A ferocious windstorm known as a derecho roared across the Upper Midwest on Monday, flattening cornfields, destroying homes and delaying flights at busy Chicago airports. The derecho produced a wind gust of 122 mph early Monday in Hays, S.D., the Weather Channel said. A derecho is a line of strong winds and severe thunderstorms that can last for hours. The system of thunderstorms produced high winds and damage for at least 300 miles, beginning over part of South Dakota late Sunday and early Monday, AccuWeather said.