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 surprising to see Flossie up to 50 knots:

EP, 06, 2013072618, , BEST, 0, 163N, 1333W, 50, 999, TS, 50, NEQ, 20, 0, 0, 0, 1009, 150, 20, 0, 0, E, 0, , 0, 0, FLOSSIE, D,
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Doesn't Dorian resemble the horsehead nebula in some way? I remember an article some days ago dealing with this resemblance of very different systems in our world, using the idea of fractals. I've tried to find it again, but I've failed. :(

Member Since: October 25, 2008 Posts: 63 Comments: 6708
Quoting 273. DataNerd:


Agreed 100%. I think ascat should put the nail in the coffin here. Could always regenerate but I think thats not going to happen. You never know though.


If it remains north of the islands then regeneration should be considered more likely than not.
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Quoting 266. washingtonian115:
Just look at it.And if that TUTT doesn't back away some it will be sheared in 4 days like the NHC is saying.It's practically chocking off dry air.
You should no better that wind shear forecasts more than 3 days out are unreliable and that all it takes is a subtle movement of the TUTT to create favorable conditions. We shall see, odds are against it though and most if not all of the computer models forecast weakening.
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Im not gonna hold my breath here but it appears that on the last image of the satellite it appears some convection MAY be trying to rewrap on it
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Quoting 269. CybrTeddy:
Back for now, but based on the current 3-6 hour trends Dorian is indeed dying. These small systems are always notorious to fall victim to conditions that wouldn't kill a larger cyclone. I expect that Dorian will open up into a wave based on the last 3-6 hour trends and the latest ASCAT pass. This was always a possibility, and it's one we've been noting could very well happen as Dorian passed through the MDR.

After Dorian's demise, which I do believe will happen, we'll have to watch it for regeneration unless of course it moves into Haiti and the DR, in which case development will be highly unlikely. If Dorian can remain north of the islands and move into the Bahamas, it may become a moderate tropical storm making landfall in Florida, or go through the Florida straits which may also become a problem.

Please note though, before everyone becomes a Debbie downer about the season, this is only happening because of Dorian's small size and a suppressive MJO phase that's causing pressures to be higher and instability to be lower. If Dorian had developed further south I have no doubt it would have become a powerful hurricane as it approached the Caribbean and we have to watch for this to happen the next few weeks.

The season hasn't even started yet, anything before August 15th for me is a bonus that we get to track. Consider Dorian and Chantal warmups because we're going to have plenty of action soon. This is the case every season.


Agreed 100%. I think ascat should put the nail in the coffin here. Could always regenerate but I think thats not going to happen. You never know though.
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18z SHIPS shows some modest strengthening in the latter part of the forecast, which I find to be very plausible, if it's able to remain a tropical cyclone throughout the next 72 hours.
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;
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Back for now, but based on the current 3-6 hour trends Dorian is indeed dying. These small systems are always notorious to fall victim to conditions that wouldn't kill a larger cyclone. I expect that Dorian will open up into a wave based on the last 3-6 hour trends and the latest ASCAT pass. This was always a possibility, and it's one we've been noting could very well happen as Dorian passed through the MDR.

After Dorian's demise, which I do believe will happen, we'll have to watch it for regeneration unless of course it moves into Haiti and the DR, in which case development will be highly unlikely. If Dorian can remain north of the islands and move into the Bahamas, it may become a moderate tropical storm making landfall in Florida, or go through the Florida straits which may also become a problem. Dorian may be down but he's certainly not out, these things can come back with a vengeance.

Please note though, before everyone becomes a Debbie downer about the season, this is only happening because of Dorian's small size and a suppressive MJO phase that's causing pressures to be higher and instability to be lower. If Dorian had developed further south I have no doubt it would have become a powerful hurricane as it approached the Caribbean and we have to watch for this to happen the next few weeks.

