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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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.
Member Since: October 2, 2004 Posts: 0 Comments: 15997
Quoting 990. StormJunkie:
Evening all.

Dorian's tiny little core just couldn't handle the modest adversity he faced. For all the talk about eyes, brains, and forecasting minds being better than models...Well, there was a reason the models never showed Dorian gaining much strength. I know many dislike the models, or say they shouldn't be relied on...But honestly, imho, they are the forecasting tool that allow us to look at the other data and make somewhat educated guesses. I don't think the models are an end all be all, but when you follow multiple models for multiple days and runs...It is pretty easy to start picking out trends and consensuses as to possible solutions. In this day and age, the models are where all forecasters begin...or should begin.

Ok, done ranting...
For your info (since the brains and eyes part was directed at me) I never had much hope to begin with for Dorian.Even when those oh so holy can do no wrong models were showing Dorian as a hurricane near the vicinity of the Bahamas I remained skeptical and from the start said that Dorian will be a struggling weak storm while cruzing through the MDR and was likely to dissipate.I pulled that plug to early but it seems that the conditions have finally caught up to him and he is on his last leg.I used my brain and eyes along with past experiences to come to the conclusion that Dorian would more than likely die and not become a hurricane.So far I have been right with just using my "eyes and brain"

Rant over.
Member Since: August 14, 2010 Posts: 10 Comments: 17073
1026. beell
Quoting 1019. moonlightcowboy:


Sure will be without some good convective lift in the fast flow.


I'm surprised it can support this blog.
But what else we gonna do on a Friday night?
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Quoting 1022. wunderkidcayman:

TS 40mph for 11pm yep


oh yeah AL, 04, 2013072700, , BEST, 0, 178N, 471W, 35, 1010, TS so much for being deactivated anyway its has some popping increasing convection starting up


yep


yeah I guess


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.
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Quoting 1020. unknowncomic:
Shear seems to be less of a problem in the future. I think it will regenerate if it gets away from the dry air and slows down a bit.


Check this out unknowncomic

Link
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1023. Patrap
2 meter Temp Loop, see QUICK Links menu
Member Since: July 3, 2005 Posts: 426 Comments: 128632
Quoting 1001. TropicalAnalystwx13:
Still a tropical storm in the 0z update.

AL, 04, 2013072700, , BEST, 0, 178N, 471W, 35, 1010, TS, 34, NEQ, 60, 30, 0, 60, 1016, 120, 20, 0, 0, L, 0, , 0, 0, DORIAN, M,

TS 40mph for 11pm yep

Quoting 1004. sar2401:

No, it's not. It's about to be deactivated.

oh yeah AL, 04, 2013072700, , BEST, 0, 178N, 471W, 35, 1010, TS so much for being deactivated anyway its has some popping increasing convection starting up

Quoting 1005. MahFL:
Some convection fireing.


yep

Quoting 1008. Seflhurricane:
looks like the NHC may give it one last shot, if Dorian does not build convection overnight then expect it to be downgraded

yeah I guess
Member Since: June 13, 2009 Posts: 2 Comments: 12144
Did more models increase future intensity at 0z ?
Member Since: Posts: Comments:
Shear seems to be less of a problem in the future. I think it will regenerate if it gets away from the dry air and slows down a bit. I was going to post "Ruby Tuesday" in honor of Mick Jagger's 70th birthday and also Dorian's demise, but I'm not so sure about that now.
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Quoting 1013. beell:
Hard to find a way to close off the SW corner.



Sure will be without some good convective lift in the fast flow.
Member Since: July 9, 2006 Posts: 184 Comments: 29610
1018. hydrus
Quoting 863. Patrap:
Ignorance precedes "ignore" sometimes as thelmores is a WUBA original.

0 Posts mean 0 Blog entries,..


Member Since: September 8, 2005 Posts: 0
Comments: 3766

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

I listen.


Yep..Someone posted yesterday that had a 2001 member initiation date.
Member Since: Posts: Comments:
no worries, this only makes it more likely to bring some rain your direction. cruise WSW for a while, redevelop in the Carib :D


Quoting 974. wunderkidcayman:
guys stop RIPing it yeah most of the convection is gone but thats mainly due to D-Min among other little less minor things but it has finished with D-Min and now starting to build on D-max
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Quoting 1004. sar2401:

No, it's not. It's about to be deactivated.

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

Look guys half and half
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1013. beell
Hard to find a way to close off the SW corner.

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Flossie still at 50 knots:

EP, 06, 2013072700, , BEST, 0, 168N, 1349W, 50, 999, TS,
Member Since: July 21, 2011 Posts: 83 Comments: 7167
1011. Patrap
Consensus been real good last 48 as to downstream with Dorian.
Member Since: July 3, 2005 Posts: 426 Comments: 128632
Flossie stays at 50kts.

