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 826. Birthmark:

No you don't.



No, really!
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Congratulations Dr. Masters, I think this is the longest winded "I told you so," in the history of your blog.

I'm sticking to my analysis yesterday that "Doris"(Dorian) was a minor Greek god, but with powerful relatives.

I do have a serious question for the experts here, and this is just from casual observation over the years.

How come we rarely see a system in the Eastern Pacific, such as Flossie, die or be as severely impeded by dry air, as often as we see in the Atlantic generally? For example, look at Flossie now, and while not a perfect comparison, I think you understand what I'm getting at.
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Quoting 797. PensacolaDoug:



I remember the predictions in 2006 of an Ice-free Arctic by summer 2013.

No you don't.
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825. VR46L
Quoting 823. KoritheMan:


It looks like an ordinary late July wave. We'll see what it does in a few days, if anything.

And not to nitpick, but the NHC surface map does not list that feature as a tropical wave yet.


They didn't have Dorian either for a while after the wave hit the water .

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Quoting 796. RTSplayer:
It's basically dead in the water here.

With this picture even myself I say its time to say goodbye.
Member Since: October 15, 2011 Posts: 0 Comments: 4010
Quoting 815. Bluestorm5:
This is why I go on here. You learn something new everyday. What are your thoughts on that CV wave?


It looks like an ordinary late July wave. We'll see what it does in a few days, if anything.

And not to nitpick, but the NHC surface map does not list that feature as a tropical wave yet.
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Quoting 811. KoritheMan:


And furthermore, just because there isn't currently any model support for development, doesn't really mean anything. How many storms this year have the models actually picked up on?


That is right about the models. The models so far have not been so good picking up things.Let's see if they do a better job in the peak months.
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Quoting 819. PensacolaDoug:




ittyboo....


We get over it and move on, eh Doug? :)
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Quoting 817. KEEPEROFTHEGATE:


low level swirl racing west out front looks like leaving behind its mid levels in the rear



that may be the case but we still keep an eye for redevelopment of convection
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Quoting 812. L1990:
very disappointed with these pathetic tropical systems this year




ittyboo....
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Quoting 767. sar2401:

Doing fine, Allan, hope you are too. For the fist time this month, we've had two consecutive days without rain. Never thought I'd want rain to stop in Alabama in July.

First e have to dispose of Dorian, who's on life support but hasn't quit breathing yet. The wave coming off Africa now looks a lot bigger and more impressive than Dorian, but sometimes they shrink when they get out over the ocean. The main indicator this year is going to be speed. If all these storms keep roaring around at 20-30 mph, all of them will struggle. As the SST's increase, we need storms traveling at no more than 10 mph, so they can develop a good moisture envelope before they meet up with the inevitable dry air and shear as they get closer to the Antilles. So, that was the long answer. Short answer is I don't know, but a slow mover has a lot better chance.
Yeah I am doing fine too,But remember a slow moving can cause upwell I believe a good speed for tc will 10 to 20mph.
Quoting 778. Bluestorm5:
It'll slow down as we go toward mid-August. Speed are always fast in July due to trades, I believe. Dorian was little high, so that why it was little slower, but it was still too fast. Dorian couldn't protect himself from dry air.
Including that dry air has diminish.Dorian so far had been protect because it was stealing moisture from the itcz.Now that he is by himself it doesn`t have it to protect him.
Member Since: October 15, 2011 Posts: 0 Comments: 4010
Quoting 806. wunderkidcayman:
Dorian has just survived the peak of D-min now we wait and see if convection increase tonight


low level swirl racing west out front looks like leaving behind its mid levels in the rear


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We better not have 8 tropical storms in a row like 2011. that would sssuuuccckkk
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Quoting 807. KoritheMan:


The MJO is less important for individual storm formation during August and September. It can still modulate the larger-scale dry air though, which can of course assist in intensification of any hypothetical tropical cyclone.
This is why I go on here. You learn something new everyday. What are your thoughts on that CV wave?
Member Since: August 1, 2011 Posts: 28 Comments: 7902
Quoting 791. Patrap:
Portlight Honored with 1 WTC Flag

Chairman Paul Timmons and New Jersey Portlight Volunteers received a Letter of Appreciation and a Flag that flew over the New WTC 1 in New York Monday morning as a token of their State's appreciation of the work done Post Sandy by Portlight and it's volunteers.