The season hasn't even started yet, anything before August 15th for me is a bonus that we get to track. Consider Dorian and Chantal warmups because we're going to have plenty of action soon. This is the case every season.
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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.


The SST's in The Bahamas have been below normal for months do to the cloud cover and rain, NW Bahamas has had over 45" of rain since May, would expect they should get back close to normal by mid August.
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Quoting 258. ncstorm:
Most of yall have been on WU for a lot of years and have seen storms come back with a vengeance after looking pitiful..this kind of attitude you would expect from newbies..with that said, you all should know better..(shaking my head over here)
Just look at it.And if that TUTT doesn't back away some it will be sheared in 4 days like the NHC is saying.It's practically chocking off dry air.
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Somewhere, the NHC forecasters are face palming, by the dozen.


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Quoting 257. wunderkidcayman:
looks like Dorian is making a comeback it had reorganized it Low Level structure all it needs is convection on top of this circulation and then bam back to new
Quoting 256. weatherlover94:
Dorian on Life support now :P ....center still closed but it may not stay closed long
LOL at the two extreme opposite thoughts.
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Quoting 258. ncstorm:
Most of yall have been on WU for a lot of years and have seen storms come back with a vengeance after looking pitiful..this kind of attitude you would expect from newbies..with that said, you all should know better..(shaking my head over here)



Nobody Listens. NEVER underestimate a tropical system.
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Dorian is done guys, dry air, shear, and trade winds have taken its toll, he be ripped worse than a stack of mail. Strong shear north of PR, combined with Hispaniola in his sights sorry Dorian its late July we shoulda seen you in Aug maybe you woulda had a shot.
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This night-time, infrared image of Tropical Storm Dorian was taken from the VIIRS instrument aboard the Suomi-NPP satellite on July 25 at 03:52 UTC. The red and purple areas are very high thunderstorms. Credit: UWI-Madison/NASA-NOAA

NASA's various views of Tropical Storm Dorian
NASA satellites analyzed Tropical Storm Dorian in infrared light, giving scientists an idea of the storm's structure, cloud heights and cloud temperatures.

Read more at: http://phys.org/news/2013-07-nasa-views-tropical-s torm-dorian.html#jCp [or link above]
Member Since: October 25, 2008 Posts: 63 Comments: 6708
Okay I am calling it. Dorian has or is dissipating right now. LLC is barely discernible and has left the main body of convection to the west, and is barely even turning on visible satellite. The remaining convective structure is collapsing and/or resembles an open wave at this time.

They should keep the system for 5 PM and possibly kill it at 11 or 5 am. Its possible that neither happens and NHC holds the system for another 12-24 hrs hours or so but unless the current trend reverse itself I find this very unlikely.

Thats all she wrote folks. Just too early yet in the season for a storm to make it through these types of conditions. We are very fortunate. Had this made it to the gulf it would have been a serious threat.

Link



Windsat confirms the collapse:




With the cape verde wave system heating I would expect another cyclone to form in the next two weeks. There is already another very vigorous wave that has just emerged off the coast, and GFS indicates something may form in that area in 100 hrs or so.

And dorian may not be totally dead yet, but I would be surprised (unless something changes) if the system is not gone in 12 hours.
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looks like Dorian is making a comeback it had reorganized it Low Level structure all it needs is convection on top of this circulation and then bam back to new
Member Since: June 13, 2009 Posts: 2 Comments: 12716
Dorian on Life support now :P ....center still closed but it may not stay closed long
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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 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.







If dorian can hang on for another 8 to 12 hours, IMO, he will find himself in a little better environment, even warmer SST, and tapping into the moisture to his NW. I wouldn't write this system off just yet.
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Meanwhile, as Dorian is weakening, a new wave is almost off with impressive convection and moisture shouldn't be an issue for the moment.



It's just on/below the 10N line, so should be over warm SSTs for a while



It also has some decent lower convergence



And there seems to be some decent upper divergence as well



It is moving towards lower wind shear too



It'll be very interesting to watch. :)
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253. JLPR2
Really easy to spot the LLC, it's exposed on the lower left side of this image. It needs to cover it ASAP.

