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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1078. sar2401
1:42 AM GMT on July 27, 2013
Quoting pottery:

Still a lot of dry air where it matters.
The convection with wave coming off now will most likely not survive for long, as there is plenty dry ahead of it.

What a Most Peculiar season !

Here in Trinidad it's still not raining
Getting a little scary....

Well, it is only the end of July. Hopefully, we're not still saying that at the end of August. I had remarked a few days ago how remarkably free both the Gulf and Caribbean were from any convection, something I've never seen before in late July. This has to mean a lot of stable, dry air covering essentially the entire basin. It's going to take a storm a lot bigger than Dorian, or a major retreat of the A-B high, to change that. At least up until the end of the first week of August, I sure don't see any major change.
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1077. CosmicEvents
1:41 AM GMT on July 27, 2013
Approaching warmer waters after 50w.
Member Since: August 3, 2005 Posts: 10 Comments: 5618
1076. pottery
1:41 AM GMT on July 27, 2013
Quoting wunderkidcayman:
looking at the water vapor floater the dry air is moistening up around Dorian area though still quite dry on the S side

Yeah, but it's losing convection.
It's surrounding environment is too dry.
Member Since: October 24, 2005 Posts: 0 Comments: 24453
1075. GeoffreyWPB
1:41 AM GMT on July 27, 2013
I know...Just what we need, another satellite loop :)

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1074. wunderkidcayman
1:39 AM GMT on July 27, 2013
looking at the water vapor floater the dry air is moistening up around Dorian area though still quite dry on the S side
Member Since: June 13, 2009 Posts: 2 Comments: 12180
1073. sunlinepr
1:38 AM GMT on July 27, 2013
Very interesting layer interaction going on... If we could see what is happening in 3D

Member Since: August 2, 2010 Posts: 21 Comments: 9825
1072. Patrap
1:38 AM GMT on July 27, 2013
Member Since: July 3, 2005 Posts: 426 Comments: 128871
1071. sar2401
1:37 AM GMT on July 27, 2013
Quoting CosmicEvents:
Cease to exist is a little harsh, maybe.
This isn't a dead parrot.

OK, how about a lonely wanderer then?
Member Since: October 2, 2004 Posts: 0 Comments: 16207
1070. SPLbeater
1:37 AM GMT on July 27, 2013
Just finished a new blog on Dorian...and a note about the new tropical wave from Africa. If you care to read...feel free. Link

I also just finished a survey from NASA/Earth Observatory....was quite interesting. Anyone else here that subscribes to them take the survey?
Member Since: August 4, 2011 Posts: 46 Comments: 4487
1069. Patrap
1:37 AM GMT on July 27, 2013


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1068. pottery
1:36 AM GMT on July 27, 2013
Quoting sar2401:
Take a look at how even Dorian's outflow is being destroyed by dry air to the NW, and what little convection there is ahead of it, and how that's also being eaten up. Short of a miracle, Dorian will cease to exist by 11:00 am, and that might be generous.

Central Atlantic IR Loop

Still a lot of dry air where it matters.
The convection with wave coming off now will most likely not survive for long, as there is plenty dry ahead of it.

What a Most Peculiar season !

Here in Trinidad it's still not raining
Getting a little scary....
Member Since: October 24, 2005 Posts: 0 Comments: 24453
1067. sar2401
1:36 AM GMT on July 27, 2013
Quoting cchsweatherman:
Just a reminder people; once a storm loses all its convection, it's gotta start from somewhere to revive itself. The fact that Dorian is popping small showers/thunderstorms should be taken as a sign of progress and attempt at reviving itself.

It's not popping up thunderstorms. What thunderstorms it had have dissipated. Can you show me a picture where it's "popping" up anything? I sure don't see it in this loop.
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1066. wunderkidcayman
1:35 AM GMT on July 27, 2013
Advisory in less than 1 hour 30 mins from now
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1065. CosmicEvents
1:33 AM GMT on July 27, 2013
Quoting 1063. sar2401:
Take a look at how even Dorian's outflow is being destroyed by dry air to the NW, and what little convection there is ahead of it, and how that's also being eaten up. Short of a miracle, Dorian will cease to exist by 11:00 am, and that might be generous.

