WeatherWhys for July 27,2016 from accuweather.com

A derecho is a widespread, long-lived wind storm that accompanies rapidly moving showers or thunderstorms. The swath of straight-line wind damage must be more than 250 miles with gusts of at least 58 mph along most of its path, with several well-separated 75 mph or higher gusts to be classified as a derecho. The majority of derechos occur from May to July.