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Stop Talking About Derechos

When a line of thunderstorms plowed through the D.C. area this morning, Bill Burton, co-founder of the Obama-aligned Super PAC “Priorities USA,” tweeted that a derecho was "moving in." He wasn’t alone. Other political tweeters, including the Huffington Post’s Sam Stein and the National Review’s Jonah Goldberg, also expressed their concern about another “derecho,” apparently expected this afternoon. The focus on a “derecho” is misplaced.

The distinction between a “derecho” and a severe thunderstorm isn’t intensity, but longevity. A derecho must possess damaging winds exceeding 58 mph, lasting for 240 miles. The definition of a severe thunderstorm, in contrast, does not suggest a less intense storm: It must have winds of at least 58 mph, or hail of 1 inch in diameter, or a tornado. Last year’s derecho was memorable in the Washington area because of its intensity, not because people in Illinois, Ohio, Indiana, and West Virginia suffered from the same storm. In other words, people in D.C. have falsely associated derecho with “extremely intense,” rather than with “long lasting.”

This morning’s thunderstorms weren’t severe, let alone a derecho. Severe thunderstorms are expected later today, and they might even last long enough to earn the dreaded derecho classification, although that seems unlikely. Severe thunderstorms might hit D.C. later today, but they probably won’t rival the intensity of last year’s derecho, which was fueled by an exceptionally unstable, hot, moist airmass and a powerful jet stream. Today, the air isn’t as unstable as it was one year ago, reducing the risk of widespread damage—regardless of whether the storms last for 100 or 500 miles.