dangerous, but they can also go poof almost as easily. Watching Dorian frame by frame and trying to make a judgement of what it will be doing three days from now is hopeless. It's going to pulse constantly as it encounters everything from dry air, warmer water, and big flock of albatross flying through. :-)


You got that right....

frame-by-frame? Useless.
Model forecast? Useless.
Trying to forecast a correct intensity? Useless!
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ok I can clearly see a LLCOC now or atleast what appears to be looking good near 17.5N 44.9W moving W
Member Since: June 13, 2009 Posts: 2 Comments: 11235


Dorian may track slightly more west/northwest for a time, but expect a shift more due west as the Atlantic high pushes southwest. The trough has also split and is lifting out, closing the escape route out to sea. If it can retain some organization despite the dry air, moderate shearing, Dorian will be a land-falling system for the islands, possibly FL, and quite possibly even becoming a GoM system.

Low level convergence has improved and outflow looks good as it tries to get sfc lift going and growing its system.

This is now less about track and more about intensity. Will it make a comeback? Warmer waters ahead, and so far has managed to still fight the dry air similarly to Chantal. Forward speed is still a bit too quick though. So, we shall see. Next 12-36 hours will be critical to any longevity as it approaches the northern Antilles.
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Quoting 201. sar2401:

At least at the surface, but it's going to take quite a while to get that warmer water down to the 60 meter level. This has been a really unusual summer so far. Looking at some of my favorite dive sites in Belize, the Virgins, the Bahamas, and Grenada/St. Vincent, I'd have to wear at least a 3mm shorty to be comfortable at 20 meters. I've never worn more than a dive skin in July at any of these places.


Well if you remember just a week or two back the Gulf had ULL's all around with convection covering I would guess about 75% of it. If we get into a normal weather pattern of 90+ degree days it won't take as long as you think. Warm water will move up from the Caribbean too. I think by mid August it will be closer to normal and storms won't have any problem intensifying when they get there. Water temps are 83-84 now, let's see what they are in the next week or two.
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Quoting 211. sar2401:

LOL. There's always alcohol and more pain pills to keep us occupied. :-)


That reminds me..its been 8 hours since my last.......... :)
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Quoting weatherlover94:
Hi everybody. Can I be the first to say RIP Dorian ?

You can be as long as you like deep fried crow. :-)
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Looks like Dorian will move into more moisture after tonight, atleast the upper and mid-levels will turn a bit more favorable here.
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Quoting 209. weatherlover94:
Hi everybody. Can I be the first to say RIP Dorian ?


Might be premature......?
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Why does it do that triplicate thingee sometimes?
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Hi everybody. Can I be the first to say RIP Dorian ?
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Quoting SPLbeater:


Remember Danielle 2010? That thing went into RI to a category 2, then in the morning dry air took it back down to a TS in 12 hours. But later, it re-intensified to a category 4.

Yeah, these small storms are dangerous, but they can also go poof almost as easily. Watching Dorian frame by frame and trying to make a judgement of what it will be doing three days from now is hopeless. It's going to pulse constantly as it encounters everything from dry air, warmer water, and big flock of albatross flying through. :-)
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Quoting 197. sar2401:

Joe, the models never initialized properly on Dorian because it's such a small storm. All the models are working off a bogus vortex, so they run out of data after about five days. I'm just ignoring the models and using what I know about these small, long track CV storms to make guesses. Once the recon has flown, then we'll have some data to hang our hats on.
Quoting 197. sar2401:

Joe, the models never initialized properly on Dorian because it's such a small storm. All the models are working off a bogus vortex, so they run out of data after about five days. I'm just ignoring the models and using what I know about these small, long track CV storms to make guesses. Once the recon has flown, then we'll have some data to hang our hats on.
Quoting 197. sar2401:

Joe, the models never initialized properly on Dorian because it's such a small storm. All the models are working off a bogus vortex, so they run out of data after about five days. I'm just ignoring the models and using what I know about these small, long track CV storms to make guesses. Once the recon has flown, then we'll have some data to hang our hats on.


We have to wait til Sunday? What will we do? :)
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Dorian decoupling. Only has hours to live. Maybe 24 hours if lucky.
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Quoting 185. SPLbeater:


Still cannot find any low-level clouds moving east on the southern periphery, and the recent convection is already fading southeast.

Waiting till 5PM for a professional look I reckon...


The ASCAT pass this morning showed the Southern side of the circulation almost open so inflow was weak even then but the rotation is better this afternoon which is saying a lot :-).
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Dorian is pathetic..And if it hits Hispaniola you can forget about it.
Member Since: August 14, 2010 Posts: 10 Comments: 16444
Seem like the Euro is trying to make Dorian a little stronger as it approaches the bahamas
96 hours
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Thanks Lee. Very informative blog post.
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Quoting 69Viking:


Just give it a week or so with clear skies like this and it will approach rocket fuel status soon enough!