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252. VR46L
Quoting 209. weatherlover94:
Hi everybody. Can I be the first to say RIP Dorian ?


I think quite a few people saw that this morning .... as they were looking at the shear and dry air .. I am surprised its not busted already .
Member Since: March 1, 2012 Posts: 0 Comments: 6998
Quoting 69Viking:


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.

Except for a patch south of Cuba, western Caribbean waters aren't as warm as usual either. Blackbird Caye, about 25 miles offshore from Belize City, is where I've done a lot of diving. I usually go in late July and August because the rates are cheap and climatology is on my side about escaping tropical storms. I think I've been there six times now. Never had surface temps of less that 87 and 20 meter temps of less than 85. The surface temperature today is 84.7 and the 20 meter temperature is 82.3. Sounds pretty good until you're in the water about 45 minutes. 82.3 is downright chilly compared to a normal 86-88. In July, 2009, the surface temperature was 91 and the 20 meter temperature was 91. That was some great diving!
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They will take Dorian down to at least 40-45mph but keep her... they will say also that this could be a open wave in 12hrs. Dorian will struggle over the next 24hrs...then have a small window to reorganize Sat night thru Mon morning. Question by Monday will be if she regained any organization and how much shear is around the Bahamas and Cuba
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Quoting 232. Tropicsweatherpr:
18z Best Track down to 40kts.

AL, 04, 2013072618, , BEST, 0, 176N, 448W, 40, 1010, TS

well see thats close to where I have it at 17.5N 44.9W however I don't agree on the pressure at most I see it at 1007/1008mbs
Member Since: June 13, 2009 Posts: 2 Comments: 12716


Exposed low-level eyewall at 17.5n,45w???????? Strange-looking indeed.
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Well...at least we know the low-level circulation is closed.

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One thing that I don't understand is that the shear is going to be backing away in the days to come. Why can't this intensify ?
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Quoting 202. K8eCane:
Thanks Lee. Very informative blog post.


You're welcome.
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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 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.


Well said!
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Quoting 237. washingtonian115:
Wouldn't be surprised if we have a open wave by the next advisory.The pressure is at 1010.


I think you may be right
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Quoting 223. wunderkidcayman:

agreed

It will not make it that far. It will likely dissapate by Saturday.
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Quoting 229. CaribBoy:


Which is a little bit south relative to the forecast track XD

yeah I know


Also I watched that area through the loop and I saw it there before but S side had looked clear and open which looked to have agreed with the scat image however that has now sealed off
Member Since: June 13, 2009 Posts: 2 Comments: 12716
11:00 AM AST Fri Jul 26
Location: 17.7°N 43.4°W

XD LOL
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this will be an open wave come morning.
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Wouldn't be surprised if we have a open wave by the next advisory.The pressure is at 1010.
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Quoting 232. Tropicsweatherpr:
18z Best Track down to 40kts.

AL, 04, 2013072618, , BEST, 0, 176N, 448W, 40, 1010, TS


Ans DOWN 0.1 degree of latitude
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Dorian going down to 45 at 5:00

Link
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Quoting 221. ecflweatherfan:


The closest thing I can see to a circulation would be near 17.9N 44.7W.


I agree...if convection cant initiate over that COC, its over. But if it can....
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18z Best Track down to 40kts.

AL, 04, 2013072618, , BEST, 0, 176N, 448W, 40, 1010, TS
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Quoting 224. MiamiHurricanes09:
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%u02DAC 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.



Pretty close, down to 40kts now

AL, 04, 2013072618, , BEST, 0, 176N, 448W, 40, 1010, TS
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Quoting 218. wunderkidcayman:
ok I can clearly see a LLCOC now or atleast what appears to be looking good near 17.5N 44.9W moving W


Which is a little bit south relative to the forecast track XD
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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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