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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Model intensity still split...Ships much more aggressive
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1677. VR46L
The Euro showing no love for storms ... No sign of Dorian .does pick up but drops the wave off Africa

Eastern US & Atlantic 00-ECMWF 500MB Heights & PMSL Forecast
Member Since: March 1, 2012 Posts: 0 Comments: 6927
Alright everyone, thanks for the warm welcome. I think I'll be around/visible a bit more often now. But I'm outta here for a bit. G' night.
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I don't think Dorian got the message that most people here officially declared him dead yesterday afternoon/evening. ;)
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1674. VR46L
Quoting 1666. Tribucanes:
NHC was really rooting for Dorian to die. He was a headache now he's a big headache.


As JLPR2 just pointed out he is relativity close to the Islands .IMO they probably feel the need to keep issuing advisories Just incase But IMO if he moves West he is dead NW not so much



Edit Just Checked shear .. Its bad to the NW not to the West ... I guess its pretty much Check mate for Dorian



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Quoting 1672. JLPR2:
Well... If Dorian is really taking advantage of its new convection it should be closing and tightening it's LLC while developing a new mid-level one. If it completes this process before tomorrow morning it should be able to sustain and develop more convection during the day now that it has hit warmer sea temperatures, has favorable shear(for the moment) and seems to be keeping the dry air at bay.

Also... I hadn't realized it was so close to the islands. I wonder if the HHs will head into Dorian to investigate like it was planned.

its getting very close!!
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1672. JLPR2
Well... If Dorian is really taking advantage of its new convection it should be closing and tightening it's LLC while developing a new mid-level one. If it completes this process before tomorrow morning it should be able to sustain and develop more convection during the day now that it has hit warmer sea temperatures, has favorable shear(for the moment) and seems to be keeping the dry air at bay.

Also... I hadn't realized it was so close to the islands. I wonder if the HHs will head into Dorian to investigate like it was planned.

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Quoting 1650. StormJunkie:


So did you ever go look at his forecasting toes?

What do you mean? How I saw Ike hitting Texas even when he was out in the Atlantic? I guess I was just a pretty good met back then for a 14 year old, ;) .
Member Since: July 8, 2013 Posts: 0 Comments: 106

Dorian looked so sickly earlier. Nice to see the flareup of convection since then.
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Quoting 1639. Tribucanes:
What are you hoping to do when your done with your degree FlaStGuy? Going for masters or doctorate?


This just made we me wonder if it was funny when Masters got his Masters.

When I went back to college to finish my degree(s), I could find only three schools in Florida with Meteorology programs... FSU (too far N for me), UM (far too expensive for me, but local enough), FIT (only 3 hours north maybe, but still didn't want to relocate).

So I stayed local, and have been at FAU since 08, studying and teaching Computer Science. BS in 10, MS in 12, about to be done with PhD coursework and just took the qualifying exam for candicacy. But alas, meteorology for me is still relegated to "hobby" status :/

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In order to make progress, Dorian still needs to manage to organize the convection it is currently firing. Otherwise, it will just continue to pulsate up and down with little fanfare. However, it has completed the first step...which is simply developing the convection at all.
Member Since: June 29, 2009 Posts: 13 Comments: 5313
Quoting 1655. yankees440:


2AM updated GFS esembles

Too far south,I see other 3 global model much further north,that is not gona be right.These models are if you change up conditions slightly from the start,the issue is the gfs has poor resolution and can't see the starting conditions well, and so even if you change it,the starting conditions may well be off.Ecmwf resolution is capable of seeing things clearer at the start and this is why it is more north along with the ukmet.
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NHC was really rooting for Dorian to die. He was a headache now he's a big headache.
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Two things to note:

1. Cloud tops are still cooling and thunderstorms are expanding

2. Note the "popcorn" thunderstorms to the west of the system. This indicates for the first time that Dorian is entering a more moist environment with lower westerly shear.

The storm could very easily still die, or may re open despite the new convection, however this is certainly a key development.

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If dorian is able to continue spreading convective coverage again and rebuilding its MLC it may have a chance yet. Hate to say it but it could end up being like Andrew (only for the GOM). Here's to hoping that doesn't happen.


