Panic in Year Zero!

Harry Baldwin (Ray Milland), his wife Ann (Jean Hagen), their son Rick (Frankie Avalon), and daughter Karen (Mary Mitchell) leave suburban Los Angeles on a camping trip. The Baldwins notice unusually bright light flashes coming from a great distance. Sporadic news reports broadcast on CONELRAD hint at the start of an thermonuclear war, later confirmed when the Baldwins see a large mushroom cloud over what was Los Angeles.