The threat of tornadoes is moving east into the Mississippi Valley and Deep South a day after a massive storm system moving across the country unleashed winds that damaged buildings, whipped up dust storms that caused deadly crashes and fanned more than 100 wildfires in several central states.

A line of severe thunderstorms and tornadoes that swept across the north-central U.S. last week has been classified by the National Weather Service as a serial derecho — the first on record December.