Portlight's mission includes providing for the needs of people with disabilities, especially in times of catastrophe.







Impressive and Commendable.
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813. JLPR2
Quoting 789. KoritheMan:
There is some pretty strong evidence to suggest that Dorian no longer has a closed surface circulation:



1445 UTC OSCAT pass.



1256 UTC ASCAT pass.

If a residual circulation does exist, visible satellite images suggest it's very small.


Even the high resolution version shows Dorian's LLC has opened up.


Though there are still some nice winds with it.
Member Since: September 4, 2007 Posts: 7 Comments: 8499
812. L1990
very disappointed with these pathetic tropical systems this year
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Quoting 805. Tropicsweatherpr:


You said it all very well. And the most important thing is El Nino wont be around to disturb the North Atlantic season so let's be prepared for anything mother nature brings in ASO.


And furthermore, just because there isn't currently any model support for development, doesn't really mean anything. How many storms this year have the models actually picked up on?
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Member Since: July 3, 2005 Posts: 421 Comments: 127559
LLC east of Southern SC growing convection over the center, wind shear is a little hostile, but it's decreasing all around it, it's more vigorous and closed than Dorian. NHC should issue a 20% chance of development with the next TWO at 8pm EDT
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Quoting 800. buzzardswrath:
I cant take it anymore



You know what to do then.
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Quoting 770. Bluestorm5:
My gut is telling me we'll see another storm by August 10th. It's too bad models and MJO doesn't support that.



If I'm reading the map right, MJO is unfavorable until favorable area arrive in late August to early September. That's the first thing a big storm need. Sometimes you don't need MJO to have a storm, though.


The MJO is less important for individual storm formation during August and September. It can still modulate the larger-scale dry air though, which can of course assist in intensification of any hypothetical tropical cyclone.
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Dorian has just survived the peak of D-min now we wait and see if convection increase tonight
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Quoting 794. KoritheMan:
Let's try to remember that, although this season has been piss poor so far, both Chantal and Dorian developed east of the Lesser Antilles in July. This is extremely remarkable, especially considering they were both able to do it without the assistance of the MJO, something that is normally a requirement during the early season.

This suggests that, if the high ever decides to weaken, eradicating its associated sinking air and fast low-level flow, things will get going in a hurry. The MDR is primed for development from the vantage point of upper-level winds, it's just the relative humidity values and trades haven't cooperated yet.

2008 was the last time a named tropical cyclone occurred east of the Lesser Antilles prior to August, and we saw two exceptionally intense hurricanes develop within that region -- Gustav and Ike. The former produced wind gusts of 230 mph in the Pinar del Rio province of Cuba, and the latter obliterated the Galveston area.

It's not over, and the high will weaken. It was even pretty strong during 2008, as evidenced by Bertha and Dolly clipping along at or over 20 mph during July. Patience guys. This season will still be very active, and based on the upper air pattern so far, will probably be a very dangerous one for the US.


You said it all very well. And the most important thing is El Nino wont be around to disturb the North Atlantic season so let's be prepared for anything mother nature brings in ASO.
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800. buzzardswrath


good you are excused to leave
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Quoting 780. flcanes:

May i remind you, Ike and Sandy generated tons of damage and neither was a major at landfall in the US.



What's yer point? Were they caused by AGW?
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Quoting PensacolaDoug:



My point, when I post things like that, is attempting to remind folks that back in the mega-seasons of '04 and '05, the AGW crowd said that storms like IVAN and KATRINA would be the new normal. No majors in years for the US and global ACE is way down. Heck of a way to run a global warm-up.

