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 were expected to exceed a foot (30 centimeters) across parts of the upper Great Lakes and as much as double that along the south shore of Lake Superior.

“Part of the storm system is getting heavy snow, other parts of the storm along the cold front are getting higher winds and much colder temperatures as the front passes,” said Bob Oravec, a lead forecaster at the National Weather Service office in College Park, Maryland. “They’re all related to each other — different parts of the country will be receiving different effects from this storm.”

The weather service warned of “dangerous wind chills” as low as minus 30 degrees Fahrenheit (minus 34.4 degrees Celsius) in North Dakota and into Minnesota from Sunday night into Monday.

In the South, meteorologists warned severe thunderstorms are likely to signal the arrival of a sharp cold front — bringing a sudden drop in temperatures and strong north winds that will abruptly end days of record warmth throughout that region.

The high temperature in Atlanta was around 72 F (22 C) on Sunday, continuing a warming trend after climbing to 78 F (about 26 C) to shatter the city’s record high temperature for Christmas Eve, the National Weather Service said. Numerous other record high temperatures were seen across the South and Midwest on the days after Christmas.

But the incoming cold front was expected to drop rain on much of the South late Sunday night into Monday, and a big drop in temperatures Tuesday. Forecasters said the low temperature in Atlanta to 25 F (minus 3.9 C) by early Tuesday morning. The colder temperatures in the South are expected to persist through New Year’s Day.

In Dallas, Sunday temperatures in the lower 80s (upper 20s C) could drop down to the mid 40s (single digits Celsius). In Little Rock, high temperatures of around 70 (21 C) on Sunday could drop down to highs in the mid-30s on Monday.

“We’re definitely going back towards a more winter pattern,” Oravec said.

The storm is expected to intensify as it moves east, drawing energy from a sharp clash between frigid air plunging south from Canada and unusually warm air that has lingered across the southern United States, according to the National Weather Service.

—Leah Willingham and Jeff Martin, Associated Press