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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Quoting 616. wunderkidcayman:

yeh plenty of times


lol


the magic will soon happen
only magic is a disappearing act take one
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627. JLPR2
Quoting 616. wunderkidcayman:

the magic will soon happen


Tonight is his last chance if he doesn't recover he will open up. A llc can only last so long closed without convection.
Member Since: September 4, 2007 Posts: 7 Comments: 8499
Quoting 619. nrtiwlnvragn:


Islands, Bahamas, Turks & Caicos...........
im talking about the states
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Byyyye Dorian.You were a waste of a good name.I'll be waiting on what Karen will do.
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Quoting 610. wunderkidcayman:
Dorian is about to hit the peak of D-Min if he survive then convection could build back up and Dorian could restrengthen
Dont you have a heart Kid? We dont want him to suffer. Lets not be selfish and greedy. There we be plenty of other TCs in the sea...Lol.. sarcasm flag way up!
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Quoting 612. Doppler22:

I'm not very familiar with average European weather, does Germany usually get quite hot or does it stay cool?


Yes, Germany can get that hot. It's not so unusual, but new records may be broken in the East of our country on Sunday.
Worst heatwave occurred in 2003. I'll never forget this and I really hope it won't happen again. Now at least we are getting some thunderstorms and the atmosphere is unstable. Back then it was rock solid for many weeks without any cloud near the main land of Western Europe.
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Quoting 610. wunderkidcayman:
Dorian is about to hit the peak of D-Min if he survive then convection could build back up and Dorian could restrengthen


WKC ya in a few hrs it will spin up to a c5 and beeline right for your front door


yeah right
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folks in Texas-Oklahoma etc pay attention to your local warnings...................here's one for Texas...........SEVERE THUNDERSTORM WATCH OUTLINE UPDATE FOR WS 447
NWS STORM PREDICTION CENTER NORMAN OK
340 PM CDT FRI JUL 26 2013

SEVERE THUNDERSTORM WATCH 447 IS IN EFFECT UNTIL 1100 PM CDT
FOR THE FOLLOWING LOCATIONS

TXC037-063-067-085-113-119-147-159-181-183-203-21 3-223-231-257-
277-315-343-365-379-387-397-401-423-439-449-459-4 67-499-
270400-
/O.NEW.KWNS.SV.A.0447.130726T2040Z-130727T0400Z/

TX
. TEXAS COUNTIES INCLUDED ARE

BOWIE CAMP CASS
COLLIN DALLAS DELTA
FANNIN FRANKLIN GRAYSON
GREGG HARRISON HENDERSON
HOPKINS HUNT KAUFMAN
LAMAR MARION MORRIS
PANOLA RAINS RED RIVER
ROCKWALL RUSK SMITH
TARRANT TITUS UPSHUR
VAN ZANDT WOOD
$$


ATTN...WFO...SHV...FWD...OUN...
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Quoting 611. weatherlover94:


Not out of the question this may live then.

Andrew and TD 10 in 2005 (helped form Katrina) struggled some as well. (Dorian wont get that strong tho)
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Quoting 603. LargoFl:
well so far it looks like the people living in south florida can breathe a sigh of relief..remember folks it was they who might have taken the first Hit from dorian...


Islands, Bahamas, Turks & Caicos...........
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Quoting 600. Civicane49:
Flossie feeding off the ITCZ from the south.



Doing what Dorian was doing up until last night...
Member Since: March 19, 2010 Posts: 0 Comments: 1147
this was the CMC on the 12z run from yesterday..

Low off Florida





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Quoting 601. weatherlover94:
Has there ever been a storm that is forecast to die and then comes back to life before it dies ?

yeh plenty of times

Quoting 608. SLU:
CaribBoyyyyyyyyy

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

lol

Quoting 609. JLPR2:
Either Dorian does a magic trick or it gets officially classified as a remnant low at 11pm.


the magic will soon happen
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Quoting 611. weatherlover94:


Not out of the question this may live then.


