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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Lol.Stop comparing this to a Katrina/Andrew come back..both were in late August and eventually became cat 5's.Something this storm will not..
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Quoting 321. fabian171017:


I think it was TD 10.
Katrina first formed as TD 10, but that depression died out. A few days later, that storm became TD 12 (not whole TD 10 so therefore, a new name). TD went to become Hurricane Katrina in a day or so and hit Florida. When she didn't weakened over Florida... you know what happened.
Member Since: August 1, 2011 Posts: 28 Comments: 8009
I would not RIP Dorian quite yet people... He may be streaking but he still has a closed low. Is still some small chance he may survive.
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Looks like Dorian is on life support the patient is gone weve lost him. Whats inline next?
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Quoting 259. DataNerd:
Okay I am calling it. Dorian has or is dissipating right now. LLC is barely discernible and has left the main body of convection to the west, and is barely even turning on visible satellite. The remaining convective structure is collapsing and/or resembles an open wave at this time.

They should keep the system for 5 PM and possibly kill it at 11 or 5 am. Its possible that neither happens and NHC holds the system for another 12-24 hrs hours or so but unless the current trend reverse itself I find this very unlikely.

Thats all she wrote folks. Just too early yet in the season for a storm to make it through these types of conditions. We are very fortunate. Had this made it to the gulf it would have been a serious threat.

Link



Windsat confirms the collapse:




With the cape verde wave system heating I would expect another cyclone to form in the next two weeks. There is already another very vigorous wave that has just emerged off the coast, and GFS indicates something may form in that area in 100 hrs or so.

And dorian may not be totally dead yet, but I would be surprised (unless something changes) if the system is not gone in 12 hours.


It might be gone in 12 hours but it will come back. Dorian has a small storm complex and he's fighting to prove his cainehood.
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Lee mentioned John Hope....He was my all time favorite guy..I cried when he passed away.
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Tropical Storm 04L

UW-CIMSS Automated Satellite-Based
Advanced Dvorak Technique (ADT)
Version 8.1.4
Tropical Cyclone Intensity Estimation Algorithm

Current Intensity Analysis



UW - CIMSS
ADVANCED DVORAK TECHNIQUE
ADT-Version 8.1.4
Tropical Cyclone Intensity Algorithm

----- Current Analysis -----
Date : 26 JUL 2013 Time : 181500 UTC
Lat : 17:51:45 N Lon : 44:29:12 W


CI# /Pressure/ Vmax
3.0 /1006.7mb/ 45.0kt


Final T# Adj T# Raw T#
3.0 3.0 3.0

Center Temp : -56.3C Cloud Region Temp : -36.3C

Scene Type : CURVED BAND with 0.54 ARC in LT GRAY

Positioning Method : FORECAST INTERPOLATION

Ocean Basin : ATLANTIC
Dvorak CI > MSLP Conversion Used : ATLANTIC

Tno/CI Rules : Constraint Limits : NO LIMIT
Weakening Flag : OFF
Rapid Dissipation Flag : OFF

C/K/Z MSLP Estimate Inputs :
- Average 34 knot radii : 43km
- Environmental MSLP : 1018mb

Satellite Name : MSG3
Satellite Viewing Angle : 54.2 degrees


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Quoting 314. MTWX:
I know it's a monthish early and it will always be a sore subject, but does anyone else see the Katrina senerio shaping up??

Let me show you what I mean. (2005 info in parenthesis)

Tropical Storm Dorian (TD 8) degenerates and moves into the Bahamas. Large potent wave comes from behind and the storm regenerates with a new name crossing the Florida Peninsula into the gulf due to the Bermuda High placement.


Now I'm not saying that someting will form, or that if it does, it's going to be the Cat 5 monter Katrina was. Just saying I see a similar setup on the table placement wise...



I think it was TD 10.
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O watch the bulls for development show up
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Quoting 302. moonlightcowboy:


The exposed low-level center I find interesting, maybe a bit unusual. Looks more like an eyewall feature. Although the whole thing is relatively small, the coc/eyewall (yeah, I'ma call it that for now) is large comparative to its size.

