An arctic air mass moved across the upper Midwest on Sunday, freezing water pipes in Minnesota and leading Chicago officials to ask residents to check on neighbors.

Due to El Nino, winter got off to a mild start. But the blast of dangerous cold moved east across the Northern Plains and Great Lakes over the weekend.

Authorities in Milwaukee said a 21-year-old woman probably died of exposure to the extreme cold. She had reportedly left a house party and was found outside by a passer-by; she was pronounced dead just before noon Sunday.

Temperatures bottomed out at 36 degrees below in Fosston in northwest Minnesota. More than 800 miles to the southeast, wind chills in Indianapolis were expected to plummet to 20 degrees below zero.

Parts of Illinois were in the single digits. The National Weather Service warned wind chill numbers could drop to near 30 degrees below zero overnight Sunday into Monday.

It was so cold in western Minnesota that traffic lights went dark Sunday morning in Montevideo when a transformer blew. One homeowner's bid to thaw pipes in West Duluth caused a fire that led to $37,000 in damage, WDIO-TV reported.

In west-central Wisconsin, the La Crosse Tribune reported a frozen pipe apparently caused a sprinkler system to fail late Friday at Gundersen Health System's Tomah Clinic, sending a gush of water into the clinic.

Many cities sought to ensure no one succumbed to the cold.

The Indianapolis Star reported that the state Department of Homeland Security would send anyone needing shelter from the weather Sunday and Monday to a Salvation Army facility.

Winter weather advisories were in effect along the eastern shore of Lake Michigan, where lake-effect snow was expected to generate more than a half-foot snow in some areas.

Some reprieve from the cold was expected in the Midwest later in the week. Highs in Chicago were expected to jump into the comparatively balmy 30s by Thursday.

But the sub-freezing temperatures were moving eastward.

In Philadelphia, a man who had been evicted from a homeless shelter returned Sunday morning and opened fire, killing one employee and injuring another, police said. Some officials feared the shooting might deter people from entering shelters as frigid weather entered the region.

"We're very, very, very concerned; this is not something that happens in the city of Philadelphia's shelters for people experiencing homelessness," Marie Nahikian, director of the city Office of Supportive Housing, told WPVI-TV.