At least at the surface, but it's going to take quite a while to get that warmer water down to the 60 meter level. This has been a really unusual summer so far. Looking at some of my favorite dive sites in Belize, the Virgins, the Bahamas, and Grenada/St. Vincent, I'd have to wear at least a 3mm shorty to be comfortable at 20 meters. I've never worn more than a dive skin in July at any of these places.
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Storm Relative 1km Geostationary Visible Imagery Loop

click image for Loop

click loop to ZOOM

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The last few visible satellite images seem to show the low level circulation of Tropical Storm Dorian becoming exposed to view just to the WSW of the main convection. Definitely looking like a struggle for Dorian for sure.
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Flossie is looking fairly organized for a tropical storm...at least on the sat view. I'm curious to see what the next update will state. The big island of Hawaii could use some rain so this might be a perfect situation as a depression or low end tropical storm.

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Quoting HurricaneHunterJoe:
I can't find any model support for Dorian continuing it's life as a Tropical Cyclone.........Anybody?

Joe, the models never initialized properly on Dorian because it's such a small storm. All the models are working off a bogus vortex, so they run out of data after about five days. I'm just ignoring the models and using what I know about these small, long track CV storms to make guesses. Once the recon has flown, then we'll have some data to hang our hats on.
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Quoting 184. sar2401:

Yes, sometimes it take several tries before I can get it to open. This has been going on for a few days, and seems to affect a lot of NOAA sites as well


No problems with NHC here
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195. JRRP
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Quoting 165. sar2401:

Indeed. With all the rain and clouds the last month, the average water temperatures have taken a dive from the Gulf to the Bahamas. The water is still plenty warm to support a TC, but the fabled "rocket fuel" we had two years just isn't the case today.


Just give it a week or so with clear skies like this and it will approach rocket fuel status soon enough!

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Quoting 187. Relix:
I have an activity on Monday. As a guy living in PR, should I worry about direct impact?


No direct impact. Only a few outer bands that can reach the northern part of the island.
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Quoting 188. sar2401:

I'm on the side of miraculous comeback, at least for now. I've seen this many times in long track CV storms.


Remember Danielle 2010? That thing went into RI to a category 2, then in the morning dry air took it back down to a TS in 12 hours. But later, it re-intensified to a category 4.
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Quoting Relix:
I have an activity on Monday. As a guy living in PR, should I worry about direct impact?

I don't know if you should worry about it, but I wouldn't rule anything out right now. Just watch the results once recon gets in there and we'll have a better idea of what might happen.
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72 hours
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Quoting JLPR2:


Watch it upset us all by making a miraculous comeback. XD

I'm on the side of miraculous comeback, at least for now. I've seen this many times in long track CV storms.
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187. Relix
I have an activity on Monday. As a guy living in PR, should I worry about direct impact?
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Quoting 181. interpreter:
Dorian getting badly sheared and consumed by very dry air.

Once it conquers that tongue of dry air, it should do a little better.
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Quoting 179. kmanislander:


Take a look at the loop in my post 169. Higher resolution. Dorian is no poster child for great structure but this morning it had one foot in the grave and one on a banana peel. Present condition marginally improved but survival still up in the air.


Still cannot find any low-level clouds moving east on the southern periphery, and the recent convection is already fading southeast.

Waiting till 5PM for a professional look I reckon...
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Quoting 19N81W:
anybody else having problems opening the nhc site these days?

Yes, sometimes it take several tries before I can get it to open. This has been going on for a few days, and seems to affect a lot of NOAA sites as well
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183. JLPR2
Quoting 173. SPLbeater:
I aint one to change my view with the wind, but looking at satellite loops...I cant find a good circulation. To me...

Dorian looks dead.


Watch it upset us all by making a miraculous comeback. XD
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I can't find any model support for Dorian continuing it's life as a Tropical Cyclone.........Anybody?
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Dorian getting badly sheared and consumed by very dry air.
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Quoting 173. SPLbeater:
I aint one to change my view with the wind, but looking at satellite loops...I cant find a good circulation. To me...

Dorian looks dead.


Take a look at the loop in my post 169. Higher resolution. Dorian is no poster child for great structure but this morning it had one foot in the grave and one on a banana peel. Present condition marginally improved but survival still up in the air.
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And because of this 18 hour weakening/disorganizing trend, I will likely post a new blog this evening(if anyone cares).
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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.