Central Atlantic IR Loop
Cease to exist is a little harsh, maybe.
This isn't a dead parrot.
Member Since: August 3, 2005 Posts: 10 Comments: 5618
1064. Patrap
1:33 AM GMT on July 27, 2013
Member Since: July 3, 2005 Posts: 426 Comments: 128871
1063. sar2401
1:31 AM GMT on July 27, 2013
Take a look at how even Dorian's outflow is being destroyed by dry air to the NW, and what little convection there is ahead of it, and how that's also being eaten up. Short of a miracle, Dorian will cease to exist by 11:00 am, and that might be generous.

Central Atlantic IR Loop
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1062. KoritheMan
1:31 AM GMT on July 27, 2013
Just did a blog update on Dorian and Flossie. Come have a look, guys.
Member Since: March 7, 2007 Posts: 587 Comments: 20877
1061. KEEPEROFTHEGATE (Mod)
1:29 AM GMT on July 27, 2013
Quoting 1057. hydrus:
Dorian will most likely have a few flare ups for the next couple days. After that, if shear doesnt lesson, they will discontinue advisories.




I think its been lobotomize


could just be sleeping


we will watch for it to wake up


it gets 12 hrs after the last of its convection and organization is gone to recover if not its gone for good or 4 years anyway
Member Since: July 15, 2006 Posts: 174 Comments: 54623
1060. CosmicEvents
1:28 AM GMT on July 27, 2013
There's a lot of "storms" around here that get tracked even when there's no real spin or center, or the center can't be found due to "air restrictions".
.
.
Whatever the NHC calls it is fine. I'll continue to watch as long as there's spin. Especially a naked spin in the Caribbean:)
Member Since: August 3, 2005 Posts: 10 Comments: 5618
1059. cchsweatherman
1:28 AM GMT on July 27, 2013
Just a reminder people; once a storm loses all its convection, it's gotta start from somewhere to revive itself. The fact that Dorian is popping small showers/thunderstorms should be taken as a sign of progress and attempt at reviving itself.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
1058. Bluestorm5
1:26 AM GMT on July 27, 2013
Sorry, but "I see it first" is little old. Only Gro is allowed to say that as he is indeed first to see anything :) That dude is a walking time machine.
Member Since: August 1, 2011 Posts: 28 Comments: 8031
1057. hydrus
1:22 AM GMT on July 27, 2013
Dorian will most likely have a few flare ups for the next couple days. After that, if shear doesnt start lessening up, they will discontinue advisories.

Member Since: September 27, 2007 Posts: 1 Comments: 21492
1056. moonlightcowboy
1:21 AM GMT on July 27, 2013
Quoting 1026. beell:


I'm surprised it can support this blog.
But what else we gonna do on a Friday night?


Hear ya! ;) I'm watching "Island In The Sky" 1953 John Wayne flick. Never seen it. Not bad. And, peeking at loops on commercial breaks! The movie is considerably more interesting. ;)
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1055. KEEPEROFTHEGATE (Mod)
1:18 AM GMT on July 27, 2013
Quoting 1025. StormJunkie:


Convection firing? Huh? This is not convection "firing"



I have no quarrels with the NHC not downgrading it to a remnant yet. If they want to wait a few hours, I don't see any harm in it. They are the experts. That said, scientifically speaking...It does not have a closed LLC right now from what I can tell.




what you can't see that sparsely sporadic convective blow up starting
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1054. unknowncomic
1:17 AM GMT on July 27, 2013
Looks to me, according to this graphic, at 18Z there was a predicted future low beginning Sunday at 60 W.


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1053. Tazmanian
1:16 AM GMT on July 27, 2013
I see this going two ways


the nhc can ues what the atcf say and keep it a ts



Or they will ues there own ways and they could vary well end advisories at 11pm
Member Since: May 21, 2006 Posts: 5091 Comments: 115252
1052. Hurricanes305
1:16 AM GMT on July 27, 2013
Dorian has very little convection at the moment but the cyclone remains vertically stacked. It would not surprised me if it were to try one last time to get its convection right.




