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 867. cchsweatherman:
I would imagine that the National Hurricane Center will likely downgrade Dorian to a tropical depression at the least at the next advisory given the absence of any convection at the moment. Unless Dorian can fire some decent convection within the next 6 to 12 hours, this tropical storm may be finished. Not writing it off completely yet and am interested in the next popcorn showers/thunderstorms just now firing in a curved band around the circulation.


I am too. There was one that popped earlier in one of the curved bands that didn't last but 15 mins before it collapsed. These are going on 45 mins (not saying much, but progress nonetheless). And now the area of dry air is diminishing, so these very well could, COULD, sustain themselves for a bit... Now that we are coming out of Dmin.
Member Since: March 19, 2010 Posts: 0 Comments: 1147
It's kind of eerie and scary to think. We have NO IDEA that there could be a Katrina or Andrew this year, and here we are getting all burned out with our D storm in July. After this current activity with the two Cape Verde storms before August and all, I can't help to think that we are going to have at least one very significant hurricane to impact the US in August and beyond.

I might do this, but it would be neat to research and see all the hurricane seasons where around 2 cape verde type storms formed before August.
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Member Since: July 3, 2005 Posts: 426 Comments: 129089
It's only a matter of time before we inflate the season total with a few mid-latitude tropical storms/hurricanes. Check out the sea surface temperature anomalies up there!



(Image is nearly a week old)
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*Poof* For now...
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Hey I spy a tiny little low-level swirl off the coast of the Carolinas on the visible satellite. Does anyone else see it??
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Quoting 859. Envoirment:
Don't rule it out just yet. Remember what happened with Chantal? lol


hello ignore list
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Member Since: July 3, 2005 Posts: 426 Comments: 129089
Quoting 866. KoritheMan:
To offer a ray of hope to those who want to see the season pick up (myself included), I select some years where there were at least five tropical cyclones during either August or September, and looked at where the MJO was in each of those years.

2004 (8 August storms):



2005 (5 August storms, 5 September storms):



2007 (8 September storms):



2010 (8 September storms):



For those unaware, Octants 8 and 1 typically affect our basin the most. Also, for these graphs, the farther away from the circle, the stronger the pulse.

As you can see, the MJO was fairly weak in all of these years, but we still saw exceptional activity for specific months, and that's not even counting the months I neglected to mention during those years. I could have also gone further and indicated years where there was a high level of major hurricane activity, but less overall monthly storms; 2008 and 2010 are good examples of this.

Don't give up so early in the game, people.
Me? Of course not! I've seen storms on TWC since 2003 so I got lot of patience :)
Member Since: August 1, 2011 Posts: 28 Comments: 8041
Just re-posting a gif of Chantal as she went over the Lesser Antilles



She almost completely dissipated but then blew up again. As Dorian moves to less dry air, we could see something similar.

(Thanks to Patrap for posting the gif previously!).
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Quoting 860. sar2401:

***cough***check join date***cough***ignore***cough***
He's right, you know... I remember his post few days ago.
Member Since: August 1, 2011 Posts: 28 Comments: 8041
I would imagine that the National Hurricane Center will likely downgrade Dorian to a tropical depression at the least at the next advisory given the absence of any convection at the moment. Unless Dorian can fire some decent convection within the next 6 to 12 hours, this tropical storm may be finished. Not writing it off completely yet and am interested in the new popcorn showers/thunderstorms just now firing in a curved band around the circulation.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
To offer a ray of hope to those who want to see the season pick up (myself included), I selected some years where there were at least five tropical cyclones during either August or September, and looked at where the MJO was in each of those years.

2004 (8 August storms):



2005 (5 August storms, 5 September storms):



2007 (8 September storms):



2010 (8 September storms):



For those unaware, Octants 8 and 1 typically affect our basin the most. Also, for these graphs, the farther away from the circle, the stronger the pulse.

