Identifying rotation

[Cris Schroeder]

I've been on the fence about posting this, but it has been bothering me and I would like to get some other opinions.

We had some severe weather in the area on June 12 and I looked out the back window to see what appeared to be a nice little supercell with a wall cloud structure on the northern end of the updraft base. By the time I got my camera set up in the back yard, there appeared to be a small funnel present. It only lasted for a short time and the storm rotation did not appear well organized. I shot some time-lapse video of different parts of the storm over the next few minutes as I was having trouble identifying any organized rotation.

After reviewing the video later, however, I've been wondering if I should have called in a report to the NWS. I did not visually observe any rapid and/or persistent rotation but the video makes me wonder if I missed something.

The video clips are available here. Clicking on the video stills will open Quicktime files, and the full-res MPG files are hyperlinked. The files are fairly big but I would like to get some feedback from other chasers/spotters on whether I missed something important.

FYI, the same storm was tornado-warned about 20 minutes later as it moved off to the east, based on a report of a rapidly rotating wall cloud.

Any feedback would be appreciated.

On Edit: I posted this in the new user section as identifying rotation seems to be a recurring theme. If it is more appropriate in another area, please move it.

 

[Shane Adams]

Rotation is easy enough to discern if you're close to it. However from a distance it can be tricky, even to seasoned observers. I wouldn't kick myself for not calling anything in; I didn't watch the videos but from the still images you didn't seem to be up close. I rarely call anything in before a tornado is down anyway, as I've seen way too many "almost tornadoes" that never happened.

 

[Cris Schroeder]

Thanks for the feedback. I certainly didn't have an ominous feeling watching the storm. Maybe it is just my perception but it seems that the number of overly conservative tornado warnings are up this year.

By the way, Shane, thanks for taking the time to weigh in on these new user threads.

 

[Tim Shriver]

As many say no report is better then a bad report.

It is important to call in true funnels or rotating wall clouds as
the NWS needs to know what the atmosphere is doing. Just because
something does not fully form into a tornado where you are
doesn't mean it wouldnt somewhere else or where your not looking.

A good rule of thumb is to count to 30 for a funnel and 60 for
a rotating wall cloud. If they continue to rotate for that
period of time, report it.

Tim

 

[Tim Shriver]

While on the subject, many times during a t-storm we will hear
about funnels being reported at the leading edge of a cell or line
of cells. We doubt these reports as most funnels form
at the base of a rotating wall cloud which is almost always
at the rear of a cell or line of t-storms.

This video is of vertical rotation at the leading edge of a line of cells.

http://www.youtube.com/watch?v=VfF3tfYhULg

This one is also at the same leading edge as the above video
but is horizontal rotation.

http://www.youtube.com/watch?v=UA3P7xm3Qqw

Gustandoes will form out of these from time to time.

Please forgive the quality as I am not a pro at filming..

Tim

 

[Jeff Richardson]

Often at the leading edge of convection you might observe a great deal of "mixing", i.e. what appears to be rotation that some people might think will lead to a tornado. More often than not, it's just harmless turbulence. Still, they can be fascinating to watch. The atmosphere is a fluid and when two fluids come together, the swirling motions are a natural result. In all the years I've chased, I've yet to see any of these harmless swirls associated with the deep sustained rotation of a mesocyclone. Just my two cents.

 

[Eric Flescher]

hook/ rain signature

<tt>Robin posted this interesting piece of infomration on wxchase. I thoght it was woul;d be a good read and tie in with this thread. Hope it is okay to post here on her behalf.

Date: Fri, 22 Aug 2008 12:49:37 -0500
From: Robin Storm <[email protected]>
Subject: Heavy Rain Triggers Destructive Tornadoes

*Heavy Rain Triggers Destructive
Tornadoes<http://dsc.discovery.com/news/2008/08/22/tornado-rain.html>
*Michael Reilly, Discovery News

*Aug. 22, 2008* -- When the spring skies blacken over the Midwestern United
States and the clouds begin to rotate ominously, residents know a
tornado<http://science.howstuffworks.com/tornado.htm>could be on its
way.

But just how the deadly twisters form is a mystery that has puzzled
scientists for decades. They know a range of atmospheric conditions must be
in place, including strong, closely paired up- and downdrafts, windshear at
high altitudes, and usually a foreboding supercell thunderstorm, spinning
slowly in the sky.

Amid this stormy cocktail, new research suggests that the heavy rains that
often accompany supercells may be key in triggering
tornadoes<http://blogs.discovery.com/news_earth/2008/01/tornadoes-globa.html>to
form. According to Robert Davies-Jones of the National
Severe Storms Laboratory <http://www.nssl.noaa.gov/> in Oklahoma, rain
falling in heavy sheets from supercell storm cloud provides a strong push to
updrafting air, causing it to spin up into a fully fledged twister.

"You can have a rotating storm but until you have rain you don't get a
tornado," Robert Davies-Jones of the National Severe Storm Laboratory said.

Scientists have known this simple fact since 1953, when radar first allowed
meteorologists to peer into supercells. In tornadic storms, they noticed the
rain swirled into a hook shape. The feature, called a 'hook echo' quickly
became known as a tell-tale radar marker for a tornado.

But the swirling rain was regarded as a by-product of a tornado, merely the
effect of its powerful corkscrewing winds. Davies-Jones believes the
opposite may be true.

"The hook echo is usually thought to be a passive feature of tornadoes," he
said. "I'm saying it's not passive, it's an active mechanism for tornado
formation."

His work is published in the August issue of *Journal of the Atmospheric
Sciences*.

Davies-Jones ran computer simulations of supercell storms to see if falling
rain could provide the needed kick that turned diffuse updrafts rising off
the warm plains into tight-spinning, lethal tornadoes. As the rain
falls<http://science.howstuffworks.com/question479.htm>out of a
rotating supercell cloud it is also twisting, and as it falls he
found it transfers the rotational energy into the updrafting air adjacent to
it.

The rain also acts as a sort of wall, confining the swirling, rising air. As
it continues to head skyward the air inside the rain curtain stretches out
like a figure skater raising her arms. The spinning speeds up, and a tornado
is born.

"The mechanism is a good one," David Lewellen of the University of West
Virginia said. "But until these things are seen more conclusively out in the
field, it's not at all clear whether rain is involved in the formation of
most tornadoes, a few, or none at all."

Lewellen points out that rain is only one of myriad weather conditions that
have to be just right for a tornado to form. A massive field campaign of
experiments is scheduled for the spring season in 2009 and 2010 that he
hopes will determine if the implications in the models are correct.




--
May fair winds be with you always,
Rob Stormer, Ch.Eng, Master Salvor
Chicago, Illinois
ROBIN STORM
http://robinstorm.blogspot.com

Severe Storm Reporter
AM 1420 WIMS Michigan City, Indiana

NASCAS Member
http://www.chasingstorms.com/

Messing About In Ships
http://messingaboutinships.com/</tt>

 

[T Melton]

I agree with the earlier statement about distance making a determination of rotation more difficult,I also agree with another statement made about time,I have often had to just take time and observe.In this paticular case I do not feel it is suitible to err on the side of caution and make a report without certainty. I find it much easier in states like Kansas where I can also observe the ground below said rotation and determine if anything is starting to occur. Being able to position yourself and being somewhat mobile helps also.