A powerful winter storm swept across Southern California on Wednesday, with heavy rain threatening mudslides and debris flows in areas scorched by wildfires, bringing near white-out snow conditions in the mountains and hazardous travel for millions of holiday drivers.

Gov. Gavin Newsom declared a state of emergency as forecasts warn of flash flooding, mudslides and debris flows in areas scorched by last January's wildfires.

Officials in Los Angeles and surrounding counties warned that a "very dangerous" holiday storm could trigger "widespread flash flooding."

Intense downpours that could trigger dangerous flood threats in California are putting a damper on Christmas Eve travel plans, while the New England area braces for heavy snow later in the week.