Airlines cancelled 1,500 US flights during the peak holiday travel period, with severe winter storm warnings and heavy snow forecast across parts of the Midwest and northeast.

More than 40 million Americans were under winter storm warnings or weather advisories on Friday, one day after Christmas, plus another 30 million under flood or storm advisories in California, where a so-called atmospheric river has brought a deluge of rain.

New York, the country's largest city, was bracing for up to 25 centimetres of snow overnight — the most in four years.

Temperatures were forecast to drop and remain chilly into the weekend, when an Arctic blast swoops down from Canada.

At least 1,490 flights had been cancelled, with more than 5,900 delays, as of 4:00 pm US Eastern time (2100 GMT) Friday, the FlightAware website said.

The National Weather Service (NWS) predicted snowfall across the upper Great Lakes region would move towards the US northeast.

"Difficult travel conditions are ongoing for the area, and those travelling from the holiday should exercise caution if you have to be on the road," it warned.

New York Mayor Eric Adams said a winter storm warning was in effect and that city crews were deployed to plough roads.

'Misery Map'

Airports in New York and Chicago were near the top of FlightAware's rankings on its "Misery Map," which tracks the spread of flight delays and cancellations. 

New York's three area airports have accounted for 850 flight cancellations, according to the website.

Weather warnings were also issued for the states of Pennsylvania, New Jersey and Connecticut, as well as parts of Michigan, Massachusetts and Rhode Island.

Meanwhile, California, on the US West coast, endured heavy rain this week, but the NWS forecast it will begin drying out over the weekend.

The fierce weather and related flooding forced the Santa Barbara Airport northwest of Los Angeles to close, but by Friday morning, after crews spent all night clearing water from runways, authorities announced it had reopened.