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 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.
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Quoting 773. SLU:
Wow, the blog is really quite. We need another genuine invest soon. I will be issuing my detailed August and updated 2013 forecast tomorrow.


Dont pay attention to the anomalies
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Quoting 746. KEEPEROFTHEGATE:
everything changes nothing stays the same



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.
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D is 17 45 I've forward it to 24 75 30 degrees west by 7 degrees north

to that locationin the middle of below image

could it be a sneak



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Quoting 764. flcanes:

I think you meant 45 west.
nice spin in the eastern a
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773. SLU
Wow, the blog is really quite. We need another genuine invest soon. I will be issuing my detailed August and updated 2013 forecast tomorrow.

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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.

We dont need the MJO in august.
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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.

Trade winds are going to need to slow down.
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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.
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Dorian is not dead yet. From what I can see on the visible satellite loop it seems to still have a closed COC. If it does not re-fire any convection by tomorrow morning then I'll buy more into the dissipation scenario.
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768. VR46L
Going ,going , Gone!!!

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Quoting allancalderini:
Hi Sar how you doing? Any bets on the new cape Verde wave?

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.
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Quoting 758. PedleyCA:


Faster and Faster ™


Just like my dancing when there is a full moon..
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Quoting 763. hurricanes2018:
nice yellow at 25 west to

that looks nice.
Member Since: July 2, 2013 Posts: 0 Comments: 1523
Quoting 763. hurricanes2018:
nice yellow at 25 west to

Yep
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nice yellow at 25 west to
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756. stormpetrol

After a second look, I think I'm wrong.
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Quoting 757. CaneHunter031472:


Dorian. The little storm that couldn't...
45 west if reforms be near 75 west when it happens
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Quoting 757. CaneHunter031472:


Dorian. The little storm that couldn't...

Lol
Link







Hurricane Michael...
The little storm that could
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What are anyone's thoughts about this little wave near Cape Verde?

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Quoting 746. KEEPEROFTHEGATE:
everything changes nothing stays the same


Faster and Faster ™
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Dorian. The little storm that couldn't...
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3 hours ago



now

850mb vort seems more pronounced,even though the LLC is racing away from what is left of the convection, the LLC appears more vigorous though exposed.
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Quoting 728. DataNerd:
Well folks I would consider this storm our first true "dry run" of the season. True that we have had other storms thus far but not even chantal came close to the potential that this storm had at one point, or the long track.

Things to take away from this as we head towards the 15th of August and the start of the real season:


1. Bermuda high is much stronger than normal. Add to this a ridge that is basically stuck out over west texas of extremely high pressure and is unlikely to go anywhere really IMO prior to the 26th of august, this sets us up for alot of storms to move further west than they should. It also means due to the stronger westerlies weaker storms will be more apt to die, however as we get further along in the season the air will become more moist and this will inhibit the displacing action of the westerlies. Chantal was killed utterly but Dorian just barely in my opinion, considering two days ago it was nearly a hurricane.

2. Guidance is unreliable. It would appear that guidance may not be able to cope with the stronger bermuda high quite the way it should. With both chantal and dorian we saw a rather high number of track errors and corrections, and in Dorian's case with the models going from total re-curve at day 5 to due west or west-southwest than back and forth between the two. Because of this, we should expect that systems later in the season will be harder to track and track errors/rapid corrections will be very likely. This means when a storm approaches the U.S. , there may be sudden track changes of a large degree, so people need to realize the "cone" really cannot be taken at face value this year.

3. Storms so far have had small cores. This is likely due to them forming in inhospitable environments early in the year but should this trend persist it will mean

1) Storms that are more capable of RI or RW, meaning intensity forecasts will be harder to figure on
2) Guidance will be complicated further due to the rapid changes in depth.



I think these are the most teachable lessons from this storm for the rest of the season, but perhaps the most unsettling thing is that we have had four storms and it is not yet august, and that the bermuda westerlies are much stronger than normal, as is the high itself. This factor, coupled with very much above average temperatures in the GOM and a lack this year, of a cold upwelling (unlike in 2008 or 2010 for example) could be a very dangerous situation. Should any storm reach the gulf, and there is no shear, RI is something that would be very likely and very treacherous, especially with the guidance possibly being weaker confidence this year.

This could turn out to be a very bad season, but we will have to see. I will keep my fingers crossed for fish storms even though I don't think there will be very many. What we have now unfortunately, resembles 2005 in alot of ways.

This means more Ikes as far as track goes.
Member Since: September 2, 2011 Posts: 0 Comments: 1149
Quoting 751. tj175:


I agree I've been tracking Dorian for the past few days and he turned out to be a complete and utter DUD!!!!!!!!!
its july its doing just like its suppose to
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Quoting 751. tj175:


I agree I've been tracking Dorian for the past few days and he turned out to be a complete and utter DUD!!!!!!!!!

Yep.
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Quoting 745. flcanes:
I am so bored. Give me something interesting besides the prospect of flossie hitting the big island as a TD.
What do you mean IF? Are you writing off the season before AUGUST?!
(Note: I dont think there has ever been a season in which no storm formed in august, september, and october)


3 full moons to go then we be on the way out

this moon has passed with one the next will come with 3
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751. tj175
Quoting 719. SecretStormNerd:


seems like foreverrrrr awayyyyyy


I agree I've been tracking Dorian for the past few days and he turned out to be a complete and utter DUD!!!!!!!!!
Member Since: September 26, 2010 Posts: 0 Comments: 103
Quoting 749. VR46L:


I will throw ya a bone!!