As you can see, the MJO was fairly weak in all of these years, but we still saw exceptional activity for specific months, and that's not even counting the months I neglected to mention during those years. I could have also gone further and indicated years where there was a high level of major hurricane activity, but less overall monthly storms; 2008 and 2010 are good examples of this.

Don't give up so early in the game, people. Even if the MJO doesn't cooperate, we're still game for a long and active season.
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865. JLPR2
Now... how many heart attacks would Dorian cause if it managed to grow into an Igor like system out of nothing.

It would be the single most ridiculous thing to happen in the Atl... ever! XD


(Hurricane Igor 2010)
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04L/XX/XX


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Ignorance precedes "ignore" sometimes as thelmores is a WUBA original.

0 Posts mean 0 Blog entries,..


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Comments: 3766

He don't speak a lot, but when thel speaks.

I listen.


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Quoting Envoirment:
Don't rule it out just yet. Remember what happened with Chantal? lol

Yes, but Chantal came back from the dead...not once, but twice. We may be tortured again by Dorian. :-)
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Quoting thelmores:


You do realize that it is only July 26th.....

I posted a few days back I thought that Dorian would face much the same challenges that Chantal did...... dry air and shear. Much was made by Doc and the NHC about the colder waters, but that was not the major inhibiting factor. This time of year, conditions approaching the Caribbean from the east are still not favorable..... particularly for relatively small storms like Chantal and Dorian.....

RIP Dorian.....

BTW, here is my post from a few days back......



***cough***check join date***cough***ignore***cough***
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Don't rule it out just yet. Remember what happened with Chantal? lol
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04L/WAVE/XX/CX
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Published on Oct 2, 2012

Defense contractor Northrop Grumman is partnering with NASA to turn former Global Hawk spy drones into hurricane hunters.



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Couple small showers north of CoC with curved bands. Reiterate, SMALL showers... We'll see what happens.
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Quoting Bluestorm5:
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.

Generally, models never pick up a wave as it's just coming off the coast. They're too weak and too small. Occasionally, the BAMS might see it, but it usually has to be added to models as a bogus vortex to seed the model and get it running. That's why initial track models of these CV storms tend to be off for at least the first five days, when some of the models can start to see the actual storm from data fed in from satellites.

There's a new unmanned aircraft, the Northrop Grumman X-47B, which just made it's first take off and landing from an underway aircraft carrier on July 17. The ideal solution to weather surveillance in the Atlantic would be a carrier with a couple of these unmanned aircraft, instruments for hurricane hunting, cruising the mid-Atlantic during hurricane season.

Maybe someday....
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..ribbitt,ribbitt'
Member Since: July 3, 2005 Posts: 426 Comments: 129089
Quoting 812. L1990:
very disappointed with these pathetic tropical systems this year


You do realize that it is only July 26th.....

I posted a few days back I thought that Dorian would face much the same challenges that Chantal did...... dry air and shear. Much was made by Doc and the NHC about the colder waters, but that was not the major inhibiting factor. This time of year, conditions approaching the Caribbean from the east are still not favorable..... particularly for relatively small storms like Chantal and Dorian.....

RIP Dorian.....

BTW, here is my post from a few days back......

Quoting 1148. thelmores:
Found the Sal Image posted in Doc's blog interesting.....

At the low and mid-levels, you see the SAL from the Sahara being funneled.... then you see 98L heading that way.....

Consider the SAL and funneling have not changed much since Chantal choked on the dry air and shear problems....

I would expect 98L will be named..... but it still will have some difficulties similar to Chantal.....

