A massive winter storm set the stage for a brutal travel day Sunday, with airlines warning of widespread cancellations and delays at some of the nation's busiest airports. Widespread snow, sleet and freezing rain threatened nearly 180 million people — more than half the U.

More than 9,000 flights across the U.S. set to take off over the weekend have been canceled as a major storm expected to wreak havoc across much of the country threatens to knock out power for days an...