Actually, I agree with Doug. There was way too much overplaying of the idea that 04-05 heralded bad hurricane seasons Ad infinitum. It's always a bad idea to hop on two years and say they are a template for things to come. We may very well turn out to have worse average hurricane season 50 years from now than we do today. I don't really know, and that's why I wouldn't drive a stake in the ground of it starting at any particular point in time. There's no doubt in my mind that the climate is changing, and some of the change has been bought on by humans. I know for sure we have an imperfect understanding of climatology, and the last seven plus years have shown us that we're not very good at all predicting changes in climate, even macro changes.
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Quoting 800. buzzardswrath:
I cant take it anymore
take it and like it
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Quoting 77. Tropicsweatherpr:


In other words,Chantal and Dorian were teasers forming in the MDR in July and preparing the scene for the biggies.
Two big teasers. Chantal and Dorian somehow survived MDR in July, which is unbelievable to start with. Dorian is the 2nd farthest east named storm in known July history which is huge. The fact Dorian made it this far west suggest that Eastern Atlantic is very favorable this season. Also, Chantal and Dorian pretty much sacrificed themselves by wetting the MDR for future storms. There will be less dry air than normally unless we go long without Erin or another storm in MDR. This should be active CV season judging by these two storms, but nothing is set in stone (a common phase on this blog it seems...)
Member Since: August 1, 2011 Posts: 28 Comments: 7902
Well, game over for Dorian, and it looks like its going to be a quiet rest of July in the Atlantic. I guess it could've been noted that Dorian wasnt expected to get very far.
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Quoting 785. KEEPEROFTHEGATE:
it was a guess outcome to the actual real thing totally different than forecasted kinda like the ice melt first it was hundreds of years but in reality its happening much faster in tens of years instead



I remember the predictions in 2006 of an Ice-free Arctic by summer 2013.
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It's basically dead in the water here.

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Quoting 789. KoritheMan:
There is some pretty strong evidence to suggest that Dorian no longer has a closed surface circulation:



1445 UTC OSCAT pass.



1256 UTC ASCAT pass.

If a residual circulation does exist, visible satellite images suggest it's very small.
it was fun while it lasted opens up it will be declassified and tagged for regeneration watch
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Let's try to remember that, although this season has been piss poor so far, both Chantal and Dorian developed east of the Lesser Antilles in July. This is extremely remarkable, especially considering they were both able to do it without the assistance of the MJO, something that is normally a requirement during the early season.

This suggests that, if the high ever decides to weaken, eradicating its associated sinking air and fast low-level flow, things will get going in a hurry. The MDR is primed for development from the vantage point of upper-level winds, it's just the relative humidity values and trades haven't cooperated yet.

2008 was the last time a named tropical cyclone occurred east of the Lesser Antilles prior to August, and we saw two exceptionally intense hurricanes develop within that region -- Gustav and Ike. The former produced wind gusts of 230 mph in the Pinar del Rio province of Cuba, and the latter obliterated the Galveston area.

It's not over, and the high will weaken. It was even pretty strong during 2008, as evidenced by Bertha and Dolly clipping along at or over 20 mph during July. Patience guys. This season will still be very active, and based on the upper air pattern so far, will probably be a very dangerous one for the US.
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Dorian is in very bad shape ATM.

I said to myself this morning it would be a wonder if it made it through the day, then come back now, and this thing has nearly been torn apart by the dry air.

It could be downgraded to depression over night at this rate.
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Not much hope for Dorian now, he had a good run
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Portlight Honored with 1 WTC Flag

Chairman Paul Timmons and New Jersey Portlight Volunteers received a Letter of Appreciation and a Flag that flew over the New WTC 1 in New York Monday morning as a token of their State's appreciation of the work done Post Sandy by Portlight and it's volunteers.








Portlight's mission includes providing for the needs of people with disabilities, especially in times of catastrophe.



