Where and how often do derechos occur?

Dear Tom,
I’m wondering where and how often derechos occur.
Margaret Robertson Chicago
Dear Margaret,
Severe thunderstorms can occur anywhere, given the ingredientsof warmth, moisture, instability, converging low-level winds and diverging winds aloft. The Midwestis prone to derechos, a high wind event in which winds,often in excess of 100 mph, plow out of a line of fast-moving thunderstorms, producing swaths of damage that extend for hundreds or even thousands of miles.The highest incidence is from midsummer through early fall. Chicago area derechos tend to occur in years when intense heat lies just to the south of the city, and seldom or not at all, when Chicago lies within the hot air dome or when the dome of heat is distant. Derecho, a Spanish word meaning straight ahead, was coined by Iowa meteorologist Gustavus Hinrichs in the 1880s.

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