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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come on dorian..just reach cuba..you can do it.....
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Dorian, I declare you a dud.
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Member Since: July 21, 2011 Posts: 82 Comments: 7056
Whether Dorian will be a hurricane or not one thing that still is kinda interesting is this is the 2nd storm in July. July usual has one named storm every two years. That is the same with June. So as of right now Hurricane Season has been pretty active.
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I pronounce Dorian dead at 4:35 pm
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TROPICAL STORM FLOSSIE DISCUSSION NUMBER 8
NWS NATIONAL HURRICANE CENTER MIAMI FL EP062013
200 PM PDT FRI JUL 26 2013

FLOSSIE HAS BECOME A LITTLE BETTER ORGANIZED DURING THE PAST SEVERAL
HOURS WITH A MORE SYMMETRICAL CENTRAL DENSE OVERCAST AND BETTER
DEFINED CENTRAL FEATURES ON MICROWAVE IMAGES. THE INITIAL
INTENSITY IS SET TO 50 KT...A BLEND OF THE OBJECTIVE AND SUBJECTIVE
DVORAK CLASSIFICATIONS AND AMSU ESTIMATES. A GRADUAL WEAKENING OF
FLOSSIE SHOULD COMMENCE TOMORROW AS IT MOVES OVER COOLER
WATERS...INTO DRIER AIR ALOFT...AND LIGHT TO MODERATE SHEAR. MODEL
GUIDANCE IS IN GOOD AGREEMENT WITH THIS SCENARIO...AND THE OFFICIAL
INTENSITY FORECAST IS BASICALLY AN UPDATE OF THE PREVIOUS ONE. ALL
OF THE GLOBAL MODELS...AND THE OFFICIAL FORECAST...NOW SHOW
FLOSSIE DISSIPATING WEST OF THE HAWAIIAN ISLANDS BY 120H DUE TO
STRONG SOUTHWESTERLY SHEAR.

FLOSSIE CONTINUES MOVING TO THE WEST-NORTHWEST...NOW 285/17. THERE
HAS BEEN NO CHANGE TO THE SYNOPTIC REASONING WITH A FIRM RIDGE TO
THE NORTH OF THE CYCLONE. THE STORM SHOULD GRADUALLY TURN TO THE
WEST IN 24-48H AS THE RIDGE STRENGTHENS SOMEWHAT. THE LATEST
GUIDANCE HAS SHIFTED A LITTLE SOUTH...BUT IS NOW TIGHTLY CLUSTERED
AROUND THE PREVIOUS OFFICIAL FORECAST. THUS...NO SIGNIFICANT
CHANGES HAVE BEEN MADE TO THE NHC TRACK FORECAST.


FORECAST POSITIONS AND MAX WINDS

INIT 26/2100Z 16.5N 134.1W 50 KT 60 MPH
12H 27/0600Z 17.1N 136.7W 50 KT 60 MPH
24H 27/1800Z 17.9N 140.0W 50 KT 60 MPH
36H 28/0600Z 18.6N 143.2W 45 KT 50 MPH
48H 28/1800Z 19.1N 146.3W 40 KT 45 MPH
72H 29/1800Z 19.5N 152.5W 35 KT 40 MPH
96H 30/1800Z 19.5N 158.5W 30 KT 35 MPH
120H 31/1800Z...DISSIPATED

$$
FORECASTER BLAKE
Member Since: July 21, 2011 Posts: 82 Comments: 7056
BULLETIN
TROPICAL STORM FLOSSIE ADVISORY NUMBER 8
NWS NATIONAL HURRICANE CENTER MIAMI FL EP062013
200 PM PDT FRI JUL 26 2013

...FLOSSIE A LITTLE STRONGER...


