On April 3, 1974, a tornado, a half mile wide, slammed into Xenia, Ohio. Railroad cars of a freight train passing through town were thrown across streets. The tornado smashed homes and businesses. It also demolished Central High School, just an hour after most students had left for the day. A teacher and students rehearsing a play in the school's auditorium escaped just seconds before the tornado threw two school buses through the walls and onto the stage.

The devastation was unbelievable, but the loss of life could have been much worse. For on April 2, 1974, National Weather Service forecasters studying weather patterns across the United States had concluded that the next day would bring potentially dangerous weather to parts of the nation. A low pressure system was forming east of the Rocky Mountains; computer forecasts predicted it would strengthen as it moved east, pushing...