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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nice tropical wave here
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2077. 7544
stays the same at 11 am imo next
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Quoting 2064. yonzabam:


My best guess is a juvenile bittern


Oh nooooooo...
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Quoting 2066. DaytonaBill:
Mikatnight,

That is a juvenille Yellow Crowned Night Heron. I have a pair that have been nesting over my car for the last 4 years. Fun to watch grow up, not so far to clean the crab parts off the roof, driveway, and car.


Great! Just when I was sure...
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2074. 62901IL
Our time of judgement is approaching.
Member Since: June 14, 2013 Posts: 2 Comments: 2004
Quoting 2060. StormJunkie:
Morning all

What is left of Dorian is going to get eaten alive once it gets just N of the Antilles.

Windsat from about an hour ago shows that very weak S side. There is a little banding indicating there may be some light W winds down there, but again; it's not a system that can handle the shear it is headed for.



Wait before pulling the plug on it as it manage to stay alive overnight so that why the NHC is the experts. Cant wait to see what they at 11am
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Quoting 2061. tc1120:


Looks like a Straited or a juvenile Tri-Colored Heron.


I'm going with straited. Seems to be the most popular pick. Thanks.
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Good morning..I see that Dorian is barely hanging on..We shall see..
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2070. hydrus
Quoting 2067. taco2me61:

Yes I do just to see what is "Really" going own LOL

Taco :o)
and i thought redwoods were tall....hhhaaa..
Member Since: Posts: Comments:
Quoting 2052. Grothar:


That is what WNW usually indicates within a certain parameter.

Not really

Quoting 2058. kmanislander:


I would be surprised if they didn't declassify it.


I wouldn't be surprised if NHC keeps it 40mph TS
Member Since: June 13, 2009 Posts: 2 Comments: 12716
Quoting 2059. Tropicsweatherpr:
Hi kman.

This is what Lixon Avila said at 11 PM about why he didn't pull the plug on Dirian at that time.

DORIAN APPEARS TO STILL HAVE A VIGOROUS LOW- TO MID-LEVEL
CIRCULATION IN CONVENTIONAL SATELLITE IMAGERY WITH 30 TO 35 KNOTS
MEASURED BY A RECENT ASCAT PASS...BUT IT HAS BEEN DEVOID OF DEEP
CONVECTION FOR SEVERAL HOURS. I WAS TEMPTED TO DECLARE DORIAN A
REMNANT LOW IN THIS ADVISORY...BUT GIVEN THE FACT THAT THE
CIRCULATION IS MOVING OVER WARMER WATERS AND SHEAR IS FORECAST TO
LESSEN...NEW CONVECTION COULD REDEVELOP


Yeah, read that in the archive. Reminds me of Dolly many years ago that opened up into a wave in the Caribbean but still had TS force winds and tons of convection. They kept it as a TS and the circulation closed off again near the Yucatan. With Dorian though the same reasoning would not apply save for the winds.

Anyway, the discussion coming up will sort this out.
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Quoting 2063. hydrus:
Do you always climb a tree when making a tropical forecast.?...jk....really:)

Yes I do just to see what is "Really" going own LOL

Taco :o)
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Mikatnight,

That is a juvenille Yellow Crowned Night Heron. I have a pair that have been nesting over my car for the last 4 years. Fun to watch grow up, not so far to clean the crab parts off the roof, driveway, and car.
Member Since: Posts: Comments:
LLC is ill-defined but is still there. I would say wait till its official before saying it dead. Still wonder if that ULL may merge with Dorian like some tried to do earlier in the season. Dorian still has a low level circulation so it has the potential to merge seemingly.
Member Since: Posts: Comments:
Quoting 2001. mikatnight:
Good Morning!

Am I the only one getting bored with these sunrise pics and the Lantana bridge?


I looked forever trying to find what kind of bird this is, but I'm still not sure. Some type of heron maybe? Perhaps a juvenile, dunno. Anybody know?


Dexter's other passion is chasing balls - little action shot here!

