Dangerously cold temperatures settled over a wide swath of the U.S. and Canada on Saturday as a massive winter storm upended Christmas holiday travel and left millions without power or facing rolling blackouts.

Utility companies from Texas to New York City were urging customers to conserve power to protect supply as people turned up their thermostats in temperatures the National Weather Service describes as "life threatening.”

Meanwhile, thousands of flights were canceled or delayed on both Saturday and Sunday, according to FlightAware, and road travel has been treacherous, forcing some to cancel Christmas plans.