A powerful Arctic blast swept into the US Northeast on Friday, pushing temperatures to dangerously low levels across the region, including New Hampshire’s Mount Washington, where the wind chill dropped to minus 79 degrees Celsius, a Reuters report cited forecasters as saying.

The National Weather Service (NWS) said the deep freeze would be relatively short-lived, but the combination of numbing cold and biting winds gripping the Northeast would pose life-threatening conditions well into Saturday.

Sub-freezing, blustery conditions spread eastward through the day, sending wind-chill factors, measuring the combined effect of wind and cold on the body, plunging into the -40s across much of Maine, NWS meteorologist Brian Hurley said.

Temperatures were expected to drop to -28 C.

Wind-chill warnings were posted for most of New York state and all six New England states – Massachusetts, Connecticut, Rhode Island, New Hampshire, Vermont and Maine – a region home to some 16 million people.

Schools in Boston and Worcester, Massachusetts, New England’s two largest cities, were among those closed on Friday over concerns about the risk of hypothermia and frostbite for children walking to school or waiting for buses.

Boston Mayor Michelle Wu declared a state of emergency through Sunday and opened warming centers to help the city’s 650,000-plus residents cope with what the weather office has warned was shaping up to be a “once-in-a-generation” cold front.

The severe cold has put more than 15 million people in the region under wind chill warnings or advisories. Wind chill indicates how cold the air may feel, and the National Weather Service issues such warnings when winds are expected to feel as cold as 25 degrees Fahrenheit below zero, according to CNN.

The cold spell is expected to begin subsiding by Sunday when temperatures will likely rise again. In the meantime, officials across several states have begun imploring residents to stay indoors and have ramped up warming center efforts to accommodate some of the most vulnerable to the cold.

In New York City, “code blue” has gone into effect from Friday evening due to the expected extreme cold temperatures, according to a tweet from the city’s Department of Homeless Services.

It’s a designation which allows people to come into the city’s homeless shelter system on an emergency basis and directs New Yorkers to report people on the street as a safety measure.