The Straight Talk about Derechos

“The Line Storm,” by John Steuart Curry, 1934

On June 11, the National Weather Service (NWS) warned the Midwest and Eastern states that a derecho was coming. Two years ago, most people wouldn’t have known what they were talking about.

After last year’s 700-mile-long super storm, however, Midwesterners were bracing themselves for the worst. Fortunately, this year’s event was a “low-end” event stretching from Iowa to Maryland.

It left Chicago in the dark, spawned tornadoes and spectacular hail, but it was far less damaging and deadly than last year. What most newspapers didn’t report is that there were three lines of storms at the same time: one in the Midwest and Mid-Atlantic and two roaring through the Southeast. They formed one enormous jumble of crummy weather.

On June 12–13, the US experienced 3 massive lines of storms! Source NOAA

Technically, a derecho is a “widespread, long-lived storm that is associated with a band of rapidly moving showers or thunderstorms.” It gets its name (Spanish for “straight”) from its powerful, straight blasts of wind that can be as damaging as a tornado.

Typically, tornados do more structural damage, twisting buildings off their foundations; derechos just flatten things. They are notorious for leveling huge areas of trees, in giant “blowdowns”. According to the NWS, the wind damage has to be at least 240 miles long with gusts are at least 58 mph, to qualify as a derecho. This June’s largest storm was 400 miles long with at least one area reporting blasts of 100 mph, spawning an estimated 25 tornadoes as well.

The storms form when cooler air crashes into almost stationary heatwaves. The ones in the Great Plains, called “Serial Derechos”, form on the western edge of the heat. They advance east like a giant line dance. Further north, the Midwestern storms are “Progressive Derechos” and stream over the heat wave like a long deadly train. Along much of the Mississippi and Ohio River Valleys, the storms are almost yearly.

Derechos frequently form around heat waves. Source NOAA

Because they are so huge, derechos frequently get rather odd names: “The Ohio Fireworks Derecho” (1969), “The ‘More Trees Down’ Derecho” (1980), “The Storm of the Century Derecho” (1993) and, more ominously “The People Chaser Derecho” (2001). We were lucky that this year we only had a relatively quiet “low-end” derecho.

Did the derecho affect you? Share your stories!