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Quoting 848. KoritheMan:


lol, it's just a really big pet peeve of mine for people to label every thunderstorm complex that rolls off Africa a tropical wave. Thunderstorms are associated with waves, yes, but their characteristics are less to do with convection and more to do with low- and mid-tropospheric winds.
I see. I'll be more careful in future, lol. Maybe they're labeling it as a wave because they didn't know the differences like I didn't until now, lol.
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Quoting 834. KoritheMan:


We do. There is usually a lot of dry air associated with the stable marine layer along the California current in the eastern Pacific, which causes a decay rate that is exceptionally faster than for Atlantic storms experiencing the same obstacle, because the water gets really cold in the Pacific on a much quicker pace.
I'd like to think that it's my imagination, or that I'm watching the Atlantic far more closely, NOT that there's a good reason why.

Where is Levi when I need him ... I need some tropical tadpoles.
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850. VR46L
Quoting 843. KEEPEROFTHEGATE:
ya its picking up


Its a good Job the Shear is so brutal in the West Atlantic



And no sign of abating

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Quoting 788. flcanes:

As i mentioned during Chantal, August is when the big boys and girls go out to "play".


Why the quotation marks around play? :P
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Quoting 840. Bluestorm5:
It's good to be picky. We'll see.


lol, it's just a really big pet peeve of mine for people to label every thunderstorm complex that rolls off Africa a tropical wave. Thunderstorms are associated with waves, yes, but their characteristics are less to do with convection and more to do with low- and mid-tropospheric wind shifts.

Tropical waves are simply anomalies within the normal lower-tropospheric flow.
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NOAA updated my Fav Page

Global Climate Change Indicators

How do we know humans are the primary cause of the warming?

A large body of evidence supports the conclusion that human activity is the primary driver of recent warming. This evidence has accumulated over several decades, and from hundreds of studies. The first line of evidence is our basic physical understanding of how greenhouse gases trap heat, how the climate system responds to increases in greenhouse gases, and how other human and natural factors influence climate. The second line of evidence is from indirect estimates of climate changes over the last 1,000 to 2,000 years.

These estimates are often obtained from living things and their remains (like tree rings and corals) which provide a natural archive of climate variations. These indicators show that the recent temperature rise is clearly unusual in at least the last 1,000 years. The third line of evidence is based on comparisons of actual climate with computer models of how we expect climate to behave under certain human influences.

For example, when climate models are run with historical increases in greenhouse gases, they show gradual warming of the Earth and ocean surface, increases in ocean heat content, a rise in global sea level, and general retreat of sea ice and snow cover. These and other aspects of modeled climate change are in agreement with observations.


Simulated global temperature in experiments that include human influences (pink line), and model experiments that included only natural factors (blue line). The black line is observed temperature change.

Member Since: July 3, 2005 Posts: 426 Comments: 129089
It's nice that Dorian died out. Blog was starting to get little unbearable and it was hard to get educated discussions like now. I know blog will be even more unbearable during August to October during hurricanes and especially a USA landfalling ones.
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It appears Dorian has slowed down quite a bit
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Quoting 837. PensacolaDoug:
Non-scientist? Like your here Al Gore?

The only things I've read by Gore are the short quotes provided by the denial of reality crowd. (Never saw his movie, either.)

But I can't help notice that you provide no source for your claims.
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Quoting 832. VR46L:
Africa is certainly producing a lot of potential at the moment

ya its picking up
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Quoting 839. Hurricanes305:
Dorian looks to be a open wave at the moment. But until the circulation dies will keep and eye on it as there is a sneaky ULL. Atlantic is not quiet thought as been pointed out earlier the MJO is going to return and thus activity well pick. In the time we have been watching Dorian. The Atlantic has warm quite  a bit. Which suggests to me that conditions are getting much more favorable. for now our last 3 storms may just be test for the true CV systems. 




SAL has diminished not as much as previous seasons. A new tropical wave may need to watch as it will likely continue west within the ITCZ and really warms SSTs and wont have to worry about mid level to upper level dry air.



Thanks to Dorian the upper level atmospheric winds is light aloft.



As long as it stay within the ITCZ natural vorticity will allow for low pressures for this and other waves over Africa. As it head for some vorticity around 25W a low may develop with it tomorrow night



With Lower convergence and SST there is a good chance it can persist longer enought to be mention 8am tomorrow.