The LLC is still remains vigorous even though it might of decrease slightly in intensity and until it dissipate I won't keep watching it.

850mb



700mb:



500mb:

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1051. RGVtropicalWx13
1:13 AM GMT on July 27, 2013
Quoting 1046. MiamiHurricanes09:
I don't see the point in continuing advisories on a couple disorganized clouds accompanied with an open ill-defined surface circulation in the middle of the Atlantic...but they're the experts.


I think they just wanted to wait for a few hours and want to see what happens by 5a. If this trend continues it'll be downgraded to a td with it's last adv.
Member Since: May 30, 2013 Posts: 0 Comments: 566
Quoting 1046. MiamiHurricanes09:
I don't see the point in continuing advisories on a couple disorganized clouds accompanied with an open ill-defined surface circulation in the middle of the Atlantic...but they're the experts.



Just not tipping their hand via ATCF
Member Since: Posts: Comments:
Quoting 1046. MiamiHurricanes09:
I don't see the point in continuing advisories on a couple disorganized clouds accompanied with an open ill-defined surface circulation in the middle of the Atlantic...but they're the experts.

Maybe its the warmer SST and decreasing shear.
Member Since: August 2, 2011 Posts: 0 Comments: 2065
I am a lurker, who absorbs the knowledge you all impart. Just moved back to Englewood, Florida; lived in Tennessee and Massachusetts the past couple of years, having left Naples. Weather is so much more interesting here. Thanks everyone for sharing and explaining to those of us who don't really know squat, but enjoy anyway.
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Quoting 1040. Hurricanes305:
Vigorous Tropical wave should emerge over water fully by tomorrow lets see if it can persist.




I saw it first!!!




You seen nothing 1st


I saw it 1st
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I don't see the point in continuing advisories on a couple disorganized clouds accompanied with an open ill-defined surface circulation in the middle of the Atlantic...but they're the experts.

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Quoting 1040. Hurricanes305:
Vigorous Tropical wave should emerge over water fully by tomorrow lets see if it can persist.




I saw it first!!!


Lol..when I begin to pay attention to a red blob coming off the African coast, please shoot me..get a freaking life
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1044. sar2401
Quoting wunderkidcayman:

I'll refuse to dignify this with a response


don't know yet

will you people stop about if it regenerate Dorian is not dead yet!!!!!!!!!!!!!! talk about that when its downgraded to a rem. low

It's awful hard to argue with N/A is 120 hours...and it's not going to better at 11:00

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1043. hydrus
Quoting 701. OrchidGrower:


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
Outflow from David was suffocating Frederick for days. Storm made a remarkable comeback as I remember, and quite a few Mets said that he would dissipate completely.
Member Since: September 27, 2007 Posts: 1 Comments: 21492
Quoting 1035. fldude99:
Dorian is a goner...let's move on. Hopefully this storm season is like the past few..all the wishcasters need to go away




all downcaster needs to go a way
Member Since: May 21, 2006 Posts: 5091 Comments: 115252
Quoting 998. BeeGone:
Dorian is just another example of the changes in the NHC dictated by NOAA!
Just a few short years ago this system would have been mostly ignored.
Our satellite capabilities now are shrinking and unless we start launching new ones we're all going to be in the dark!

Changes have been made regarding our ability to interpret the satellite presentation y'all.
Then it just trickles down the pipe.


This needs to be said...and I said it...What you see is not always reality!


LOL..whatta nut
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Vigorous Tropical wave should emerge over water fully by tomorrow lets see if it can persist.




I saw it first!!!
Member Since: May 25, 2012 Posts: 0 Comments: 2043
Sorry washingtonian, it was not directed at just you. But that said, I do not recollect a single model run showing it becoming a hurricane. Matter of fact, outside of the GFS, many of them were dissipating it. Hell, even the bullish CMC never initialized it as a real storm.