LLC appears to be closed again now underneath the new convective structure. We will have to see how it does tomorrow but its not quite dead yet, was a near thing there for sure.
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Dorian still is closed and by looking at dvorak images looks like 40-45mph TS still. Dorian has rapidly reformed from absolutely nothing; it decoupled earlier today it looked like. I think Dorian will be alive and kicking and maybe intensifying tomorrow quickly. This may not technically qualify for RI but it's not RIP either for the fighter.
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1662. vis0
Quoting 631. GTstormChaserCaleb:
Bold prediction time: Dorian will become a hurricane and pull an Andrew, not the same strength though.

see something similar yet a disheveled cat1.35 (i know no such cat, figure out the %) if it stays between ~270° & 320° (Directional degrees).

If it heads between 321° & 355° it'll be pulled by a front BUT might reach a cat 2.11 towards the Carolinas.  JUST FOR ENTERTAINMENT & LAUGHING PURPOSES, only on SA, Su & Mn.
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Quoting 1653. Tribucanes:
Hawaii watch out. Flossie was the last TS to affect Hawaii too.


Iniki was the most recent Hurricane strike correct?
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Quoting 1647. Civicane49:
Eye becoming apparent on satellite image.



Ah, new thought. Where's Taz when you need him.

Now I'm out, lol.
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Quoting 1657. Camille33:

Thanks for the nice comments,I try to control myself and my reactions,and try to be steady,but it's hard.I get triggered a lot by people disagreeing and can't control it. I don't want to start arguments,I hope people understand me.


Hey, Camille. Do the best you can. Night blog's always the nicest in my opinion. Wish you the best of luck.


Rest o' blog: I'm out. Till tomorrow.
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Quoting 1648. Tribucanes:


Well there you go, sorry to hear that. Lots of cool people here you could talk one on one with if you need. Feel free to IM me if you want. That really does explain a lot, most here have a pretty good capacity to forgive and forget. You've probably made many ignore lists, but that's not the end of the world. Reactive tendencies are hard to overcome, thanks for being honest and good luck.

Thanks for the nice comments,I try to control myself and my reactions,and try to be steady,but it's hard.I get triggered a lot by people disagreeing and can't control it. I don't want to start arguments,I hope people understand me.
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Quoting 1646. VR46L:


I personally don't enjoy reading about isolating and ignoring bloggers or telling others to. I only once ignored a blogger ever on the internet and that time I really had no choice but it goes against everything I believe in .




I'm with you there. I've been mostly lurking here for the past 8 years, and have never even flagged a post. Ever. I love reading. Everything. Even the funny trolls and even some of the spats. It's all part of the experience.

I think the whole idea of flagging/reporting users/posts is kind of silly outside of controlling deliberate (like bot-generated) spam. This is a weather blog, but it doesn't have to be only weather, only accurate weather, every moment. In fact, it would be more boring if it were true.

Everyone should savvy enough to get an idea of what is true, what is false, what is fact, what is speculation, what is logical, what is goofy, etc.

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2AM updated GFS esembles
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1654. VR46L
Quoting 1651. Tribucanes:
I'm really curious about several of the intensity models showing Dorian growing into a hurricane in the coming days. Convection has grown impressively fast from when Dorian was likely an open wave earlier. Hasn't been just one quick burst of convection, it's steadily grown over the last five hours. Banding next?


I think thats why the NHC have kept his classification for a little longer ... It doesnt really matter, that to most watchers eye its a low now ! but to keep the Islands alert to the possibility of regeneration Just my take on it .
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Hawaii watch out. Flossie was the last TS to affect Hawaii too.
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Quoting 1647. Civicane49:
Eye becoming apparent on satellite image.



Looks like Flossie wants to surpass NHC's predictions for her. Unlike some other tropical storms I know...
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I'm really curious about several of the intensity models showing Dorian growing into a hurricane in the coming days. Convection has grown impressively fast from when Dorian was likely an open wave earlier. Hasn't been just one quick burst of convection, it's steadily grown over the last five hours. Banding next?
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Quoting 1645. UNThurricane2019:

No... TropicalAmanda went out in 2008, about a week after Ike crashed ashore in Galveston.

Of course, during that first week after the storm a vast number of under-25 males on this blog frantically wondered why "she" was no longer posting here, and were genuinely worried for "her" well-being, as she had declared that she was to be riding out the storm in Galveston.

