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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2128. Sfloridacat5
3:55 PM GMT on July 27, 2013
Quoting 2124. eddiedollar:
I am fairly new to this blog and I have learnt a lot about the weather. I see many bloggers have written off this season already and we haven't finished July yet. So far this season is on par with some of the most active seasons ever eg 2011,2010,2008 or 1969!!


And it only takes 1 storm. Andrew, the 1st storm of the season, wasn't until Aug.24th.

I bet people really thought that year was a bust at this time with no named storms.
Member Since: September 16, 2007 Posts: 0 Comments: 7389
2127. scottsvb
3:55 PM GMT on July 27, 2013
Dorian is a exposed LLC...We are thinking of dropping her on the next adv at 5pm. MLC is just south off the LLC. Dry air, shear ahead will give this a 20% chance of redevelopment in the next 48hrs (as of 5pm). Still discussing the exact details.
Member Since: January 22, 2006 Posts: 0 Comments: 1690
2126. HuracanTaino
3:39 PM GMT on July 27, 2013
Quoting 2112. Grothar:



I wouldn't expect to see more than 10-20%. It takes a while for these, although this is the one I have been worried about since last week.





Dorian look bad

img src="http://www.ssd.noaa.gov/PS/TROP/floaters/04L/ imagery/rb-animated.gif" style=b"max-width:e Look bads but way bettter than yesterday
Member Since: May 31, 2011 Posts: 0 Comments: 917
2125. stormpetrol
3:23 PM GMT on July 27, 2013
Member Since: April 29, 2007 Posts: 0 Comments: 7931
2124. eddiedollar
3:07 PM GMT on July 27, 2013
I am fairly new to this blog and I have learnt a lot about the weather. I see many bloggers have written off this season already and we haven't finished July yet. So far this season is on par with some of the most active seasons ever eg 2011,2010,2008 or 1969!!
Member Since: June 2, 2013 Posts: 0 Comments: 42
2123. hydrus
3:05 PM GMT on July 27, 2013
A long way out, but interesting.. CMC, GFS at 240..



Member Since: September 27, 2007 Posts: 1 Comments: 21416
2122. weatherlover94
3:05 PM GMT on July 27, 2013
could see a yellow circle on the new african wave at 2:00.....but likely to suffer the same fate as Dorian down the road.
Member Since: September 8, 2010 Posts: 0 Comments: 2209
2121. SLU
3:02 PM GMT on July 27, 2013
Poor CaribBoy. He's having another bad season.

GIVEN THE TRACK AND INTENSITY FORECAST...ALONG WITH THE WEAK WIND
FIELD OVER THE SOUTHERN SEMICIRCLE OF THE CYCLONE...WATCHES OR
WARNINGS ARE NOT NEEDED FOR THE ISLANDS OF THE NORTHEAST CARIBBEAN.
Member Since: July 13, 2006 Posts: 13 Comments: 5259
2120. RascalNag
3:01 PM GMT on July 27, 2013
Link

While Dorian chugs along straddling the line between life and death, these two surface lows continue to churn off the East Coast. The NC one has a beautiful circulation and is really trying hard to get some convection going along with it, but its western side is dropping the ball there. The FL one is still busy improving its circulation, and seems to be doing a good job so far. The former of the two may not have enough time left in warm waters to do much, but if shear continues to decrease across the coast then perhaps the surface low currently off of Florida can make progress. Both are moving NNE.
Member Since: October 24, 2012 Posts: 0 Comments: 209
2119. Some1Has2BtheRookie
3:00 PM GMT on July 27, 2013
Quoting 2054. Grothar:



I could have said, ragged and cranky, but I didn't. I still have modicum of kindness. (Although it has been running thin lately)


