Severe thunderstorm warning

Let’s say that you are listening to your favorite program when, all of a sudden, it is interrupted to advise you that your area is under a severe thunderstorm warning. While it’s great to be able to receive the warning, I have found that quite a few folks are not familiar with what it exactly means. Turns out a severe thunderstorm warning has nothing to do with lightning (ALL thunderstorms produce lightning.). It does not have to do with flash flooding (if there was flash flooding expected or occurring, a flash flood warning would be issued).

So if we know what a severe thunderstorm warning isn’t, well then, what is it?

A severe thunderstorm warning means that the area being warned should expect winds of 50 knots (58 mph) or stronger, and/or hail one inch in diameter or larger. A U.S. quarter is exactly one inch in diameter.

These thresholds were determined because that is where significant storm damage typically begins to occur — for instance, trees felled from the wind, or vehicles damaged from the hail.

In the warning, you will be informed which of these hazards you should expect, and sometimes both hazards will be warned for. In addition, a severe thunderstorm can occasionally produce a tornado. This, too, will be included in the warning with words to the effect of “this storm is showing signs of rotation and may produce a tornado”.

If a tornado develops, a tornado warning will be issued. Tornado warnings are issued based on either Doppler radar indications, or a spotter seeing a tornado. The warning will specify whether it is doppler radar-indicated, or has been seen by a spotter. To be called a tornado, it must be making contact with the ground. If you ever see a funnel cloud, look carefully at the ground beneath it and if you see debris being raised, even if you do not see the funnel making contact with the ground, you would still report a tornado.

However, please remember that your chances in our part of the U.S. of being killed or injured by a tornado are FAR LESS than your chances of being killed or injured by driving across a flooded road or being struck by lightning.

Fortunately, there are precautions you can take for both of those. NEVER drive across flooded roads, since the roadway may no longer be beneath the floodwaters and also, given that water weighs about 1700 pounds per cubic yard, when it is flowing, it has tremendous force, and it only needs to be about halfway up your tires to sweep you off the road. With regard to lightning, as soon as you hear thunder, seek safe shelter. Lightning can strike up to 12 miles away from the parent thunderstorm.

Finally, in terms of visual clues, a trend toward puffy cumulus clouds getting significantly taller should be watched closely, as it may signal that the atmosphere is “set up” for thunderstorms. This trend is especially relevant on a warm and humid day, although you don’t need a warm and humid day for a thunderstorm, so the baseline clue is a trend toward cumulus clouds getting significantly taller. Note: this does not always reveal itself clearly, as other cloud layers may interfere with your view, but in general, it is a very handy visual clue to keep an eye on.

Ted Shapiro holds the Broadcast Seal of Approval from both the American Meteorological Society and the National Weather Association. An Alexandria, Va. native, he has been chief meteorologist at WAGM-TV since 2006. Email him at tshapiro@wagmtv.com.  

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