Flight cancellations mounted Sunday as the U.S. was hit with dangerous winter weather from a storm that moved across the country.

More than 10,000 flights canceled as bitter cold blankets the U.S. As a dangerously cold weather front plunges into the U.S., tens of millions across much of the country can expect heavy snow, sleet and freezing rain.

The 29 January midair collision involving a PSA Airlines MHIRJ CRJ700 appears to have happened when the regional jet was flying at less than 1,000ft over the Potomac River. Data from flight-tracking website FlightAware.com shows the flight ending a few ...