Polar Vortex Triggers Subzero Temperatures, 'Life-Threatening' Wind Chill

A brutal polar vortex sent temperatures plummeting to "life-threatening" lows across the Midwest and Northeast on Saturday, with millions of Americans warned of frostbite and other dangers.

Related: Polar Vortex to Bring 'Coldest Air for More Than a Decade' to Northeast

The coldest air mass of the winter brought the thermometer down to minus 6 degrees overnight in Minneapolis, while New Yorkers were urged to take "extreme precautions" against wind chill.

New York City Mayor Bill de Blasio urged residents to check on vulnerable friends, relatives and neighbors.

"Extremely cold weather can be life threatening — especially for seniors, infants and people with medical conditions," he said.

Frigid Polar Vortex Brings Subzero Temperatures to Millions 2:28

Forecasters warned it would be Tuesday before temperatures return to normal, with Sunday morning likely to be the coldest for many.

The mercury went so low, it forced organizers to cancel an ice festival featuring ice-carving artists and music scheduled for Saturday in New York's Central Park. Horse racing at New York's Aqueduct Racetrack was also canceled.

A wind chill advisory was in effect for New York City beginning Saturday afternoon and continuing to noon Sunday. With the actual temperatures falling as low as 4 degrees below, the weather service says the city could see wind chills of minus 18 to minus 24. Wind gusts may reach 45 mph.

Elsewhere, outdoor events went on as scheduled, despite the bone-chilling temperatures.

Polar plunges went on in Madison, Wisconsin, and Raleigh, North Carolina, among other places, and Cupid's Undie Run, a charity race in which participants strip down to their skivvies, happened in Pittsburgh and Washington, D.C.

In Grand Rapids, Michigan, there was a fundraising walk to help pay for heat for the needy.

The arctic blast is a result of a piece of the polar vortex — the cold air system that sits over the North Pole year-round — coming further south and combining with a cold front.

How Long Will the Deep Freeze Last? 1:24

"An eddy of the polar vortex over Quebec, along with a reinforcing cold front, is expected to bring the coldest weather of this winter season from the Great Lakes to New England," the National Weather Service said in an update early Saturday.

"Wind chill warnings and lake effect snow warnings are in effect for these areas, with wind chill readings dropping below minus 30 degrees by Saturday night. Actual temperatures will also be frigid with highs in the single digits and teens, and subzero lows across much of upstate New York and New England."

Boston, Philadelphia, D.C., Hartford, Connecticut, and Albany, New York, could see record lows over the weekend. In the Boston area, wind chills could make it feel like 35 below in places.

However, apart from lake effect snow over the Great Lakes area, there won't be a repeat of the heavy snow that struck the Northeast earlier this month.

"The biggest danger is from frostbite," said John Rowe, lead forecaster at The Weather Channel.

Meanwhile, in Chicago, where the high was only forecast to reach 14 degrees Saturday, snow was predicted to fall on Sunday. Anywhere from 2 to 4 inches is expected, reported NBC Chicago.

FEB. 3, 2014: What's All of This Talk About the Polar Vortex? 1:29