'Epic' winter storm wallops US, leaving 1 million without power

CHICAGO - More than a million US power customers were in the dark as a "bomb cyclone" winter storm walloped the country, closing highways, grounding flights and causing misery for Christmas travellers.

Heavy snow, howling winds and air so frigid it instantly turned boiling water into ice took hold of much of the nation, including normally temperate southern states.

Over 200 million Americans were under weather warnings, as wind chills sent temperatures down as low as -48 Celsius, according to the National Weather Service (NWS).

The biting cold is an immediate concern for hundreds of thousands of electricity customers who were without power, according to tracker poweroutage.us.

Transportation departments in North and South Dakota, Oklahoma, Iowa and elsewhere reported near-zero visibility whiteouts, ice-covered roads and blizzard conditions, and strongly urged residents to stay home.

Around 5,000 US flights were cancelled Friday and another 7,600 delayed, according to flight tracking website FlightAware, many at international hubs in New York, Seattle and Chicago's O'Hare.

By Friday afternoon, the storm had acquired the status of "bomb cyclone" after air pressure dropped precipitously over 24 hours.

Bomb cyclones produce heavy rain or snow. They can also cause flooding at coasts, and generate hurricane-force wind.

Meteorologist Kelsey McEwen in Toronto tweeted that waves of up to eight metres have been reported in Lake Erie, while in Ohio's Fairport Harbor, winds gusted to 120km/h, the NWS tweeted.