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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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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I am not writing him off yet. I am kind of thinking towards what levi was thinking.
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726. Relix
SW side of Dorian is definitely open in Visible.
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Quoting 721. Ameister12:
Good evening everybody.

Looks like Flossie could impact the Big Island as a weak tropical storm early next week.

Much less of an eyesore than Dorian.

Yep. Interesting because of that.
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Dorian down, but not completely out just yet.
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Quoting 712. weatherlover94:
eventually we will have a major hurricane to track...thats a given in any season.

It's highly unlikely we go without one, but no, it's not a "given" that we see a major in any particular year. The Atlantic hurricane seasons of 1972, 1994, 1968, and 1986 featured no Category 3 or higher storms. Heck, last year, we only had two major hurricanes -- Michael and Sandy -- that remained at that intensity for 6 hours each.
Member Since: July 6, 2010 Posts: 113 Comments: 31460
So...Since this is headed to the gulf...will it come back in the gulf? Or disappear?
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Good evening everybody.

Looks like Flossie could impact the Big Island as a weak tropical storm early next week.

Not a huge eyesore like Dorian is right now.
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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.

Quoting 714. Grothar:
Things can regenerate. Just look at me.

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Quoting 712. weatherlover94:
eventually we will have a major hurricane to track...thats a given in any season.


seems like foreverrrrr awayyyyyy
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Really quite surprised NHC kept this system at 5 PM.

We should be at depression or remnant status by 11 pm as long as no further deep convection forms over the center.
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717. xcool
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Quoting 687. KEEPEROFTHEGATE:
enhanced vapour


dry air is to blame and a little shear to boot

Outran the protective environment.
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Well... back to real and boring life, lol. At least there's nothing else going on while I do my 4 days in row of working starting tomorrow.
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Things can regenerate. Just look at me.

Member Since: July 17, 2009 Posts: 69 Comments: 25369
713. JLPR2
Pure Dmin for Dorian.

Member Since: September 4, 2007 Posts: 7 Comments: 8499
eventually we will have a major hurricane to track...thats a given in any season.
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With each new frame and no new convection forming, it looks like Dorian is done for now.
Member Since: June 29, 2009 Posts: 13 Comments: 5300
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.
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18z GFS is running- low off florida-into 18 hours





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too much dry air, but she did help the moisten the atmosphere for the next one.
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706. Relix
Dorian looks pretty sickly now. It may be a goner.
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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.



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. Double post, sorry.
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Quoting 695. GTstormChaserCaleb:
Agree with you Allan, it is like this time when I told this girl I would never give up on her, well she ended up breaking my heart and took a next man. Oh well plenty of fishes in the sea. :)
Sorry man you will find another pretty soon.
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Quoting 654. SomersetSquall:


Many examples, but one that sticks in my mind is Hurricane Kristy in 2006 in the East Pacific, also Hurricane Cosme of 2007 in the East Pacific. Both survived against forecasts of dissipation within 12 hours, both lost all convection but then refired some again and again, with Kristy even restrengthening to a tropical storm after being one advisory away from being declared a remnant low because of having no convection. I always find it a fascinating watch when a storm fights a hostile environment.

If you are interested, read the discussions from NHC under this link here for Kristy:

http://www.nhc.noaa.gov/archive/2006/KRISTY.shtml ?


We called Frederic the "born-again hurricane" because he went from being a hurricane in the Atlantic, down to a tropical depression and almost perished over Cuba -- then sprang back to life and hit our area as a Major. 1979
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Quoting 693. sar2401:

I can see it now. Was that actually confirmed as a tornado ? It looks more like a wall cloud to me, but it's so rain wrapped it's hard to tell for sure.


Four houses roofs were taken off and some trees roots were taken off but no injuries.We don't have those like in the U.S but sometimes,if an afternoon thunderstorm is very strong it can happen.
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Quoting 690. allancalderini:
You are from Germany right?