So, what's that mean? Definitely not dead. Still has a vigorous circulation, closed system. It just doesn't have any convection overhead, likely more due to the fast, low-level easterly flow than it is about shearing considering its position. Certainly, seems the disproportionate coc to its size may be quite capable of churning out any infiltrating dry air. Still, it's gotta have that convection overhead to mature. Again, convergence seemed to be improving, it's trying to get its act together.

And, I say unusual in that if Dorian finds its sea legs soon in warmer waters, the small system has a potential to grow in size (not necessarily intensity) fairly quickly.
One thing we all should have learned through the years, and that is not to disregard a storm by its appearance. They have a way of saying surprise. Just like Dorian, that looks like an eye popping up to say "I am not dead, just peeking out to see who is watching" LOL.Just a little humor, but the fact remains ,don't turn your eyes off of Dorian
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Quoting 311. canehater1:
Will have to be some large changes in the environment
ahead of Dorian for it to intensify or regenerate,
which ever applies...


There is a reason some models intensify Dorian in the Bahamas...the environment should change. Now, in the near term, the fate of Dorian is certainly in question.
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.."faint's'

Klunk!
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Quoting 258. ncstorm:
Most of yall have been on WU for a lot of years and have seen storms come back with a vengeance after looking pitiful..this kind of attitude you would expect from newbies..with that said, you all should know better..(shaking my head over here)
Oops, I minus it by accident, but I agree. I wouldn't consider myself being on here long though... I got to admit it's hard to not be disappointed but I'm glad we got something to do in July. Also glad no one is going to get hurt from this unless Dorian hang on.
Member Since: August 1, 2011 Posts: 28 Comments: 8009
Interesting.

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314. MTWX
I know it's a monthish early and it will always be a sore subject, but does anyone else see the Katrina senerio shaping up??

Let me show you what I mean. (2005 info in parenthesis)

Tropical Storm Dorian (TD 10) degenerates and moves into the Bahamas. Large potent wave comes from behind and the storm regenerates with a new name crossing the Florida Peninsula into the gulf due to the Bermuda High placement.


Now I'm not saying that someting will form, or that if it does, it's going to be the Cat 5 monter Katrina was. Just saying I see a similar setup on the table placement wise...

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Quoting 308. Boco12:


Sorry newbie here, I was wondering where I can get those numbers?
Many thanks!


ATCF

Link
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Quoting 302. moonlightcowboy:


The exposed low-level center I find interesting, maybe a bit unusual. Looks more like an eyewall feature. Although the whole thing is relatively small, the coc/eyewall (yeah, I'ma call it that for now) is large comparative to its size.

So, what's that mean? Definitely not dead. Still has a vigorous circulation, closed system. It just doesn't have any convection overhead, likely more due to the fast, low-level easterly flow than it is about shearing considering its position. Certainly, seems the disproportionate coc to its size may be quite capable of churning out any infiltrating dry air. Still, it's gotta have that convection overhead to mature. Again, convergence seemed to be improving, it's trying to get its act together.

And, I say unusual in that if Dorian finds its sea legs soon in warmer waters, the small system has a potential to grow in size (not necessarily intensity) fairly quickly.
Such an interesting storm, one of its kind, hardly can even find an analog for it, so if it survives it might take one of those weird tracks which would be kind of cool to see.
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Will have to be some large changes in the environment
ahead of Dorian for it to intensify or regenerate,
which ever applies...
Member Since: September 8, 2008 Posts: 1 Comments: 1069
Dorian still has a rather well defined center, but it needs convection in a hurry.
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Even if Dorian dies, his open wave should still be watched. Chantal split into two and both parts of her wave were both given chances to develop later down the line. Things are more favourable now than when Chantal split, so if Dorian does open out, he could easily re-form later, especially if he makes it into the GOM.
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Quoting 278. Civicane49:
Not surprising to see Flossie up to 50 knots:

EP, 06, 2013072618, , BEST, 0, 163N, 1333W, 50, 999, TS, 50, NEQ, 20, 0, 0, 0, 1009, 150, 20, 0, 0, E, 0, , 0, 0, FLOSSIE, D,


Sorry newbie here, I was wondering where I can get those numbers?
Many thanks!
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Quoting 304. ncstorm:
When is DMAX?
Before sunrise. This storm is going to be fighting DMIN in few hours.
Member Since: August 1, 2011 Posts: 28 Comments: 8009
Quoting 296. MiamiHurricanes09:
Completely agree; it has to remain a tropical cyclone though, and I honestly don't see that happening unless it can start firing at least some sort of convection in the very near future.