Cranky?!?! What makes you think I am cranky??? lol

Just to show that I am not cranky, I will admit to the "ragged" comment. Sadly, coffee will resolve the cranky, but I will still be ragged. I have yet to find the drink that solves the ragged look. Wild Turkey doesn't do it, but then again it does get me to where I no longer care about my ragged look. :)
Member Since: August 24, 2010 Posts: 0 Comments: 4745
2118. wunderkidcayman
2:58 PM GMT on July 27, 2013
guys NEW BLOG!!!!!!
Member Since: June 13, 2009 Posts: 2 Comments: 12157
2117. JrWeathermanFL
2:57 PM GMT on July 27, 2013
People..
Shear is decreasing.
Dorian is entering warmer waters.
It looks better convective wise than last night.
Dry air isn't extremely bad.
Dorian should hang on a while...unless it don't have a closed circulation..
Member Since: July 19, 2011 Posts: 12 Comments: 2455
2116. canehater1
2:57 PM GMT on July 27, 2013
11am advisory is not out early, so probably a lot of
thought going into it.
Member Since: September 8, 2008 Posts: 1 Comments: 1078
2115. VR46L
2:57 PM GMT on July 27, 2013
New Blog LOL !!!!!!!
Member Since: March 1, 2012 Posts: 0 Comments: 6927
2114. weatherlover94
2:56 PM GMT on July 27, 2013
I wonder whats taking so long for the 11:00 update ?
Member Since: September 8, 2010 Posts: 0 Comments: 2209
2113. StormJunkie
2:55 PM GMT on July 27, 2013
Quoting 2073. Hurricanes305:


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


Wait, don't wait; it doesn't really matter at this point. It is headed for a very detrimental environment and that isn't going to change quick enough. Even if the ULL were moving or dissipating, it is not doing it at nearly the speed Dorian is headed for it. I would put it at under a less than 10% chance that it survives that shear. And if it does, I would put it at about a 1% chance that it does it with a closed circulation.
Member Since: August 17, 2005 Posts: 26 Comments: 16872
2112. Grothar
2:54 PM GMT on July 27, 2013
Quoting 2086. redwagon:


I expect a 70% red circle with pre-Erin in a few minutes.



I wouldn't expect to see more than 10-20%. It takes a while for these, although this is the one I have been worried about since last week.





Dorian look bad

Member Since: July 17, 2009 Posts: 71 Comments: 26499
2111. theyoungmet
2:54 PM GMT on July 27, 2013
Quoting 2101. taco2me61:


I'm not sure they would even go Because of the Sequester..... They are saying they will not fly into many of the Storms that are small and no threat to CONUS....

Taco :o)

Dang sequester! But I want to knowwww (sorry my weather weenie side is coming out)
Member Since: July 26, 2013 Posts: 0 Comments: 35
2110. Vlad959810
2:52 PM GMT on July 27, 2013
Quoting 2089. kmanislander:
The 925 mb vorticity ( 2500 feet ) shows the rotation with Dorian is located away from where the low center is supposed to be, suggestion that whatever circulation remains is either not due to a closed surface low or that Dorian is not vertically aligned.


This situation reminds EARL 3 years ago while approaching the Leeward Islands
Member Since: July 14, 2011 Posts: 0 Comments: 89
2109. TheDawnAwakening
2:51 PM GMT on July 27, 2013
Quoting 2103. stoormfury:
well it is beginning of the period to look towards the African coast for the E storm. right now, there are no other threat areas, except the pathetically looking Dorian. The tropical wave se of the cape verdes have a little spin .but nothing that looks like an organizing system.


