A woman is seen bundled up from the cold in New York, Feb. 12, 2016. Eduardo Munoz/Reuters

A dangerous cold snap seized the Northeastern United States Saturday, with temperatures in some areas set to fall below zero and Boston facing its coldest Valentine’s Day in almost four decades.

Officials warned people to stay indoors away from what the National Weather Service described as “life threatening” cold. Wind chill advisories were in effect over parts of nine states extending from northern Pennsylvania to western Maine, with forecasters expecting gusts up to 45 mph.

“Wind chills will be getting colder and colder as the day goes on,” said Alan Dunham, a meteorologist with the National Weather Service in Taunton, Massachusetts.

The temperature in Boston was expected to drop to -7 degrees Fahrenheit overnight but feel as cold as -30 degrees Fahrenheit with the wind chill.

That would be below the record low of -3 degrees Fahrenheit set in 1979.

“That one looks quite breakable,” Dunham said.

New York was bracing for its coldest night in 20 years. Mayor Bill de Blasio said officials had put on extra staff to help respond to residents who had lost their heat and had expanded efforts to bring homeless people into shelters.

“It’s so important to take this seriously, to stay indoors to the maximum extent possible, go out for as little time as possible. Do not have skin exposed. These are tough conditions,” de Blasio told reporters. “Be really careful.”

He ordered construction crews to put cranes into secure positions following the collapse of a huge crane in high winds this month, which killed one person and injured three.

The lower-than-average temperatures for mid-February come after a mostly mild winter and higher-than-average temperatures in the U.S. Northeast and Midwest.

In Boston, some were hurrying through their mornings to get outdoor chores done before the worst cold set in.

“Right now I’m going to drink a coffee” to stay warm, said Carmen Pichente, 40, en route to her job at a Boston restaurant. “Tomorrow I’m going to stay at home all day.”

Others brushed it off as an inevitable part of life in New England.

“To me, it’s nothing. I lived in Boston all my life.” said Eddie Brown, 51, a delivery truck driver who was out on his rounds. Asked why he wasn’t wearing a coat, Brown replied, “I got long underwear on.”