Doesn't have to remain a cyclone, just has to remain a closed LLC.
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I finally went and get a life for first time in a week by working on my newest Volvo S60 from early 2000s with my dad. We sold my first car, a Honda Accord from early 90s since we both doubt car's ability to make 300 miles journeys back and forth to Asheville. My Honda barely made it over the mountains east of Asheville on I-40 last time. I got to say I won't be predicting a hurricane in my next forecast, but I'm not giving up on this storm. Maybe 50 or 60 mph peak few days from now. It's a treat that mother nature gave us from Chantal and Dorian. These two are a surprise for being early. Well, back to working on my Volvo with the heat in the mid-90s.
Member Since: August 1, 2011 Posts: 28 Comments: 8009
When is DMAX?
Member Since: August 19, 2006 Posts: 13 Comments: 15300
Quoting 286. nrtiwlnvragn:
O




Quoting 289. AllStar17:


I'd argue that given the visible satellite presentation of Tropical Storm Dorian that, while the convection has weakened and become disorganized in nature, the storm has actually done a great job throughout the day of getting much better organized in the lower levels as the low level circulation appears very well defined and established and low level banding develops.
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The exposed low-level center I find interesting, maybe a bit unusual. Looks more like an eyewall feature. Although the whole thing is relatively small, the coc/eyewall (yeah, I'ma call it that for now) is large comparative to its size.

So, what's that mean? Definitely not dead. Still has a vigorous circulation, closed system. It just doesn't have any convection overhead, likely more due to the fast, low-level easterly flow than it is about shearing considering its position. Certainly, seems the disproportionate coc to its size may be quite capable of churning out any infiltrating dry air. Still, it's gotta have that convection overhead to mature. Again, convergence seemed to be improving, it's trying to get its act together.

And, I say unusual in that if Dorian finds its sea legs soon in warmer waters, the small system has a potential to grow in size (not necessarily intensity) fairly quickly.
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Quoting 295. Tazmanian:
.


Is that a eye


Not a pinhole
Member Since: September 23, 2005 Posts: 14 Comments: 11179
Quoting 290. washingtonian115:
Hispaniola comes into the picture by then.
That track wouldn't pan out if the ULL moves Southwestward like the Euro is showing it would block it and keep it on a heading of WNW. Most of the models now take it north of Hispaniola as well. I know I seem to be flip-flopping like the models, but the strength and positioning of the ridge will determine whether it hits Hispaniola or track north of there.

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maybe invest 90L IN five days from now.
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Definite "eye" like center visible on the RGB at 17.9N 45W. Convection on the east. But looks like it may be trying to hang in there. Give it a few hours, see what happens.
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Well, even though Dorian is looking like it's on its last leg, it's not over yet, and we have 6 more days until August.

And when August comes, then we should be expecting storms like Dorian to thrive not waste away.
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Quoting 293. TropicalAnalystwx13:
IF Dorian can survive the next 36 hours or so, and that's a *huge* if, it could become a threat to become a strong tropical storm or hurricane in the Bahamas. There, water temperatures are 28-29C, wind shear is expected to be below 15 knots, atmospheric pressures should be lower, and vertical instability is much higher. The 18z SHIPS is under the impression this storm will survive, and the model brings it up to 53kt (61 mph) at 120 hours; the LGEM brings the storm up to 70 mph.
Completely agree; it has to remain a tropical cyclone though, and I honestly don't see that happening unless it can start firing at least some sort of convection in the very near future.
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Quoting 286. nrtiwlnvragn:
O


.


Is that a eye
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Although Dorian's pressure is up to 1010 MB, Andrew's pressure in 1992 was at 1015 MB at one point, unheard of for a 40 MPH tropical storm, even if it was just barely holding its own when it was a few hundred miles north of Puerto Rico.