No matter, the point is if you can forecast storms and not look at forecast shear, forecast sea temps, forecast MJO, forecast steering maps...Than you sir, should be making a whole boat load of money somewhere.

BTW, "forecast" is just another name for the model data output which it derives from. Good forecasting can't be done by looking at a sat image and non-existent surface condition readings out in the middle of the Atl. It takes eyes and a brain to utilize the tools available to be a good forecaster.

I am by no means a good forecaster btw.
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Anybody remember Andrea....it was suppose to be a 45 mph storm at landfall.....actual intensity 60 mph in An over night period....lots tend to happen in the over night hours.
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Quoting 998. BeeGone:
Dorian is just another example of the changes in the NHC dictated by NOAA!
Just a few short years ago this system would have been mostly ignored.
Our satellite capabilities now are shrinking and unless we start launching new ones we're all going to be in the dark!

Changes have been made regarding our ability to interpret the satellite presentation y'all.
Then it just trickles down the pipe.


This needs to be said...and I said it...What you see is not always reality!


There's always the problem of WX sats seeing things on accident. And when you see something you shouldn't see, you fake your own death. And when you fake...etc etc.
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1036. sar2401
Quoting RGVtropicalWx13:

Deactivated is for invests, that would be really called reminent low..anyways it's still. Holding on for atleast the 11pm

My mistake. I believe it will remain a TS, possibly a TD for one more round, then become a remnant low. This really is Chantal all over again.
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Dorian is a goner...let's move on. Hopefully this storm season is like the past few..all the wishcasters need to go away
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1033. sar2401
Quoting BeeGone:
Dorian is just another example of the changes in the NHC dictated by NOAA!
Just a few short years ago this system would have been mostly ignored.
Our satellite capabilities now are shrinking and unless we start launching new ones we're all going to be in the dark!

Changes have been made regarding our ability to interpret the satellite presentation y'all.
Then it just trickles down the pipe.


This needs to be said...and I said it...What you see is not always reality!

You're gone.
Member Since: October 2, 2004 Posts: 0 Comments: 16207
Quoting 1024. weatherlover94:


Check this out unknowncomic

Link
That's probably because of not hitting land and environmental situation in the future.
May happen.
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Even on enhanced IR, it looks like a weak scattering of sprinkles...



Could that change at some point, sure I guess so...But right now it is not the case.
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Quoting 1016. evilpenguinshan:
no worries, this only makes it more likely to bring some rain your direction. cruise WSW for a while, redevelop in the Carib :D



I'll refuse to dignify this with a response

Quoting 1021. weatherlover94:
Did more models increase future intensity at 0z ?

don't know yet

will you people stop about if it regenerate Dorian is not dead yet!!!!!!!!!!!!!! talk about that when its downgraded to a rem. low
Member Since: June 13, 2009 Posts: 2 Comments: 12180
1029. SRQfl
RIP QuikSCAT! I miss you so...
Quoting 1006. Patrap:
What we need is a few good Polar Scatterometer's,

The former QUIKSCAT



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1028. sar2401
Quoting KoritheMan:


I've said before that humans are drawn toward tragedy. When someone is murdered or raped, we are more interesting in hearing that than we are that a 5 year old found his favorite toy after he'd lost it.

Even if some people don't want a major hurricane, they will definitely be more attentive to it than a Category 3 fish spinner, if only to protect their life. But that still proves people are drawn toward tragedy, regardless of the reason.

People need to stop hiding it.

People need to stop hiding that they want to see a major hurricane cause untold death and destruction for their amusement, so they won't be bored? Maybe you can clarify that.

People are always interested in any event out of the ordinary. That's why traffic backs up from rubberneckers looking at an accident. That's rather different that someone driving around with a police scanner looking for accidents, hoping to find a multicar pileup with multiple injuries. Well balanced people pay attention to anything that could be risk to them their family, or their community. Unbalanced people want to see any disaster because it meets a sick desire to see destruction and suffering. Really, there are enough people that think we're all a little nuts without giving them even more reasons to believe they're right.
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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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