LOL.
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749. VR46L
Quoting 745. flcanes:
I am so bored. Give me something interesting besides the prospect of flossie hitting the big island as a TD.
What do you mean IF? Are you writing off the season before AUGUST?!
(Note: I dont think there has ever been a season in which no storm formed in august, september, and october)


I will throw ya a bone!!

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Member Since: July 3, 2005 Posts: 428 Comments: 129841
Quoting 744. opal92nwf:

Hey Levi, do you think that we will have a significant lull between now and that "classic" first big burst of activity in August?

I believe that in two weeks time, we may have Erin.
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Quoting 739. PensacolaDoug:



AGW
everything changes nothing stays the same
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I am so bored. Give me something interesting besides the prospect of flossie hitting the big island as a TD.
Quoting 734. buzzardswrath:
ERIN and FERNAND and GABRIEllE will all suffer the same fate as chantal and doeian if they even form this season....... the dry air all through the atlantic id here to stay and the whole atlantic m if covered by dry air as you can see on the maps from TWC

What do you mean IF? Are you writing off the season before AUGUST?!
(Note: I dont think there has ever been a season in which no storm formed in august, september, and october)
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Quoting 704. Levi32:
If you're really desperate for some hope for Dorian, the 12z UKMET shows slight regeneration north of the Caribbean. Compare the surface wind signature valid in 3 hours to the one valid in 3 days. It is true that naked swirls, if they make a big enough dent in the wind field, can refire convection farther west, but realistically, prospects for Dorian are much dimmer than they were previously.




Hey Levi, do you think that we will have a significant lull between now and that "classic" first big burst of activity in August?
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Quoting 716. unknowncomic:
Outran the protective environment.
yep then again it was maybe the semi-fast forward motion that did it in either way it goes to sleeper cell now

hopefully it does not wake up
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742. xcool
ha okay lol
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Quoting 737. xcool:
weatherlover94 ya try come back to life


Lol I never died.
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Quoting 712. weatherlover94:
eventually we will have a major hurricane to track...thats a given in any season.

Yes, it's just a matter of time.
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Quoting 709. allancalderini:
I am amaze that there has only been 1 major in the three basins.(Atlantic,Epac and Wpac) and there have 8 storms in the pacific 6 in the Epac and 4 in the Atlantic that is 18 storms and only 1 major.



AGW
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737. xcool
weatherlover94 ya try come back to life
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Quoting 724. stormpetrol:
Dorian down, but not completely out just yet.

yep

I think now that the peak of D-min as almost past Dorian dorian can soon start refiring those convection
Member Since: June 13, 2009 Posts: 2 Comments: 12716
The week ahead!


Member Since: July 17, 2009 Posts: 71 Comments: 27208
Quoting 711. Patrap:
Once I thought I saw you, in a crowded Hazy Bar, dancing on the light from Star to Star.


Thank you, Patrap, one of my all time favourites.
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.
Member Since: July 21, 2011 Posts: 83 Comments: 7167
Quoting 720. GrumpyWeather:
Hey Gro...never chatted with you before. Long time lurker. Seems that you are one of the most civilized ones on the blog...so, did you take the blue or the red pill for that?

btw...very hot and partly cloudy in Miami.



Robin's egg blue ones. And I never write off a system until!
Member Since: July 17, 2009 Posts: 71 Comments: 27208
Quoting 722. Stormchaser121:
So...Since this is headed to the gulf...will it come back in the gulf? Or disappear?


It should be dead before it ever gets there.
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Quoting 717. xcool:


Is it trying to come back to life ?
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Well folks I would consider this storm our first true "dry run" of the season. True that we have had other storms thus far but not even chantal came close to the potential that this storm had at one point, or the long track.

Things to take away from this as we head towards the 15th of August and the start of the real season:


1. Bermuda high is much stronger than normal. Add to this a ridge that is basically stuck out over west texas of extremely high pressure and is unlikely to go anywhere really IMO prior to the 26th of august, this sets us up for alot of storms to move further west than they should. It also means due to the stronger westerlies weaker storms will be more apt to die, however as we get further along in the season the air will become more moist and this will inhibit the displacing action of the westerlies. Chantal was killed utterly but Dorian just barely in my opinion, considering two days ago it was nearly a hurricane.

2. Guidance is unreliable. It would appear that guidance may not be able to cope with the stronger bermuda high quite the way it should. With both chantal and dorian we saw a rather high number of track errors and corrections, and in Dorian's case with the models going from total re-curve at day 5 to due west or west-southwest than back and forth between the two. Because of this, we should expect that systems later in the season will be harder to track and track errors/rapid corrections will be very likely. This means when a storm approaches the U.S. , there may be sudden track changes of a large degree, so people need to realize the "cone" really cannot be taken at face value this year.

3. Storms so far have had small cores. This is likely due to them forming in inhospitable environments early in the year but should this trend persist it will mean

1) Storms that are more capable of RI or RW, meaning intensity forecasts will be harder to figure on
2) Guidance will be complicated further due to the rapid changes in depth.



I think these are the most teachable lessons from this storm for the rest of the season, but perhaps the most unsettling thing is that we have had four storms and it is not yet august, and that the bermuda westerlies are much stronger than normal, as is the high itself. This factor, coupled with very much above average temperatures in the GOM and a lack this year, of a cold upwelling (unlike in 2008 or 2010 for example) could be a very dangerous situation. Should any storm reach the gulf, and there is no shear, RI is something that would be very likely and very treacherous, especially with the guidance possibly being weaker confidence this year.

This could turn out to be a very bad season, but we will have to see. I will keep my fingers crossed for fish storms even though I don't think there will be very many. What we have now unfortunately, resembles 2005 in alot of ways.
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