(CNN) Sundays are for staying in -- especially this week, as a winter storm sweeps across parts of the US.

Over 20 million people are under winter weather alerts across the northern Plains, Midwest and Northeast.

Heavy snow and possible blizzard conditions will stretch from northern Nebraska to northern Minnesota Sunday, the National Weather Service said. Heavy snow up to a foot in some places has already fallen across the Northern Plains and Upper Midwest over the last 24 hours.

Minnesota is grappling with slippery roads -- so slippery that school buses were sliding off roadways.

The winter storm will center over northern Wisconsin and the Upper Peninsula of Michigan by Monday.

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