SUMMARY OF 200 PM PDT...2100 UTC...INFORMATION
----------------------------------------------
LOCATION...16.5N 134.1W
ABOUT 1395 MI...2245 KM E OF HILO HAWAII
MAXIMUM SUSTAINED WINDS...60 MPH...95 KM/H
PRESENT MOVEMENT...WNW OR 285 DEGREES AT 20 MPH...32 KM/H
MINIMUM CENTRAL PRESSURE...999 MB...29.50 INCHES
Member Since: July 21, 2011 Posts: 82 Comments: 7056
Quoting 564. Patrap:

What you are seeing is the CCC convective cloud cover...this is a sign of a storm that has died.There is nothing to see here,move along.
Member Since: July 2, 2013 Posts: 0 Comments: 1256
TROPICAL STORM DORIAN DISCUSSION NUMBER 11
NWS NATIONAL HURRICANE CENTER MIAMI FL AL042013
500 PM AST FRI JUL 26 2013

UNDER THE INFLUENCE OF WESTERLY SHEAR AND DRIER AIR...DORIAN HAS
BECOME EVEN LESS ORGANIZED THAN IT WAS EARLIER TODAY. VISIBLE
SATELLITE IMAGES INDICATE THAT THE LOW-LEVEL CENTER HAS BECOME
DISPLACED TO THE WEST OF AN AREA OF WEAKENING DEEP CONVECTION.
ALSO...ANIMATION OF THE IMAGERY SHOWS THAT CIRCULATION IS BARELY
CLOSED AT THIS TIME. USING A BLEND OF DVORAK ESTIMATES FROM SAB
AND TAFB...THE ADVISORY INTENSITY IS REDUCED TO 40 KT. THE
OFFICIAL WIND SPEED FORECAST NOW SHOWS GRADUAL WEAKENING WITH
DISSIPATION BY DAY 5...IN GENERAL AGREEMENT WITH THE GFDL AND HWRF
GUIDANCE. GIVEN THE CURRENT TRENDS...HOWEVER...THE SYSTEM COULD
DEGENERATE INTO AN OPEN WAVE MUCH SOONER THAN THAT.

THE LATEST CENTER FIXES INDICATE THAT DORIAN IS MOVING RAPIDLY
WESTWARD...SLIGHTLY TO THE SOUTH OF THE PREVIOUS TRACK. A STRONG
LOW- TO MID-LEVEL HIGH PRESSURE SYSTEM TO THE NORTH OF DORIAN IS
FORECAST TO BUILD WESTWARD WITH TIME BY THE GLOBAL MODELS. THIS
SHOULD MAINTAIN A GENERALLY WESTWARD MOTION FOR THE NEXT SEVERAL
DAYS. THE OFFICIAL TRACK FORECAST IS SHIFTED A BIT TO THE SOUTH OF
THE PREVIOUS ONE AND IS ABOUT IN THE MIDDLE OF THE GUIDANCE
ENVELOPE. THE LATEST ECMWF MODEL TRACK IS A LITTLE FARTHER NORTH
BUT THAT MODEL DOES NOT INITIALIZE NOR FORECAST A CLOSED SURFACE
CIRCULATION.



FORECAST POSITIONS AND MAX WINDS

INIT 26/2100Z 17.8N 45.8W 40 KT 45 MPH
12H 27/0600Z 18.2N 48.8W 40 KT 45 MPH
24H 27/1800Z 18.6N 52.8W 35 KT 40 MPH
36H 28/0600Z 19.0N 56.7W 35 KT 40 MPH
48H 28/1800Z 19.5N 60.5W 30 KT 35 MPH
72H 29/1800Z 20.0N 67.0W 30 KT 35 MPH
96H 30/1800Z 20.5N 74.0W 25 KT 30 MPH
120H 31/1800Z...DISSIPATED

$$
FORECASTER PASCH
Member Since: July 21, 2011 Posts: 82 Comments: 7056
Correction, east of SE SC.
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BULLETIN
TROPICAL STORM DORIAN ADVISORY NUMBER 11
NWS NATIONAL HURRICANE CENTER MIAMI FL AL042013
500 PM AST FRI JUL 26 2013

...DORIAN WEAKENS WHILE MOVING RAPIDLY WESTWARD...


SUMMARY OF 500 PM AST...2100 UTC...INFORMATION
----------------------------------------------
LOCATION...17.8N 45.8W
ABOUT 1135 MI...1830 KM E OF THE NORTHERN LEEWARD ISLANDS
MAXIMUM SUSTAINED WINDS...45 MPH...75 KM/H
PRESENT MOVEMENT...W OR 280 DEGREES AT 22 MPH...35 KM/H
MINIMUM CENTRAL PRESSURE...1010 MB...29.83 INCHES
Member Since: July 21, 2011 Posts: 82 Comments: 7056
Quoting 559. VR46L:
Might be wrong ! Looks DOA to me



Two steps from KIA.
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BULLETIN
TROPICAL STORM DORIAN ADVISORY NUMBER 11
NWS NATIONAL HURRICANE CENTER MIAMI FL AL042013
500 PM AST FRI JUL 26 2013

...DORIAN WEAKENS WHILE MOVING RAPIDLY WESTWARD...