Collected 0.6" of rain yesterday in a brief, but intense little downpour, and ended our rainless streak at 4 days. NWS calling for a 60% chance today mostly between 5 & 8 pm. Maybe I'll find a 4x4 and mount that rain gauge today. There's always hope.



My best guess is a juvenile bittern
Member Since: Posts: Comments:
2063. hydrus
Quoting 2056. taco2me61:
I might go out on a limb now but I'll say TD Dorian at the next TWO :o)

Taco :o)
Do you always climb a tree when making a tropical forecast.?...jk....really:)
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Quoting 2037. kmanislander:


How do you know it closed up again ??

late yesterday when the LLCOC became visible if you examined it closer you could see that the S side was open and you an then see it closing off then it became very clear as it was clearly naked and devoid on convection and then night fell and wasn't fully visible however it could have been seen on the RGB night and Shortwave ir as very faint clouds due to its low level and night time but watching it it eventually got higher and the convection started to pop and the it got covered and is still covered yeah the scat showed it open but that was before all what I just said
Member Since: June 13, 2009 Posts: 2 Comments: 12716
2061. tc1120
Quoting 2001. mikatnight:
Good Morning!

Am I the only one getting bored with these sunrise pics and the Lantana bridge?


I looked forever trying to find what kind of bird this is, but I'm still not sure. Some type of heron maybe? Perhaps a juvenile, dunno. Anybody know?


Dexter's other passion is chasing balls - little action shot here!

Collected 0.6" of rain yesterday in a brief, but intense little downpour, and ended our rainless streak at 4 days. NWS calling for a 60% chance today mostly between 5 & 8 pm. Maybe I'll find a 4x4 and mount that rain gauge today. There's always hope.



Looks like a Straited or a juvenile Tri-Colored Heron.
Member Since: Posts: Comments:
Morning all

What is left of Dorian is going to get eaten alive once it gets just N of the Antilles.

Windsat from about an hour ago shows that very weak S side. There is a little banding indicating there may be some light W winds down there, but again; it's not a system that can handle the shear it is headed for.

Member Since: Posts: Comments:
Hi kman.

This is what Lixon Avila said at 11 PM about why he didn't pull the plug on Dorian at that time.

DORIAN APPEARS TO STILL HAVE A VIGOROUS LOW- TO MID-LEVEL
CIRCULATION IN CONVENTIONAL SATELLITE IMAGERY WITH 30 TO 35 KNOTS
MEASURED BY A RECENT ASCAT PASS...BUT IT HAS BEEN DEVOID OF DEEP
CONVECTION FOR SEVERAL HOURS. I WAS TEMPTED TO DECLARE DORIAN A
REMNANT LOW IN THIS ADVISORY...BUT GIVEN THE FACT THAT THE
CIRCULATION IS MOVING OVER WARMER WATERS AND SHEAR IS FORECAST TO
LESSEN...NEW CONVECTION COULD REDEVELOP
Member Since: April 29, 2009 Posts: 75 Comments: 14890
Quoting 2047. Seflhurricane:
Very interested to see what the NHC says in 30 MIN


I would be surprised if they didn't declassify it.
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2057. hydrus
Quoting 2052. Grothar:


That is what WNW usually indicates within a certain parameter.
I,m from the west side of south eastern North Carolina...:)
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I might go out on a limb now but I'll say TD Dorian at the next TWO :o)

Taco :o)
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Quoting 2030. Grothar:


The middle one looks like a striated crane. The top one looks like regular crane and the bottom picture looks like little Dexter.


You got all three!
"Don, what do we have for our contestant today?"
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2054. Grothar
Quoting 2051. Some1Has2BtheRookie:


What does that graphic have to do with how I feel. ... Give me a chance to get some coffee before making those comments.