Then, about a week later, "she" came back and admitted to everyone that "she" was just fine and had been trolling all along. That the picture was of a random myspace girl and "she", the lovely astute weather girl, was, in fact, not a girl after all... and not even JFV... but rather... Altestic the Great. The user currently posts occasionally today under the username of "UNThurricane2019", for his college and pharmacy school graduating class.


So did you ever go look at his forecasting toes?
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1649. vis0
CREDIT: Unisys


watch the GTcc blob that graduated, generate some assistance that DORIAN will indirectly use.
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Quoting 1643. Camille33:

I try to be,but I hope people understand I have borderline personality disorder and mood swings.That is part of my issues on why I have a lot of arguments in my daily life.I hope people don't get the wrong image of me.


Well there you go, sorry to hear that. Lots of cool people here you could talk one on one with if you need. Feel free to IM me if you want. That really does explain a lot, most here have a pretty good capacity to forgive and forget. You've probably made many ignore lists, but that's not the end of the world. Reactive tendencies are hard to overcome, thanks for being honest and good luck.
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Eye becoming apparent on satellite image.

Member Since: July 21, 2011 Posts: 83 Comments: 7167
1646. VR46L
Quoting 1638. Tribucanes:
Your allowed to make whatever call you want, but yours have been outrageously high and doom worthy predictions over and over again. Saying you know more than most everyone and that everyone here except you can't be objective is just crass and doesn't exactly endear you to anyone. Camille33 just be nicer, this blog could certainly use more of that. When a system fades the blog gets quite tense it seems.


It does doesn't it ! I don't know if its due to disappointment of the storms failure or something else

I personally don't enjoy reading about isolating and ignoring bloggers or telling others to. I only once ignored a blogger ever on the internet and that time I really had no choice but it goes against everything I believe in .

I see Dorian resembles a remnant low surprised its still listed as a storm unless the NHC are seeing possible regeneration near the Bahamas .

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Quoting 1584. StormJunkie:


I don't know if it was about promoting himself...I think he tried to tip toe around some stuff...Or maybe it was about promoting himself...lol I guess TrapicalAmanda went out in that ban too...

Dorian is quite the little system for sure...


No... TropicalAmanda went out in 2008, about a week after Ike crashed ashore in Galveston.

Of course, during that first week after the storm a vast number of under-25 males on this blog frantically wondered why "she" was no longer posting here, and were genuinely worried for "her" well-being, as she had declared that she was to be riding out the storm in Galveston.

Then, about a week later, "she" came back and admitted to everyone that "she" was just fine and had been trolling all along. That the picture was of a random myspace girl and "she", the lovely astute weather girl, was, in fact, not a girl after all... and not even JFV... but rather... Altestic the Great. The user currently posts occasionally today under the username of "UNThurricane2019", for his college and pharmacy school graduating class.
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Quoting 1639. Tribucanes:
What are you hoping to do when your done with your degree FlaStGuy? Going for masters or doctorate?


Well I went into college as a meteorology major, but changed majors my freshman year to international relations and history, I'm working on a dual masters degree in International Relations and Urban & Regional Planning.

Goal is to work in local government as a city planner focusing on land use and zoning. Though I'll do anything in the field I enjoy, have been looking at planning tracks in emergency management as of late as well.
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Quoting 1638. Tribucanes:
Your allowed to make whatever call you want, but yours have been outrageously high and doom worthy predictions over and over again. Saying you know more than most everyone and that everyone here except you can't be objective is just crass and doesn't exactly endear you to anyone. Camille33 just be nicer, this blog could certainly use more of that. When a system fades the blog gets quite tense it seems.

I try to be,but I hope people understand I have borderline personality disorder and mood swings.That is part of my issues on why I have a lot of arguments in my daily life.I hope people don't get the wrong image of me.
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Yikes...Look where the 1016 line is.......also the forecast remnants of Dorian in Fla Straits in 120 hours

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He's trying to make that comeback , I give him credit ! Who knows ? A long shot at the betting window, that's for sure !
Member Since: July 6, 2013 Posts: 0 Comments: 460
Still a tropical storm in the 06z update.

AL, 04, 2013072706, , BEST, 0, 180N, 490W, 35, 1011, TS,

Flossie up to 55 knots.