There are two solid surface lows off the East Coast of the US. There are developing nice spin to them, and low level convergence is strengthening with developing cumulus cloud field, with low level banding trying to get going soth of the center east of NC, it's a closed circulation
Member Since: October 21, 2008 Posts: 246 Comments: 3947
2108. wunderkidcayman
2:51 PM GMT on July 27, 2013
New
Last NHC advisory issued on FLOSSIE
Member Since: June 13, 2009 Posts: 2 Comments: 12157
2107. WunderAlertBot (Admin)
2:51 PM GMT on July 27, 2013
JeffMasters has created a new entry.
2106. wxchaser97
2:48 PM GMT on July 27, 2013
At least we have TS Flossie to watch. It is still expected to hit the Hawaiian Islands as a TS. The latest advisory was just issued and Flossie has 65mph, down from 70mph on the last advisory.
Member Since: March 16, 2012 Posts: 127 Comments: 7948
2105. mcluvincane
2:48 PM GMT on July 27, 2013
Still 40mph TS 11:00 advisory
Member Since: June 2, 2010 Posts: 0 Comments: 1347
2104. wunderkidcayman
2:47 PM GMT on July 27, 2013
Epac TS no longer issuing Advisorys
Member Since: June 13, 2009 Posts: 2 Comments: 12157
2103. stoormfury
2:46 PM GMT on July 27, 2013
well it is beginning of the period to look towards the African coast for the E storm. right now, there are no other threat areas, except the pathetically looking Dorian. The tropical wave se of the cape verdes have a little spin .but nothing that looks like an organizing system.
Member Since: August 22, 2006 Posts: 0 Comments: 2713
2102. theyoungmet
2:46 PM GMT on July 27, 2013
Quoting 2089. kmanislander:
The 925 mb vorticity ( 2500 feet ) shows the rotation with Dorian is located away from where the low center is supposed to be, suggestion that whatever circulation remains is either not due to a closed surface low or that Dorian is not vertically aligned.


Dorian is not vertically aligned I think. The center is dislocated from the main convective structure. It's not gonna help when the TUTT introduces 40 knot shear once again. I wouldn't expect him to maintain a stacked center until he gets to the Bahamas or near Cuba.
Member Since: July 26, 2013 Posts: 0 Comments: 35
2101. taco2me61
2:46 PM GMT on July 27, 2013
Quoting 2093. theyoungmet:
Many of you are using the ASCAT to assume Dorian has no circulation, but Dorian's core might be so small, the satellite pass may not be able to pick up any west winds. If only we had a recon...


I'm not sure they would even go Because of the Sequester..... They are saying they will not fly into many of the Storms that are small and no threat to CONUS....

Taco :o)
Member Since: July 7, 2005 Posts: 6 Comments: 3261
2100. hurricanes2018
2:45 PM GMT on July 27, 2013
water temp getting warmer
Member Since: March 12, 2013 Posts: 27 Comments: 53241
2099. JRRP
2:44 PM GMT on July 27, 2013
Member Since: August 16, 2007 Posts: 0 Comments: 5981
2098. hydrus
2:44 PM GMT on July 27, 2013
Quoting 2087. Stoopid1:
Sunset at Puerto Barrios, Guatemala


Riverine boats onboard the USS Oak Hill. Note the beautiful blue waters of the Southern Caribbean
Blue and warm..A hurricane would slurp it up quick.
Member Since: September 27, 2007 Posts: 1 Comments: 21416
2097. LargoFl
2:44 PM GMT on July 27, 2013
Member Since: August 6, 2011 Posts: 4 Comments: 39248
2096. kmanislander
2:43 PM GMT on July 27, 2013
Back in a few minutes
Member Since: August 19, 2006 Posts: 0 Comments: 15842
2095. wunderkidcayman
2:43 PM GMT on July 27, 2013
Quoting 2089. kmanislander:
The 925 mb vorticity ( 2500 feet ) shows the rotation with Dorian is located away from where the low center is supposed to be, suggestion that whatever circulation remains is either not due to a closed surface low or that Dorian is not vertically aligned.


if this is correct and I know its not correct all the time I don't know why but I have noticed it
Member Since: June 13, 2009 Posts: 2 Comments: 12157
2094. kmanislander
2:43 PM GMT on July 27, 2013
Quoting 2083. centex:
I'm thinking last one more six hour period than dropped at 5. But hate when post something like this and proved wrong minutes later.


