An intense winter storm has battered large sections of the U.S., killing at least 50 people as of Monday morning, according to an NBC News tally.

The deaths were recorded in 12 states: Colorado, Illinois, Kansas, Kentucky, Michigan, Missouri, Nebraska, New York, Ohio, Oklahoma, Tennessee and Wisconsin. 

The extreme weather stretched from the Great Lakes near Canada to the Rio Grande along the border with Mexico. About 60 percent of the U.S. population faced some sort of winter weather advisory or warning, and temperatures plummeted drastically below normal from east of the Rocky Mountains to the Appalachians.

The National Weather Service (NWS) said Sunday the frigid arctic air "enveloping much of the eastern half of the U.S. will be slow to moderate."

Meanwhile, three power substations in East Pierce County operated by Tacoma Public Utilities (TPU) and Puget Sound Energy were attacked and vandalized early Sunday, resulting in power outages affecting thousands of customers. The affected area was centered in Graham.

The attacks were preceded by a warning from federal law enforcement citing a possible threat to the local electrical grid, according to a statement from TPU. The utility did not describe the specific nature of the attack, apart from saying it occurred early Sunday morning. According to the Pierce County Sheriff's office, unknown individuals broke into the two substations and vandalized equipment.

More than 1,700 domestic and international flights had been canceled as of Sunday morning, according to the tracking site FlightAware.

"In some areas, being outdoors could lead to frostbite in minutes," the NWS said in a bulletin, which advised anyone traveling or going outside "to prepare for extreme cold by dressing in layers, covering exposed skin and packing winter safety kits in vehicles."

(With input from agencies)