A polar vortex of brutally cold air sent temperatures plummeting to "life-threatening" lows across the Midwest and Northeast Saturday, with millions of Americans warned of dangers including frostbite.
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.
Forecasters warned it would be Tuesday before temperatures return to normal, with Sunday morning likely to be the coldest for many.
"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."
In New York, it was so cold that organizers canceled an Ice Festival scheduled for Saturday in Central Park.
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.
And in Washington, D.C., weather was expected to impact Saturday's Cupid's Undie Run, a charity race in which participants strip down to their skivvies.
Boston, Philadelphia, D.C., Hartford, Connecticut, and Albany, New York, could see record lows over the weekend.
"The biggest danger is from frostbite," said John Rowe, lead forecaster at The Weather Channel.
This article originally appeared on NBCNews.com.