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Storm spotter training

March 31, 2010

I’ve mentioned weather a few times on this blog. It’s a big topic of conversation out here in the panhandle. My introduction to the Scottsbluff area included storm warnings and time spent in a National Park Service basement. Two requirements when I was shopping for a home here: storm-season shelter of a garage for the car and a basement for me.

So, when the tornado siren test went off last week during Nebraska Severe Weather Awareness Week, I was reminded of a news item on a free storm spotter training that evening, sponsored by the Cheyenne National Weather Service office and Scotts Bluff County Emergency Management. I decided it would be a good idea to attend. (I wasn’t the only one; the Gering Civic Center ballroom was packed.) I learned a few new things, and thought I’d share them with you.

Number-one killer in storms

One thing I didn’t realize is that the majority of storm deaths occur not from tornadoes or high wind or lightning or hail, but from flooding — mostly from people who try to drive through floodwaters. To give an idea of how fast a flash flood can cover a dry road under a dangerous, roiling torrent, the presentation included this video of a February 2007 flash flood in Australia. I found another 2007 video online, this one from Oklahoma, which shows the power of water from a flash flood as it rages over a highway. You get water pounding against asphalt for awhile, and you can get this, which may not be visible under the water. (And you thought winter potholes were bad?) When you’re out driving around, take flash flood warnings seriously. And if you see water over the road, “turn around, don’t drown“.

Anatomy of a severe storm

Here are a couple of cool things to note when watching storms develop, with pictures pulled from the Basic Spotters’ Field Guide (with my added doodles).

Overshooting top

When you see the classic anvil shape of a thunderstorm from afar, look to the flat top. If you see sustained bubblings up (which in a time lapse video from Oklahoma reminds me of watching dry ice bubble in water), that “overshooting top” means there is strong updraft in the storm and hail and tornadoes are possible.

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Precipitation, the rain-free base, wall clouds, and tail clouds

If you’re looking out on the horizon at a thunderstorm (something very easy to do in Nebraska big-sky country), and you can see a curtain of rain, that’s an area of downdraft. And see what looks to be a clear, rain-free area next to that? That area of cloud is called the rain-free base (RFB). There is no visible rain because there are strong updrafts at that location that are pulling the moisture up into the storm. If you’re looking for a wall cloud, the typical cradle of a tornado, look between the downdraft of the precipitation and the updraft of the rain-free base. That’s where the air mixes, and if there is rapid condensation, you get that wall cloud dropping down from the rain-free base. If you see that wall cloud rotating, a tornado may form.

Wall clouds can also be associated with little projections called “tail clouds” that some people may mistake for a tornado. A tail cloud does not rotate, but points out towards the leading edge of the storm and indicates moist air from the precipitation downdraft flowing up into the updraft. Here is a picture of a wall cloud that has a tail cloud (NOT a tornado) projecting to the right:

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Where NOT to stand when a tornadic storm is coming

Here’s a diagram looking down on a tornadic storm. (Gee, it looks an awful lot like the “hook echo” I remember TV weathercasters yammering on about during an evening of intense storms.) See the dark blue in the middle? That’s the wall cloud. Not sure if you can see it, but the tornado in this storm is indicated by a little red triangle. Look at where the green, black, and orange bands of rain are relative to the tornado.

Now, imagine that this storm is moving on a typical northeast track and that you are to the north or northeast of the storm. Will you see the tornado coming? No you will not! There’s all kinds of rain in the way! Don’t go out and watch for it, or you many find yourself on beyond Kansas.

(Oh, yeah . . . and don’t try to shelter under a highway overpass either. Bad, bad idea.)

Dry downburst

A dangerous wind phenomenon more common out here on the high, dry plains is virga and associated dry micro/macro downbursts. Here’s the scenario: you see a cloud dropping rain, but the rain never hits the ground because the air is so dry it evaporates. That’s called virga. What happens when the virga water evaporates? It causes cooling. What happens to that cooler air? It accumulates until it reaches a critical mass, and BLLURRRP! the cold air pocket splashes down onto the ground, creating strong straight-line winds radiating out from the splashdown point.

Shelf cloud

Here is something you do not want to see coming at you when you are on a boat on a lake in Minnesota:

It is called a “shelf cloud”. It’s at the leading edge of a storm and is caused by a wedge of cool air plowing towards you and lifting up the warm, moist air that’s around you, which then condenses into clouds. The risk here is often severe straight-line winds followed by heavy precipitation. Yep. That’s pretty much what we got that day. Glad we made it to shore before that happened! (Incidentally, that shelf cloud was preceded by very clear mammatus clouds.)

For some good pictures and good information about severe storms, see the website of storm chaser and fellow Nebraskan Mike Hollingshead.

Soo . . . hope this helps you skywatchers in big sky country, wherever that big sky may be.

Text and shelf cloud image copyright 2010 by Katie Bradshaw

4 Comments leave one →
  1. March 31, 2010 10:40 pm

    Fantastic research!

  2. April 1, 2010 9:14 am

    Once again…I am left speechless…you are amazing!!

  3. April 1, 2010 9:22 am

    Since I’ve been in this part of the country I’ve learned that weather watching is the number one pastime. When I first moved here I noticed that the locals talked about the wind like they’d never experienced it before: “Is it windy?” “How bout that wind today?” “When do you think the wind will start (stop)?” I thought it was kind of quaint and regional.

    But now, I have to admit, I’m as consumed with the weather as any native! Wind, blizzards, rain (“we sure could use the moisture”) hail & tornados, I’m as obsessed as my neighbors! It’s the sky here. Katie’s right, it’s as big a sky as you’ll see in Montana, and man, you can see that weather coming a long way!

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