A massive winter storm sweeping across the U.S. has caused at least three fatalities, left more than a million utility customers without power and impacted millions of Americans traveling for the holidays. Bitter cold temperatures are being recorded across the midwest, great plains and south—hitting -16 degrees in Montana and -4 degrees in Colorado— with six inches of snow already having fallen around the Great Lakes. Airports in Chicago and Denver are seeing severe delays, and Seattle airport closed and reopened after a runway had to be de-iced. As of Friday morning, more than 3,600 flights were canceled nationally.