EP, 06, 2013072706, , BEST, 0, 172N, 1367W, 55, 998, TS,
Member Since: July 21, 2011 Posts: 83 Comments: 7167
What are you hoping to do when your done with your degree FlaStGuy? Going for masters or doctorate?
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Your allowed to make whatever call you want, but yours have been outrageously high and doom worthy predictions over and over again. Saying you know more than most everyone and that everyone here except you can't be objective is just crass and doesn't exactly endear you to anyone. Camille33 just be nicer, this blog could certainly use more of that. When a system fades the blog gets quite tense it seems.
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Quoting 1632. HurricaneHunterJoe:


When does school start again?


In Florida August 26th for FSU
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Quoting 1632. HurricaneHunterJoe:


When does school start again?
August 19th. Move in August 11th.
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Quoting 1632. HurricaneHunterJoe:


When does school start again?


For me, August 1. I hate my school district.
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Quoting 1626. Camille33:

The anticyclone protected it from dry air,and it would have kept protecting it. Imagine a center co-located with an upper high,increasing the latent heat release,and further strengthening it for 2 days. Then boom the llc finally is moved away from it,the upper higher that build up,imparted a heavy dose of sw shear that was not detected well by models. This is a classic dose of storm scale shear. I strongly believe dry air is a factor for developing systems which don't have an upper high,like this one. This one is not a cyclone anymore, and for shear translated dry air into the system, where the anticyclone can't do anything.


Well here was my prognosis on Dorian yesterday morning explaining why the LLC outran the upper anticyclone...what do you think?

"My previous expectation was that Dorian's upper anticyclone would have enough thunderstorm latent heat release behind it such that the anticyclone would push out unfavorable mid-ocean upper vorticity to the degree that it would merge with upper vortex south of Bermuda...afterwards the anticyclone pushing out that upper vortex as Dorian strengthened further into a hurricane. Instead it appears the mid-ocean upper vorticity could no longer be pushed westward and away as the vorticity became stopped up by upper anticyclone in the vicinity of the Lesser Antilles...and now Dorian has crashed into the upper vorticity and left its upper anticyclone behind to the east. This has allowed the thunderstorms supported by the outflow of the anticyclone to lag behind to the east such that the Saharan dry air mentioned in paragraph P6 has worked it way into the west side of the circulation. Based on studying the 5-day upper wind forecast from the 00Z GFS...my updated intensity forecast shows Dorian weakening further in the next 24 hours while passing below the unfavorable mid-ocean upper vorticity...then Dorian recovering below the favorable upper outflow of Lesser Antilles upper anticyclone by 48 and 72 hrs...then the storm struggling again by 96 hrs while encountering the upper vortex south of Bermuda..."
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Just looked at the latest satellite imagery of Dorian and I understand why the cyclone was not downgraded to a remnant low at 11:00 pm.
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Quoting 1604. Bluestorm5:
I dunno why I'm up this late... work at 10:30. At least only one more week of Walmart.


When does school start again?
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Quoting 1564. wunderkidcayman:
come on guys you got to give it to me I did say that Dorian will make a comeback tonight most of you were RIPing and Downcasting but I had called it



hugs and smooches to you
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Quoting 1628. Tribucanes:


You know more than most anyone on here? Like Chantal becoming a cat4 and hitting Haiti. And Dorian roaring to major status yesterday as you predicted. Perhaps we're not the one's who don't know what objective means.

Yes,I don't dismiss the comments...but that was not forecast on the models.Every model showed an anticyclone over the system,so please don't blame me for that,I don't control model input. Given the conditions I was presented on the models, and the expectation of an anticyclone in tandem for 5 days on top of a 60 mph system,you are allowed to make that call.
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we have a few adults here you might expect just graduated sixth grade at the bottom of their class. :-)

Was I that easy of a giveaway?
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Quoting 1601. Camille33:

Go ahead and ignore,I try to be objective,people don't know what that means.I know quite a bit more meteorological stuff than everyone on here,perhaps except Dr.Masters and a few others.


You know more than most anyone on here? Like Chantal becoming a cat4 and hitting Haiti. And Dorian roaring to major status yesterday as you predicted. Perhaps we're not the one's who don't know what objective means.
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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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