Member Since: July 3, 2005 Posts: 421 Comments: 127559
Quoting 784. sar2401:

Looks to me like another in a long line of disorganized blobs without much evidence of circulation. It kind of looks like a picture of ITCZ in July - just convection but not doing anything. I don't know why, but things just don't look "right" with what's coming off the African coast. Maybe it's still too early in the season, and we would normally be minutely examining every little wave off Africa. I can't put my finger on it. but it's just not "right" yet.
Maybe it's the lack of models supporting the storm? Maybe it's because most waves don't become a storm no matter which month? We'll see, but I do have the same feeling. I just don't feel like storm is doing anything and maybe other feel the same because no one is talking about it. I'll have to ask folks I chat with in chat room about that.
Member Since: August 1, 2011 Posts: 28 Comments: 7902
There is some pretty strong evidence to suggest that Dorian no longer has a closed surface circulation:



1445 UTC OSCAT pass.



1256 UTC ASCAT pass.

If a residual circulation does exist, visible satellite images suggest it's very small.
Member Since: Posts: Comments:
Quoting 787. Tropicsweatherpr:


In other words,Chantal and Dorian were teasers forming in the MDR in July and preparing the scene for the gies.

As i mentioned during Chantal, August is when the big boys and girls go out to "play".
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Quoting 783. Bluestorm5:
I know, but it's more likely to have storms during MJO Atlantic phase. The fact it's going to be here during the peak of season is worrying.


In other words,Chantal and Dorian were teasers forming in the MDR in July and preparing the scene for the biggies.
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got my mojo working
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Quoting 776. PensacolaDoug:



My point, when I post things like that, is attempting to remind folks that back in the mega-seasons of '04 and '05, the AGW crowd said that storms like IVAN and KATRINA would be the new normal. No majors in years for the US and global ACE is way down. Heck of a way to run a global warm-up.
it was a guess outcome to the actual real thing totally different than forecasted kinda like the ice melt first it was hundreds of years but in reality its happening much faster in tens of years instead
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Quoting Bluestorm5:
What are anyone's thoughts about this little wave near Cape Verde?


Looks to me like another in a long line of disorganized blobs without much evidence of circulation. It kind of looks like a picture of ITCZ in July - just convection but not doing anything. I don't know why, but things just don't look "right" with what's coming off the African coast. Maybe it's still too early in the season, and we would normally be minutely examining every little wave off Africa. I can't put my finger on it. but it's just not "right" yet.
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poting 772. flcanes:

We dont need the MJO in august.
I know, but it's more likely to have storms during MJO Atlantic phase. The fact it's going to be here during the peak of season is worrying.
Member Since: August 1, 2011 Posts: 28 Comments: 7902
Quoting 779. hurricanes2018:
new tropical wave got a good spin to it

We may have Erin sooner than i thought.
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Quoting 768. VR46L:
Going ,going , Gone!!!



rock-a-bye-baby
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Quoting 776. PensacolaDoug:



My point, when I post things like that, is attempting to remind folks that back in the mega-seasons of '04 and '05, the AGW crowd said that storms like IVAN and KATRINA would be the new normal. No majors in years for the US and global ACE is way down. Heck of a way to run a global warm-up.

May i remind you, Ike and Sandy generated tons of damage and neither was a major at landfall in the US.
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new tropical wave got a good spin to it
Member Since: March 12, 2013 Posts: 8 Comments: 13421
Quoting 767. sar2401:

Doing fine, Allan, hope you are too. For the fist time this month, we've had two consecutive days without rain. Never thought I'd want rain to stop in Alabama in July.

First e have to dispose of Dorian, who's on life support but hasn't quit breathing yet. The wave coming off Africa now looks a lot bigger and more impressive than Dorian, but sometimes they shrink when they get out over the ocean. The main indicator this year is going to be speed. If all these storms keep roaring around at 20-30 mph, all of them will struggle. As the SST's increase, we need storms traveling at no more than 10 mph, so they can develop a good moisture envelope before they meet up with the inevitable dry air and shear as they get closer to the Antilles. So, that was the long answer. Short answer is I don't know, but a slow mover has a lot better chance.
It'll slow down as we go toward mid-August. Speed are always fast in July due to trades, I believe. Dorian was little high, so that why it was little slower, but it was still too fast. Dorian couldn't protect himself from dry air.
Member Since: August 1, 2011 Posts: 28 Comments: 7902

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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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