A winter storm system with heavy snow and ‘dangerous wind chills’ is sweeping the U.S. 

A potent winter storm threatened blizzard-like conditions, treacherous travel, and power outages in parts of the Upper Midwest as other areas of the country braced Monday for plunging temperatures, strong winds, and a mix of snow, ice, and rain.

The snow and strengthening winds began spreading Sunday across the northern Plains, where the National Weather Service warned of whiteout conditions and possible blizzard conditions that could make travel impossible in some areas. Snowfall totals...