SUMMARY OF 500 PM AST...2100 UTC...INFORMATION
----------------------------------------------
LOCATION...17.8N 45.8W
ABOUT 1135 MI...1830 KM E OF THE NORTHERN LEEWARD ISLANDS
MAXIMUM SUSTAINED WINDS...45 MPH...75 KM/H
PRESENT MOVEMENT...W OR 280 DEGREES AT 22 MPH...35 KM/H
MINIMUM CENTRAL PRESSURE...1010 MB...29.83 INCHES


WATCHES AND WARNINGS
--------------------
THERE ARE NO COASTAL WATCHES OR WARNINGS IN EFFECT.

INTERESTS IN THE NORTHERN LEEWARD ISLANDS SHOULD MONITOR THE
PROGRESS OF DORIAN.


DISCUSSION AND 48-HOUR OUTLOOK
------------------------------
AT 500 PM AST...2100 UTC...THE CENTER OF TROPICAL STORM DORIAN WAS
LOCATED NEAR LATITUDE 17.8 NORTH...LONGITUDE 45.8 WEST. DORIAN IS
MOVING TOWARD THE WEST NEAR 22 MPH...35 KM/H...AND THIS GENERAL
MOTION IS EXPECTED TO CONTINUE FOR THE NEXT 48 HOURS.

MAXIMUM SUSTAINED WINDS HAVE DECREASED TO NEAR 45 MPH...75
KM/H...WITH HIGHER GUSTS. SATELLITE IMAGES SHOW THAT DORIAN IS
DISORGANIZED...AND ADDITIONAL WEAKENING IS FORECAST DURING THE NEXT
48 HOURS.

TROPICAL STORM FORCE WINDS EXTEND OUTWARD UP TO 70 MILES...110 KM
FROM THE CENTER.

THE ESTIMATED MINIMUM CENTRAL PRESSURE IS 1010 MB...29.83 INCHES.

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There is a low level circulation east of NE Florida, with convection being somewhat sheared off to the east.
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Quoting 554. Camille33:

Go ahead and believe the crappy gfs and it's associated models.


LOL!!! Well, generally I favor ecmwf. However, it's not performing well this go around, and that's okay. That's why we use multiples and go with a consensus. ;-) It can't always be correct and it's the outlier with this storm, and the GFS suite has been really well performing this time.
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Will they keep a TS at 5 , or open it to a wave.
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what in the flying heck are you talking about? The GFS Was the first one to show a t.d or t.s near the cape verde, and hold it consistently as well, last I checked this isnt going to become a tropical wave, nice try though.
Quoting 556. Camille33:

No the ecmwf has been good,it was the first to show a td near cv and the first to show this beocmming a tropical wave,stop dissing it.Gfs is never gona surpress it.

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559. VR46L
Might be wrong ! Looks DOA to me

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558. MTWX
Quoting 546. stormpetrol:


Yet on the visible there is more LL turning than earlier, I'm confused about Dorian, strange little fellow, have to watch him til the lid is on him!


Almost looks like he decoupled and the surface low is trying to reconsolidate to the SW while the mid level died to the N...
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Quoting 540. Camille33:

No he is not,nice try though.Ecmwf scrapes florida and takes this out to sea.


I doubt that seriously......
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Quoting 551. dfwstormwatch:
the EURO has been trash with this storm, throw it out.


No the ecmwf has been good,it was the first to show a td near cv and the first to show this beocmming a tropical wave,stop dissing it.Gfs is never gona surpress it.
Member Since: July 2, 2013 Posts: 0 Comments: 1256
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 : 194500 UTC
Lat : 17:55:37 N Lon : 44:59:16 W


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


Final T# Adj T# Raw T#
3.0 3.1 3.1

Center Temp : -15.5C Cloud Region Temp : -27.8C

Scene Type : CURVED BAND with 0.58 ARC in DK 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 : 50km
- Environmental MSLP : 1018mb

Satellite Name : MSG3
Satellite Viewing Angle : 54.8 degrees



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Quoting 547. huntsvle:


And is the outlier of the guidance this time around

Go ahead and believe the crappy gfs and it's associated models.
Member Since: July 2, 2013 Posts: 0 Comments: 1256
553. rco3
Quoting 530. CybrTeddy:


Let's just hope you're not wrongasusual.