It's not particularly likely that a storm will just randomly blow up, but it happens every year at least once, so ignoring it as a possibility, no matter how remote, is just setting yourself up for a serving of crow.
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Quoting 603. LargoFl:
well so far it looks like the people living in south florida can breathe a sigh of relief..remember folks it was they who might have taken the first Hit from dorian...


Would assume you are talking about first U.S. hit as it is pretty much impossible for SoFlo to take the first hit from a storm originating in the Atlantic Ocean since it is surrounded by islands to its south and east that would take the first hit.
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Quoting 605. barbamz:
I see, not only Germany is quite a hot place right now. With that I'm diving into a sweltering tropical night in Germany and wish you and poor Dorian all the best!

It's 90 Degrees in Siberia and People Are Sunbathing

Your mental image of Siberia is probably a snowy, wind-whipped expanse, perhaps with a cluster of buildings to house those banished from Russian society. Not this week. This week, Norilsk, the northernmost large city in the world, the second largest city north of the Arctic Circle, and the site of one of those gulags, hit a balmy 32 degrees Celsius — about 90 Fahrenheit. It's normally in the mid-60s.

The online outlet The Siberian Times ("up-to-date information in English from across Siberia's six time zones") featured a photo of people sunbathing on the shores of Lake Baikal in its report on what may be a new record high.


More see link above with reference to the last post of our WU weather historian

I'm not very familiar with average European weather, does Germany usually get quite hot or does it stay cool?
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Quoting 607. yqt1001:


It happens quite a bit. Biggest example would be major hurricane Michael from last year. Was expected to peak at a minimal tropical storm and die within a day or two, but instead became a major hurricane.


Not out of the question this may live then.
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Dorian is about to hit the peak of D-Min if he survive then convection could build back up and Dorian could restrengthen
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609. JLPR2
Either Dorian does a magic trick or it gets officially classified as a remnant low at 11pm.

Member Since: September 4, 2007 Posts: 7 Comments: 8499
608. SLU
CaribBoyyyyyyyyy

INTERESTS IN THE NORTHERN LEEWARD ISLANDS SHOULD MONITOR THE
PROGRESS OF DORIAN.
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Quoting 601. weatherlover94:
Has there ever been a storm that is forecast to die and then comes back to life before it dies ?


It happens quite a bit. Biggest example would be major hurricane Michael from last year. Was expected to peak at a minimal tropical storm and die within a day or two, but instead became a major hurricane.

Another big example is major hurricane Ophelia from 2011.
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low has formed east of Florida

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I see, not only Germany is quite a hot place right now. With that I'm diving into a sweltering tropical night in Germany and wish you and poor Dorian all the best!

It's 90 Degrees in Siberia and People Are Sunbathing

Your mental image of Siberia is probably a snowy, wind-whipped expanse, perhaps with a cluster of buildings to house those banished from Russian society. Not this week. This week, Norilsk, the northernmost large city in the world, the second largest city north of the Arctic Circle, and the site of one of those gulags, hit a balmy 32 degrees Celsius — about 90 Fahrenheit. It's normally in the mid-60s.

The online outlet The Siberian Times ("up-to-date information in English from across Siberia's six time zones") featured a photo of people sunbathing on the shores of Lake Baikal in its report on what may be a new record high.


More see link above with reference to the last post of our WU weather historian
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Dorian its ok...you fought hard, and you can leave with your self respect...Im throwing the towel in buddy...ding ding.
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well so far it looks like the people living in south florida can breathe a sigh of relief..remember folks it was they who might have taken the first Hit from dorian...
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Has there ever been a storm that is forecast to die and then comes back to life before it dies ?
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Flossie feeding off the ITCZ from the south.

Member Since: July 21, 2011 Posts: 83 Comments: 7167
Quoting 555. Patrap:
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



What does that mean? Dorian is dead for a day and has a new life on the 27th?
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oh well..Maybe the GFS had it right all along..we'll see....................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
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Quoting 587. 62901IL:
I see dorian is on life support. Let's see if he can make it to more favorable conditions.


