Along the East Coast, residents are waking up to the coldest morning of the winter so far.
The wind chill -- or what temperature it feels like -- plunged to 16 degrees in New York City, 7 in Boston, 17 in Philadelphia and 24 in Raleigh, North Carolina.
The Northeast will stay cold through Tuesday morning, with temperatures 5 to 10 degrees below normal, before the region warms up for the rest of the week.
Some models predict Christmas Day temperatures reaching 52 degrees in Chicago, 60 in Washington, D.C., and 51 in New York City.
Meanwhile, the West Coast is bracing for a series of storms. Up to 10 feet of snow is expected in the Sierra Nevada mountains while flooding will threaten a large swath of California, from the Bay Area to Los Angeles.
The heavy rain is expected Tuesday through Thursday, which could cause flash flooding, mudslides and debris flow, especially in the wildfire burn scar areas.
By Christmas Day, the Sierra Nevada mountains will have 5 to 10 feet of fresh snow.