I could have said, ragged and cranky, but I didn't. I still have modicum of kindness. (Although it has been running thin lately)
Member Since: July 17, 2009 Posts: 71 Comments: 27208
2052. Grothar
Quoting 2032. wunderkidcayman:

actually its W but just a tad N of W but still moving W


That is what WNW usually indicates within a certain parameter.
Member Since: July 17, 2009 Posts: 71 Comments: 27208
Quoting 2045. Grothar:


It looks like it is on its way out. Really putrid and weak.



What does that graphic have to do with how I feel. ... Give me a chance to get some coffee before making those comments.
Member Since: August 24, 2010 Posts: 0 Comments: 4772
2050. centex
I think they didn't want to declassify and maybe need to reclassify in short time frame. Anyway they much more pessimistic and now pegged when it will be gone for good. If it keeps firing convection and maintains LLC they may have a tough time declassifying until when forecasted. But if it drops convection this afternoon, would not be surprised if they pull the plug.
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2049. baulas
Quoting 2027. WWPR:


Loos like a Juvenile Yellow Crowned Night Heron...but a juvenile for sure.


Yes, this is correct.


And hoping that Dorian dissipates. No one needs a hurricane.
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Quoting 2004. TylerStanfield:

You do realize Danielle was almost a month further into the season then Dorian. The Cape Verde hurricanes don't really ramp up until the meat of the season. The fact that Dorian did form out near the Cape Verdes in July is something that is still a likely harbinger for an active Cape Verde season.


Also the 3rd one to form in the deep tropics this year (see Masters' post for July 8).
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Very interested to see what the NHC says in 30 MIN
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Quoting 2017. TylerStanfield:

The 5AM discussion mentioned that ASCAT and OSCAT passes suggested that the circulation may no longer be closed, but they resisted declassifying because they wanted to monitor it further before classifying it as a remnant low.


Quite clear from ASCAT that there were no West winds at all with Dorian last night. Wide open circulation on the South side.

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2045. Grothar
Quoting 2042. 7544:


thats a good looking open wave may be can just call it a blob for now groo


It looks like it is on its way out. Really putrid and weak.

Member Since: July 17, 2009 Posts: 71 Comments: 27208
1.93" of rain over night at my house in S. Ft. Myers. 3"-5" of rain reported in the area from Ft. Myers down to Naples Fl.
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Quoting 2027. WWPR:


Looks like a Juvenile Yellow Crowned Night Heron...but a juvenile for sure.

Interesting, I also looked at those. I agree, definitely a juvenile...something. Thanks for your input! Where's IndianRivGuy? he'd know fer sure.
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2042. 7544
Quoting 2033. Seflhurricane:
Visible images indicate that Dorian is very likely degenerating into an open wave, i see no LLC and if there is one its super weak


thats a good looking open wave may be can just call it a blob for now groo
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2041. LargoFl
1. Tampa-St. Petersburg, Fla.

The Tampa-St. Petersburg area, on the west coast of Florida, is extremely vulnerable to storm surge flooding, and has been fortunate to escape a direct hit from a strong hurricane for many years. Climate Central calculates the 100-year flood height in this area is 6.5 feet above the high tide line. Using Climate Central’s Surging Seas sea level rise mapping tool — it becomes clear that there are about 125,000 people currently living below this flood level. In St. Petersburg alone, there are more than 45,000 homes that lie below 6 feet in elevation, and would likely be vulnerable to a storm surge of that magnitude or greater.

Click image to enlarge. Radar image of Hurricane Charley as it swept ashore south of Tampa in 2004. Credit: National Hurricane Center.

According to Climate Central’s research, sea level rise is escalating the threat of damaging storm surge flooding in the Tampa area. The odds that a flood exceeding 6.5 feet would occur in Tampa before 2030 are about 14 percent without global warming, but these odds increase to 20 percent with the effects of global warming-related sea level rise factored in.

Tampa is no stranger to hurricanes, including storms that came very close to the city but avoided a worst-case scenario track during the past decade. According to the Hurricane City website, tropical storms and hurricanes have passed within 60 miles of Tampa on 68 occasions since 1871. These included Hurricanes Frances and Jeanne, both of which struck during 2004. However, Tampa has not suffered a direct hit by a strong hurricane since way back in 1921, when a storm surge of 10.5 feet occurred in Tampa Bay.