Oh, no - not anotherwrongyear. I CANTBELIEVEIT. ITSNOTGONNAHAPPEN. WHATNONSENSE. TOTALLYWRONG. CANTTAKEITANYMORE.

It's almost as if there's a pattern there...
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Quoting 530. CybrTeddy:


Let's just hope you're not wrongasusual.
He gave himself away yesterday he said..."My local mets see high shear and dry air for the rest of the season".
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the EURO has been trash with this storm, throw it out.
Quoting 540. Camille33:

No he is not,nice try though.Ecmwf scrapes florida and takes this out to sea.

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Compare Dorian with the area we're interested in.
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Dorian is dead... RIP Dorian
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It will be nice when Dorian passes 50W so we can get a plane in there.

BTW. I know it is a bit late, but thanks Lee Grenci for the blog update.
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Quoting 540. Camille33:

No he is not,nice try though.Ecmwf scrapes florida and takes this out to sea.


And is the outlier of the guidance this time around
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Yet on the visible there is more LL turning than earlier, I'm confused about Dorian, strange little fellow, have to watch him til the lid is on him!
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Quoting 539. stormpetrol:


I acknowledeged that I think Oscat is messing up, see previous post


Gotcha, I see it now! :-)
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Quoting 542. nrtiwlnvragn:
Mess of clouds


maybe wind shear. to
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Tropical Storm Dorian will have winds of 40 mph at 5pm.
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Mess of clouds


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Quoting 529. LargoFl:
he is headed straight for the Gulf..


Good. Means more surf in the Gulf!
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Quoting 529. LargoFl:
he is headed straight for the Gulf..

No he is not,nice try though.Ecmwf scrapes florida and takes this out to sea.
Member Since: July 2, 2013 Posts: 0 Comments: 1256
Quoting 537. ecflweatherfan:


Wait a sec... The first image is ASCAT, the second is OSCAT. Apples to oranges IMO.


I acknowledeged that I think Oscat is messing up, see previous post
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Dry air is definitely a major problem for Dorian.



Member Since: July 21, 2011 Posts: 82 Comments: 7056
Quoting 521. stormpetrol:


earlier



most recent = Dorian now an open wave


Wait a sec... The first image is ASCAT, the second is OSCAT. Apples to oranges IMO.
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Quoting 521. stormpetrol:


earlier



most recent = Dorian now an open wave

yeah at the time this was taken 14Z and if you follow the visible and rgb and watch the time stamp yes indeed Dorin's LLCOC was open but as you loop it through you can see that it then closes back after 14Z infact it was 15:45Z that it closed and it strengthed its LLCOC at 17:15Z
Member Since: June 13, 2009 Posts: 2 Comments: 9549
535. MTWX
Quoting 529. LargoFl:


Don't think the CLPS has it out for Bermuda do ya??
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Quoting 493. ProgressivePulse:


For what it's worth, EFL is typically not under the gun until Aug. Aug 15th - Oct 15th to be precise. Rare for storms to hit the east coast of FL in July.


don't feed the trolls...and yes, he IS a troll, esp after yesterdays blog when he was claiming to be correctly predicting dorians movements while at the same time his handle DID NOT exist! he is lucky #87 on my ban list.

on a weather note, another rainy day here in sw florida. not as bad as last week, but the ground here is already saturated to the max
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Quoting 525. Dakster:


I'm in the tropics and I am depressed. Does that count?

I needed a laugh, thanks for helping me :p
Member Since: February 13, 2012 Posts: 10 Comments: 3262
Quoting 523. ecupirate:
poof there she goes.


Who is the 'she' you are referring to? Flossie?
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last night Oscat was messing up too had the center at 14N
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Quoting 512. buzzardswrath:
one last note.... the dry air and shear will remain high for quite some time, so any storms comming from africa will play the same games as chantal and dorian.... maybe by late september we might get something real


Let's just hope you're not wrongasusual.
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he is headed straight for the Gulf..
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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.