EP, 06, 2013072700, , BEST, 0, 168N, 1349W, 50, 999, TS
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Quoting 982. KEEPEROFTHEGATE:


WKC would beg to differ




this is perfect conditions for the D-Max


umm not really yeah it ain't the perfect of conditions but its still can work

Quoting 983. Gearsts:
It has only a few hours of life, if nothing happens then is dead.

ok I can kinda Agree with that though if it becomes a rem. low it still could redevelop that is why I won't call it dead more like a sleeper at that time

Member Since: June 13, 2009 Posts: 2 Comments: 12144
Quoting 1001. TropicalAnalystwx13:
Still a tropical storm in the 0z update.

AL, 04, 2013072700, , BEST, 0, 178N, 471W, 35, 1010, TS, 34, NEQ, 60, 30, 0, 60, 1016, 120, 20, 0, 0, L, 0, , 0, 0, DORIAN, M,
looks like the NHC may give it one last shot, if Dorian does not build convection overnight then expect it to be downgraded
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1007. scott39
Quoting 1003. 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.
You bet...Human nature.
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1006. Patrap
What we need is a few good Polar Scatterometer's,

The former QUIKSCAT



Member Since: July 3, 2005 Posts: 426 Comments: 128632
1005. MahFL
Some convection fireing.

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1004. sar2401
Quoting weatherlover94:
it's still holding on

No, it's not. It's about to be deactivated.
Member Since: October 2, 2004 Posts: 0 Comments: 15997
Quoting 978. scott39:
There is such a hunger for a Major Hurricane(myself included.) My prediction is this site will crash when we get the first one this season. I want to see a bona-fide cape verde beautiful Major Hurricane. Oh, and please dont respond with, "oh Scott, you want death and destruction." 99.9999% of you who say that, want to see a major too.


I've said before that humans are drawn toward tragedy. When someone is murdered or raped, we are more interested 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.
Member Since: March 7, 2007 Posts: 583 Comments: 20802
1002. scott39
Quoting 990. StormJunkie:
Evening all.

Dorian's tiny little core just couldn't handle the modest adversity she faced. For all the talk about eyes, brains, and forecasting minds being better than models...Well, there was a reason the models never showed Dorian gaining much strength. I know many dislike the models, or say they shouldn't be relied on...But honestly, imho, they are the forecasting tool that allow us to look at the other data and make somewhat educated guesses. I don't think the models are an end all be all, but when you follow multiple models for multiple days and runs...It is pretty easy to start picking out trends and consensuses as to possible solutions. In this day and age, the models are where all forecasters begin...or should begin.

Ok, done ranting...
Our lack of support for him when he was down, didnt help him either......J/K Lol and ha ha
Member Since: Posts: Comments:
Still a tropical storm in the 0z update.

AL, 04, 2013072700, , BEST, 0, 178N, 471W, 35, 1010, TS, 34, NEQ, 60, 30, 0, 60, 1016, 120, 20, 0, 0, L, 0, , 0, 0, DORIAN, M,
Member Since: Posts: Comments:
1000. ncstorm
Quoting 990. StormJunkie:
Evening all.

Dorian's tiny little core just couldn't handle the modest adversity she faced. For all the talk about eyes, brains, and forecasting minds being better than models...Well, there was a reason the models never showed Dorian gaining much strength. I know many dislike the models, or say they shouldn't be relied on...But honestly, imho, they are the forecasting tool that allow us to look at the other data and make somewhat educated guesses. I don't think the models are an end all be all, but when you follow multiple models for multiple days and runs...It is pretty easy to start picking out trends and consensuses as to possible solutions. In this day and age, the models are where all forecasters begin...or should begin.

Ok, done ranting...


that was a rant? :)
Member Since: August 19, 2006 Posts: 13 Comments: 15664
Down to 35 knots:

AL, 04, 2013072700, , BEST, 0, 178N, 471W, 35, 1010, TS,
Member Since: July 21, 2011 Posts: 83 Comments: 7167
Wave, at least in my opinion


Member Since: September 23, 2005 Posts: 14 Comments: 11211
Quoting 984. Patrap:
Hard to crash the wunderground server, as the blogs only use at best 3-5% of the capacity on the Buisiet days


We've come a long way since back in the day...I remember quite a few occasions with majors out there or bearing down when we'd crash the blogosphere.
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Quoting 980. nrtiwlnvragn:


Two already have, hey you never know.
Ugh... waste of money on short budget :\ It's really depressing that Hurricane Hunters have to struggle with furloughs and skinny budget.
Member Since: August 1, 2011 Posts: 28 Comments: 8027
Quoting Birthmark:

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

Really, you have no memory of some scientists saying that AGW would make hurricane seasons more like 04-05? And no memory of linking AGW to 04-05 itself? OK, here are a few links:

Phys.org

2010 Study from Nature Geoscience

SAL and Hurricanes

Link

Union of Concerned Scientists

Link

Just kidding about the hairspray, but the now infamous aerosol study. Plus almost every TV show, newspaper and magazine, from the end of 2005 until about 2011, when the media finally got bored that the next season that would flatten everything was just around the corner.