As you can tell I'm optimistic about the wave as I was monitoring it along with Dorian. But notice some similarities. When Dorian moved across cooler SST and it continue to have a small size it would struggle but the high is really potent than before so I not expecting a turn WNW anytime soon.



In anycase these waves are getting bigger better and move ready to develop and the Greater Antilles, Gulf, and SE. US (Florida through NC) is a great risk when the peak comes
If this one develop it will probably have the same fate as Dorian.
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Quoting 833. PensacolaDoug:




you're funny.

You've made a claim. Can you back it up with evidence or not? If it's simply your opinion, then no evidence is needed, but evidence *is* needed if you are claiming that your statement is factual.
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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.
It's good to be picky. We'll see.
Member Since: August 1, 2011 Posts: 28 Comments: 8041
Dorian looks to be a open wave at the moment. But until the circulation dies will keep and eye on it as there is a sneaky ULL. Atlantic is not quiet thought as been pointed out earlier the MJO is going to return and thus activity well pick. In the time we have been watching Dorian. The Atlantic has warm quite  a bit. Which suggests to me that conditions are getting much more favorable. for now our last 3 storms may just be test for the true CV systems. 




SAL has diminished not as much as previous seasons. A new tropical wave may need to watch as it will likely continue west within the ITCZ and really warms SSTs and wont have to worry about mid level to upper level dry air.



Thanks to Dorian the upper level atmospheric winds is light aloft.



As long as it stay within the ITCZ natural vorticity will allow for low pressures for this and other waves over Africa. As it head for some vorticity around 25W a low may develop with it tomorrow night



With Lower convergence and SST there is a good chance it can persist longer enought to be mention 8am tomorrow.




As you can tell I'm optimistic about the wave as I was monitoring it along with Dorian. But notice some similarities. When Dorian moved across cooler SST and it continue to have a small size it would struggle but the high is really potent than before so I not expecting a turn WNW anytime soon.



In anycase these waves are getting bigger better and move ready to develop and the Greater Antilles, Gulf, and SE. US (Florida through NC) is a great risk when the peak comes
Member Since: May 25, 2012 Posts: 0 Comments: 2043
A few small showers beginning to develop in the thicker low level moisture pivoting around whatever remains of the LLC. Still arced banding of the low clouds around it. And it is beginning to run out of real estate as far as the driest of the air (although still dry, but beginning to move into a moister environment). Lets see, it is what 9pm out there, give it 2 more hours, lets see what happens.
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Non-scientist? Like your hero Al Gore?
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Quoting 830. PensacolaDoug:



Oddly enough, we do.


One year from now, and the struggles of 2013 won't even be a vivid memory.

Hell, if we have a major US landfall one month from now, these peewee systems will be a distant memory.

It's all relative.
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Member Since: July 3, 2005 Posts: 426 Comments: 129089
Quoting 827. OracleDeAtlantis:
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.


We do. There is usually a lot of dry air associated with the stable marine layer along the California current in the eastern Pacific, which causes a decay rate that is exceptionally faster than for Atlantic storms experiencing the same obstacle, because the water gets really cold in the Pacific on a much quicker pace.
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Quoting 829. Birthmark:

Who exactly said such things? And in what time-frame was this new normal to appear? Sources needed.




you're funny.
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832. VR46L
Africa is certainly producing a lot of potential at the moment

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Quoting 828. PensacolaDoug:



No, really!

No, really, you don't...unless you mean non-scientists. And if you're taking your science predictions or projections from non-scientists you really mustn't complain about the lack of quality.
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Quoting 821. KoritheMan:


We get over it and move on, eh Doug? :)



Oddly enough, we do.
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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.

Who exactly said such things? And in what time-frame was this new normal to appear? Sources needed.
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Quoting 826. Birthmark:

No you don't.



No, really!
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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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