I got a feeling it may regenerate but im not holding my breath.
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Dorian...weve lost the pulse the patient has died.
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Quoting 581. Civicane49:

So, in a day or two watches may be needed for the big island of Hawaii
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Link


a lot of models calling for it to weaken to a TD then coming back to life....it's not a straight line down hill...but it may come to that....who knows.
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Closed low level low can be seen on visible satellite imagery east of Charleston, SC. Could become a tropical storm and impact SNE this Sunday.
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TROPICAL STORM DORIAN FORECAST/ADVISORY NUMBER 11
NWS NATIONAL HURRICANE CENTER MIAMI FL AL042013
2100 UTC FRI JUL 26 2013

THERE ARE NO COASTAL WATCHES OR WARNINGS IN EFFECT.

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

TROPICAL STORM CENTER LOCATED NEAR 17.8N 45.8W AT 26/2100Z
POSITION ACCURATE WITHIN 30 NM

PRESENT MOVEMENT TOWARD THE WEST OR 280 DEGREES AT 19 KT

ESTIMATED MINIMUM CENTRAL PRESSURE 1010 MB
MAX SUSTAINED WINDS 40 KT WITH GUSTS TO 50 KT.
34 KT....... 60NE 30SE 0SW 60NW.
12 FT SEAS.. 60NE 30SE 0SW 60NW.
WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL
MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT.

REPEAT...CENTER LOCATED NEAR 17.8N 45.8W AT 26/2100Z
AT 26/1800Z CENTER WAS LOCATED NEAR 17.6N 44.8W

FORECAST VALID 27/0600Z 18.2N 48.8W
MAX WIND 40 KT...GUSTS 50 KT.
34 KT... 60NE 40SE 0SW 60NW.

FORECAST VALID 27/1800Z 18.6N 52.8W
MAX WIND 35 KT...GUSTS 45 KT.
34 KT... 60NE 40SE 0SW 60NW.

FORECAST VALID 28/0600Z 19.0N 56.7W
MAX WIND 35 KT...GUSTS 45 KT.
34 KT... 50NE 40SE 0SW 50NW.

FORECAST VALID 28/1800Z 19.5N 60.5W
MAX WIND 30 KT...GUSTS 40 KT.

FORECAST VALID 29/1800Z 20.0N 67.0W
MAX WIND 30 KT...GUSTS 40 KT.
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Thanks Lee for the steering lesson.
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The supercomputers are each 213 teraflop systems, running a Linux operating system on Intel processors.

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sleep now you look tucker out

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I see dorian is on life support. Let's see if he can make it to more favorable conditions.
I'll say this,tho. He may be crippled but he's not dead.
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NOAA goes 'live' with new weather supercomputers (7/25/2013)

The National Oceanic and Atmospheric Administration (NOAA) Thursday switched on two new supercomputers that are expected to improve weather forecasting.

The "go live" switch over to the new systems was made today without any fanfare, just a box of donuts on hand to mark 18 months of preparation and testing. The new IBM systems are now responsible for producing forecast data that's relied on in the U.S. and around the world.

The agency had planned to go live next Tuesday, but strong storms now in the Pacific and Atlantic Oceans prompted the decision to move the date up, said Ben Kyger, director of central operations at the National Centers for Environmental Prediction.

NOAA's National Weather Service (NWS) runs the two identical, or clone systems, one in Reston, Va., and in Orlando, Fla. They can switch over in about six minutes.

The supercomputers are each 213 teraflop systems, running a Linux operating system on Intel processors.
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Quoting 579. will40:


NHC doesnt agree with ya


True. But it is dying
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well IF he reaches the gulf as a depression..conditions there are highly favorable for redevelopment...we have 5 days to wait lol
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and Dorian lives like I expected
Dorian is moving W
cone shifted S
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Quoting 542. nrtiwlnvragn:
Mess of clouds


nothing left going sleeper mode


we did not need it anyway too early maybe in mid august it will be better
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Member Since: July 21, 2011 Posts: 83 Comments: 7167
Maybe Erin will be our first Hurricane.
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Quoting 573. weatherlover94:
I pronounce Dorian dead at 4:35 pm


NHC doesnt agree with ya
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Next !!
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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.