I know you don't have early onset Alzheimer's, so you must remember some of this talk, unless you had your head stuck in some arcane science journals for the last seven plus years.

Fire when ready, Gridley.

Edit:
Well, the site certainly did mess up that post but the links are there nonetheless.
Member Since: October 2, 2004 Posts: 0 Comments: 15997
Quoting 946. nrtiwlnvragn:
TXNT28 KNES 270003
TCSNTL

A. 04L (DORIAN)

B. 26/2345Z

C. 17.9N

D. 47.1W

E. THREE/GOES-E

F. TOO WEAK

G. IR/EIR/SWIR

H. REMARKS...THIS SYSTEM IS TOO WEAK TO CLASSIFY DUE TO BANDING LESS
THAN .2

I. ADDL POSITIONS

NIL


...SCHWARTZ


Link
Nil I think here means that we can officially declare whatever evolves as a zombie storm. The remnants and or remains will need to be watched, if you can.
Member Since: August 3, 2005 Posts: 10 Comments: 5606
it's still holding on
Member Since: Posts: Comments:
Evening all.

Dorian's tiny little core just couldn't handle the modest adversity he faced. For all the talk about eyes, brains, and forecasting minds being better than models...Well, there was a reason the models never showed Dorian gaining much strength. I know many dislike the models, or say they shouldn't be relied on...But honestly, imho, they are the forecasting tool that allow us to look at the other data and make somewhat educated guesses. I don't think the models are an end all be all, but when you follow multiple models for multiple days and runs...It is pretty easy to start picking out trends and consensuses as to possible solutions. In this day and age, the models are where all forecasters begin...or should begin.

Ok, done ranting...
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Quoting 970. Seflhurricane:
despite everything against it dorian has held its LLC rather good , the issue has been convection if it can fire some good convection overnight it has a chance if not there appears to be another opportunity north of Hispaniola. Remember Katrina TD#10

yeh but I think tonights convection will help greatly

Quoting 973. weatherlover94:


They won't declassify it no matter how naked it is if it remains closed


yep well they could but its less likely if it can get just a small amount of convection near or over it
which its starting to do

Member Since: June 13, 2009 Posts: 2 Comments: 12144
Quoting 984. Patrap:
Hard to crash the wunderground server, as the blogs only use at best 3-5% of the capacity on the Buisiet days
Cool, I did not know.
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Quoting 963. Seflhurricane:
very strong likelihood dorian will be downgraded to a remnant low

As of right now , very doubtful it will yet
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Quoting 978. scott39:
There is such a hunger for a Major Hurricane(myself included.) My prediction is this site will crash when we get the first one this season. I want to see a bona-fide cape verde beautiful Major Hurricane. Oh, and please dont respond with, "oh Scott, you want death and destruction." 99.9999% of you who say that, want to see a major too.
Three more weeks and things should get busy.
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well that little bit of convection is going away now.
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Hard to crash the wunderground server, as the blogs only use at best 3-5% of the capacity on the Buisiet days
Member Since: July 3, 2005 Posts: 426 Comments: 128632
Quoting 974. wunderkidcayman:
guys stop RIPing it yeah most of the convection is gone but thats mainly due to D-Min among other little less minor things but it has finished with D-Min and now starting to build on D-max
It has only a few hours of life, if nothing happens then is dead.
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Quoting 975. Seflhurricane:
Looking at water vapor super dry air right over Dorian .


WKC would beg to differ




this is perfect conditions for the D-Max
Member Since: July 15, 2006 Posts: 173 Comments: 54292
Quoting 973. weatherlover94:


They won't declassify it no matter how naked it is if it remains closed
must have convection to remain a tropical cyclone , which currently has none
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Quoting 972. Bluestorm5:
With the budget being tight, Hurricane Hunters shouldn't bother making the trip to St. Croix...


Two already have, hey you never know.
Member Since: September 23, 2005 Posts: 14 Comments: 11211
Member Since: July 3, 2005 Posts: 426 Comments: 128632
There is such a hunger for a Major Hurricane(myself included.) My prediction is this site will crash when we get the first one this season. I want to see a bona-fide cape verde beautiful Major Hurricane. Oh, and please dont respond with, "oh Scott, you want death and destruction." 99.9999% of you who say that, want to see a major too.
Member Since: Posts: Comments:

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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.