Gust front “shelf cloud” (or “arcus”) on the leading edge of a derecho-producing convective system. The photo was taken on the evening of July 10, 2008 in Hampshire, Illinois as the storm neared the Chicago metropolitan area. The derecho had formed around noon in southern Minnesota. (Courtesy of Brittney Misialek via the SPC website)

Many people learned the meteorological term, “derecho” for the first time after a large bowing line of thunderstorms did massive straight-line wind damage in the Fort Wayne area and elsewhere on June 29 and July 1, 2012. Since then, some people equate the term with “really, really bad thunderstorm.” These folks want to know if there will be a derecho tonight, because they think that only a derecho would produce the kind of thunderstorm damage they saw last June. They’re wrong.

According to the National Weather Service Storm Prediction Center website, a derecho (pronounced similar to “deh-REY-cho” in English) is a “widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms.” By definition, if the wind damage swath extends more than 240 miles and includes wind gusts of at least 58 mph (93 km/h) or greater along most of its length, then the event may be classified as a derecho.

What some people probably don’t realize is that you don’t need a derecho to get wind gusts of 58 mph or greater and the same kind of damage we saw last June. A bowing line of strong thunderstorms can do such damage, even if the line is less than 240 miles long and therefore not technically a derecho.

So if you know someone who is worried about whether there will be a derecho, tell them to forget it. Also tell them to pay attention to any thunderstorm warnings that come out, because any storm that generates such a warning can cause the kind of damage we saw last June.

Last June’s storm demonstrated that people don’t pay enough attention to thunderstorm warnings. They don’t take action unless a tornado warning comes out. That’s foolish, and last June’s storms proved it.

Derecho or no derecho, we could have some really, really bad thunderstorms tonight. Make sure you’re able to receive any tornado or thunderstorm warning that comes out, regardless of the time of day, and don’t ignore any thunderstorm warnings, just because they aren’t tornado warnings!