CHICAGO (AFP) - A massive winter storm pounding the northern United States grounded over 1,100 flights, closed hundreds of schools and made roadways impassible on Tuesday.

More than a dozen states from Minnesota to Virginia were in the path of the huge storm which had already dumped as much as 60 centimetres of snow in Montana and 38 centimetres in North Dakota.

The heavily populated Chicago area was expected to get as much as 2.5 centimetres of snow an hour during the evening rush, the National Weather Service said.

Hundreds of plows were working the Windy City's roads and freeways but with up to 25 centimetres of snow expected, there was no way they could keep up.

"Significant amounts of snow are forecast that will make travel dangerous," the weather service warned. "Consider only travelling if in an emergency."

Nearly 800 flights were grounded at Chicago's O'Hare airport - a major hub - while another 240 were cancelled at Chicago Midway. Over 100 flights were cancelled in Minneapolis, Minnesota, according to FlightAware.

The storm was expected to hit the nation's capitol late Tuesday or early Wednesday, and some Congressional meetings were already being cancelled in Washington.