Really does not matter who turns out to be right or wrong. We all have an opinion and often there is an element of subjectivity in deciding whether to declassify or not. Indeed, we saw that last night when on a purely technical basis Dorian was no longer closed.
Member Since: August 19, 2006 Posts: 0 Comments: 15842
2093. theyoungmet
2:42 PM GMT on July 27, 2013
Many of you are using the ASCAT to assume Dorian has no circulation, but Dorian's core might be so small, the satellite pass may not be able to pick up any west winds. If only we had a recon...
Member Since: July 26, 2013 Posts: 0 Comments: 35
2092. hydrus
2:42 PM GMT on July 27, 2013
Quoting 2081. taco2me61:


We have "Really" Tall Pine Trees :o) LOL

Taco :o)
I " had " some pines..They died when Charley reconfigured them horizontally.
Member Since: September 27, 2007 Posts: 1 Comments: 21416
2091. LargoFl
2:41 PM GMT on July 27, 2013
Member Since: August 6, 2011 Posts: 4 Comments: 39248
2090. SLU
2:40 PM GMT on July 27, 2013
Quoting 2084. hydrus:
Where is the butter.?....all toast should have somethin on it..:)


hehe
Member Since: July 13, 2006 Posts: 13 Comments: 5259
2089. kmanislander
2:41 PM GMT on July 27, 2013
The 925 mb vorticity ( 2500 feet ) shows the rotation with Dorian is located away from where the low center is supposed to be, suggestion that whatever circulation remains is either not due to a closed surface low or that Dorian is not vertically aligned.

Member Since: August 19, 2006 Posts: 0 Comments: 15842
2088. wunderkidcayman
2:39 PM GMT on July 27, 2013
according to SST Dorian is about to step into hot waters 27 degrees and higher
Member Since: June 13, 2009 Posts: 2 Comments: 12157
2087. Stoopid1
2:39 PM GMT on July 27, 2013
Sunset at Puerto Barrios, Guatemala


Riverine boats onboard the USS Oak Hill. Note the beautiful blue waters of the Southern Caribbean
Member Since: August 6, 2007 Posts: 24 Comments: 2745
2086. redwagon
2:39 PM GMT on July 27, 2013
Quoting 2078. hurricanes2018:
nice tropical wave here


I expect a 70% red circle with pre-Erin in a few minutes.
Member Since: August 4, 2007 Posts: 0 Comments: 3276
2085. aislinnpaps
2:39 PM GMT on July 27, 2013
The next step in google searching will be to put in a photo of something and click on search...
Member Since: August 22, 2009 Posts: 0 Comments: 3148
2084. hydrus
2:39 PM GMT on July 27, 2013
Quoting 2080. SLU:
Dorian is toast.
Where is the butter.?....all toast should have somethin on it..:)
Member Since: September 27, 2007 Posts: 1 Comments: 21416
2083. centex
2:38 PM GMT on July 27, 2013
I'm thinking last one more six hour period than dropped at 5. But hate when post something like this and proved wrong minutes later.
Member Since: August 10, 2007 Posts: 0 Comments: 3270
2082. slavicthunder
2:38 PM GMT on July 27, 2013
I wonder if Dexter has looked at the low pressure system by Cape Verde this morning.
Member Since: August 5, 2012 Posts: 0 Comments: 110
2081. taco2me61
2:37 PM GMT on July 27, 2013
Quoting 2070. hydrus:
and i thought redwoods were tall....hhhaaa..


We have "Really" Tall Pine Trees :o) LOL

Taco :o)
Member Since: July 7, 2005 Posts: 6 Comments: 3261
2080. SLU
2:37 PM GMT on July 27, 2013
Dorian is toast.

Should open up officially at 11am.
Member Since: July 13, 2006 Posts: 13 Comments: 5259
2079. Stoopid1
2:37 PM GMT on July 27, 2013
Quoting 2069. wunderkidcayman:



I wouldn't be surprised if NHC keeps it 40mph TS


This should be the case.
Member Since: August 6, 2007 Posts: 24 Comments: 2745
2078. hurricanes2018
2:37 PM GMT on July 27, 2013
nice tropical wave here
Member Since: March 12, 2013 Posts: 27 Comments: 53241

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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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