Sure. I thought everybody would know this until now, lol. But now I'm really out for the night (with greetings to Sar - a pity that you didn't came around Mainz back 2003, near to Frankfurt, so we could have met, hypothetically ;)
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Is there the potential for 90 L to be declared from the new wave at 8:00 ?
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So...Since this is headed to the gulf...will it come back in the gulf? Or disappear?
Member Since: September 2, 2011 Posts: 0 Comments: 1143
Quoting 676. allancalderini:
He is not out Washi.Until his last breath.
Agree with you Allan, it is like this time when I told this girl I would never give up on her, well she ended up breaking my heart and took a next man. Oh well plenty of fishes in the sea. :)
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Slowly but surely coming to an agreement on a goodbye to Dorian

Member Since: August 18, 2011 Posts: 0 Comments: 598
Quoting Tropicsweatherpr:


Now. Link

I can see it now. Was that actually confirmed as a tornado ? It looks more like a wall cloud to me, but it's so rain wrapped it's hard to tell for sure.
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Quoting 689. sar2401:

LOL. We keep going from killer hurricane to it's still breathing but dead. I'm not willing to go either way. Until we get some recon in there, we don't know what the actual structure is. Until it's a remnant, I'm not going to bet against it. Too many of these long track CV's have come back to life. Now, a whole bunch of them have died too, so that's not impossible either. I don't have enough information either way to make anything more than an educated guess, which is I have no idea, but it's not dead yet.
Hi Sar how you doing? Any bets on the new cape Verde wave?
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691. JLPR2
Quoting 683. allancalderini:
I imagine September is not a peak month in the Epac because the Atlantic steal most of the waves? or why?


That is an interesting question... Sounds possible, maybe the Atl uses up the nicest waves so the Epac gets left out.
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Quoting 682. barbamz:


Yes, it must be hot in Russia. Its president is half naked again. You remember him beeing the sharknator some posts ago, lol? New feed for the russian satirists ... But with that I'm really out before I become banned! Bye, all.
Quoting 682. barbamz:


Yes, it must be hot in Russia. Its president is half naked again. You remember him beeing the sharknator some posts ago, lol? New feed for the russian satirists ... But with that I'm really out before I become banned! Bye, all.
You are from Germany right?
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Quoting allancalderini:
He is not out Washi.Until his last breath.

LOL. We keep going from killer hurricane to it's still breathing but dead. I'm not willing to go either way. Until we get some recon in there, we don't know what the actual structure is. Until it's a remnant, I'm not going to bet against it. Too many of these long track CV's have come back to life. Now, a whole bunch of them have died too, so that's not impossible either. I don't have enough information either way to make anything more than an educated guess, which is I have no idea, but it's not dead yet.
Member Since: October 2, 2004 Posts: 0 Comments: 13138
Quoting 678. sar2401:

I think something went wrong with your image. :-)


Now. Link
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enhanced vapour


dry air is to blame and a little shear to boot

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686. JLPR2
Quoting 667. Tropicsweatherpr:


Look what happened in Moca. A tornado.

Link


They have a few good videos in there, those are a pretty rare occurrence here.
Member Since: September 4, 2007 Posts: 7 Comments: 8499
I guess this makes it painfully clear just how dry our dry season normally is here:

Downtown Los Angeles has received 0.09 inches of rainfall so far this month. This places July 2013 in a tie with July 1918 for the 4th wettest since records began in downtown Los Angeles in July
1877. The all time wettest July was in 1886 when 0.24 inches of rain was recorded during the month... followed by July 1986 with 0.18 inches of rain and July 1991 with 0.13 inches. There is a slight chance of additional rain today... so the monthly rainfall
total may climb.


Link
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Quoting 669. GTstormChaserCaleb:
That's why it's called a bold prediction. I'll go 40% and bank on that ULL filling in and backing southwestward. With an ULL to the south and a ridge to the north to cause good ventilation.


That really was a legitimate question. I'm sort of thinking the same thing myself, that's why I wanted to know what you were thinking regarding that.
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Quoting 675. JLPR2:
Going by climatology the Eastern Pacific is about to enter its busiest month.



The Atl is still a month away from its busiest month, which is September.
I imagine September is not a peak month in the Epac because the Atlantic steal most of the waves? or why?
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Yes, it must be hot in Russia. Its president is half naked again. You remember him being the sharknator some posts ago, lol? New feed for the russian satirists ... But with that I'm really out before I become banned! Bye, all.
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Brutal artic blast hitting Greenland,daily record ice at south pole,snowing in Buenos aires ,major snow first since 1918!105 years.
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Quoting 670. unknowncomic:
Dorian could be an omen.


I think you are thinking of Damian. ;-)
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Dorian is about an hour from the Peak of D-Min after that we could see convection start to increase
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Quoting Tropicsweatherpr:


Look what happened in Moca. A tornado.

a href="" target="_blank">Link

I think something went wrong with your image. :-)
Member Since: October 2, 2004 Posts: 0 Comments: 13138

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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.