Hurricane Charley in 2004 was initially forecasted to strike Tampa-St. Petersburg as a Category 2 storm, but shortly before landfall it intensified rapidly into a Category 4 storm, and turned eastward sooner than expected, inflicting its worst damage on the Port Charlotte area, south of Tampa. Charley caused $16 billion in damage.
Member Since: August 6, 2011 Posts: 4 Comments: 42064
2039. 62901IL
Quoting 2038. Doppler22:
So, Dorian finally got thunderstorms but lost its center... What a shame :p

I know.
Member Since: June 14, 2013 Posts: 2 Comments: 2004
So, Dorian finally got thunderstorms but lost its center... What a shame :p
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Quoting 2016. wunderkidcayman:

yeah but then it closed up and started to pop some convection

I said it would reorganise and well it did do that


How do you know it closed up again ??
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2036. 62901IL
Quoting 2033. Seflhurricane:
Visible images indicate that Dorian is very likely degenerating into an open wave, i see no LLC and if there is one its super weak

Bye bye dorian...nice knowing ya...
Member Since: June 14, 2013 Posts: 2 Comments: 2004
Quoting 2021. 62901IL:
I bet you guys anything that dorian will be an open wave at the next advisory.

nah 40mph TS
Member Since: June 13, 2009 Posts: 2 Comments: 12716
Quoting 2029. washingtonian115:
Be funny after all this foreshadowing you all are doing nothing significant came from this cape verde season.And what I meant was that a more deserving storm should come down the line and get the name not this weakling..

Oh washi, you and your hurricane names...
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Visible images indicate that Dorian is very likely degenerating into an open wave, i see no LLC and if there is one its super weak
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Quoting 2022. Grothar:
Coordinates: 18.3N 51.1W 12Z
Coordinates: 18.0N 49.0W 06Z
Coordinates: 17.8N 47.1W 00Z

Wow. Seems to still be moving WNW. Interesting to see what the 18Z will be.

actually its W but just a tad N of W but still moving W
Member Since: June 13, 2009 Posts: 2 Comments: 12716
2011. Stoopid1

2013. Seflhurricane

I have reviewed the 11 PM Discussion in the NHC storm archive and they mention that Dorian still had a vigorous low and mid level circulation so it would appear they did not want to declassify too soon in the event the low closed off again later last night.

To my eye Dorian looks like an open wave again this morning but hopefully ASCAT will give us a good look soon.
Member Since: Posts: Comments:
2030. Grothar
Quoting 2001. mikatnight:
Good Morning!

Am I the only one getting bored with these sunrise pics and the Lantana bridge?


I looked forever trying to find what kind of bird this is, but I'm still not sure. Some type of heron maybe? Perhaps a juvenile, dunno. Anybody know?


Dexter's other passion is chasing balls - little action shot here!

Collected 0.6" of rain yesterday in a brief, but intense little downpour, and ended our rainless streak at 4 days. NWS calling for a 60% chance today mostly between 5 & 8 pm. Maybe I'll find a 4x4 and mount that rain gauge today. There's always hope.



The middle one looks like a striated crane. The top one looks like regular crane and the bottom picture looks like little Dexter.
Member Since: July 17, 2009 Posts: 71 Comments: 27208
Quoting 2004. TylerStanfield:

You do realize Danielle was almost a month further into the season then Dorian. The Cape Verde hurricanes don't really ramp up until the meat of the season. The fact that Dorian did form out near the Cape Verdes in July is something that is still a likely harbinger for an active Cape Verde season.
Be funny after all this foreshadowing you all are doing nothing significant came from this cape verde season.And what I meant was that a more deserving storm should come down the line and get the name not this weakling..
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Quoting 2021. 62901IL:
I bet you guys anything that dorian will be an open wave at the next advisory.


No, It will still be at 40 with pressure of 1011 mb
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