If Dorian is degenerating into a tropical wave, it could be done for, as it may not have enough critical mass to get going again. If it can retain a circulation, it may be able to re-intensify down the road.
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IF Dorian can survive the next 36 hours or so, and that's a *huge* if, it could become a threat to become a strong tropical storm or hurricane in the Bahamas. There, water temperatures are 28-29C, wind shear is expected to be below 15 knots, atmospheric pressures should be lower, and vertical instability is much higher. The 18z SHIPS is under the impression this storm will survive, and the model brings it up to 53kt (61 mph) at 120 hours; the LGEM brings the storm up to 70 mph.
Member Since: July 6, 2010 Posts: 113 Comments: 32067
Just my opinion so grain of salt. Even if Dorian dissipates and is nothing more than a tropical wave, those remnants could still end up in the GOM and at that time August will be here. Lows in the GOM during that time are indicative of tropical generation so we all might be somewhat right with this, one way or another. At the end of the day whether or not it was a named storm or a wave to begin with is what it is. In this case cyclone genesis might just be a heads up a little early.

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Quoting 270. ncstorm:


the NHC also said in 96 hours it could re-intensify..I wouldnt sleep on Dorian just yet..
Quoting 275. GTstormChaserCaleb:
You should no better that wind shear forecasts more than 3 days out are unreliable and that all it takes is a subtle movement of the TUTT to create favorable conditions. We shall see, odds are against it though and most if not all of the computer models forecast weakening.
Hispaniola comes into the picture by then.
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Well, Dorian has been a rather artistic storm at least. It's given us the image of an Elephant (rather impressive one at that!), a smiley face and now barbamz has picked up that it looks similar to the horsehead nebular on the wv image LOL.
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Quoting 276. CybrTeddy:


If it remains north of the islands then regeneration should be considered more likely than not.


Reason why I don't think this will happen:

1. Dry air. Still alot of dry air in the way.

2. The westerly flow does not really relax until a significant distance away from the Island, and that is likely whats killing it now more than anything.

3. Upper level shear is going to be like 50Kt or greater. A hurricane could survive that not a naked low pressure system.
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O


Member Since: September 23, 2005 Posts: 14 Comments: 11179
216 hours
Member Since: August 19, 2006 Posts: 13 Comments: 15300
Quoting 265. Patrap:
Somewhere, the NHC forecasters are face palming, by the dozen.





yep
Member Since: September 8, 2008 Posts: 1 Comments: 1069
Quoting 278. Civicane49:
Not surprising to see Flossie up to 50 knots:

EP, 06, 2013072618, , BEST, 0, 163N, 1333W, 50, 999, TS, 50, NEQ, 20, 0, 0, 0, 1009, 150, 20, 0, 0, E, 0, , 0, 0, FLOSSIE, D,
Need to keep an eye on it FIM-9 keeps it a tropical storm over the Big Island.
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Part of me wants to quit tracking Dorian for a day or so and see if some organization occurs into next week. I'm tired of this boring mess. xD
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Dorian moving west now at 280 degrees.

LATCUR = 17.6N LONCUR = 44.8W DIRCUR = 280DEG SPDCUR = 19KT
Member Since: April 29, 2009 Posts: 75 Comments: 14267
Quoting 225. sar2401:

I think you mean tripole. Check out this UK Met page for a pretty good explanation of the tripole.


LOL....No the triple posting my reply to a post.....or did you already know that? :) and btw....oh what a relief it is!
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279. JLPR2
Circulation is basically heading alone into the dry air.



LLC is right at 45W, below 18N.

I find it a bit hard to believe Dorian will recover from this, the dry air was bad enough alone and then the shear arrived. But who knows, maybe it will surprise us.
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Not surprising to see Flossie up to 50 knots:

EP, 06, 2013072618, , BEST, 0, 163N, 1333W, 50, 999, TS, 50, NEQ, 20, 0, 0, 0, 1009, 150, 20, 0, 0, E, 0, , 0, 0, FLOSSIE, D,
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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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