A monster U.S. winter storm pounded the Northern Plains and Upper Midwest on Thursday, killing a firefighter, knocking out power to more than 9,00,000 people and canceling or delaying thousands of flights.

A broad swath of the northern United States from Washington state to New England remained under winter weather advisories with another 18 inches (46 cm) of snow, winds up to 50 miles per hour (80 kilometers per hour) and wind chills equivalent to minus 40 degrees Fahrenheit (minus 40 Celsius) possible through the day, the National Weather Service said.