A massive blast of bitterly cold air plunged temperatures across much of the United States on Monday to life-threatening lows not seen in nearly two decades.

Gusty winds in combination with the frigid temperatures will produce “dangerously cold” wind chills of minus 50°C across States in the northern tier and Midwest, the National Weather Service said, referring to the cold as “historic and life-threatening.” Thousands of households were without power on Monday due to a snowstorm that stretched from St. Louis in the Midwest into Pennsylvania and New York in the northeast. Some areas received up to 25 cm of fresh snow on top of as much as 40 cm that fell earlier.

The extreme weather also snarled air travel, with more than 4,100 cancelled flights and more than 11,200 delays, according to the flight tracker FlightAware.com.

NASA postponed a commercial resupply mission to the International Space Station (ISS) due to the low temperatures. The launch of Orbital Sciences’ Cygnus cargo spacecraft from a NASA site in Virginia was put off until at least Wednesday.

Bitter cold and snow are not an unknown in the region, particularly in Chicago, Minneapolis and Indianapolis, but the cold arriving from Canada has a different dimension.

“It’s the cold that really scares us,” said Greg Ballard, Mayor of Indianapolis, where thermometers stood at the lowest temperature in 20 years. He worried that people could get frostbite after just a few minutes outside and may not even know it.

“You can die in 10 minutes if you’re not properly clothed,” Mr. Ballard said.

City officials put travel restrictions into affect, but softened them on Monday morning. Mr. Ballard said he still wanted people to stay off the roads if possible due to the dangerously low temperatures.

Meteorologists were having a difficult time believing how low the temperature was heading. In Fargo, North Dakota, it was expected to drop to minus 51°C.

Schools in the north-central State of Minnesota were closed on Monday by order of the Governor. It was the first time in 17 years that Minnesota, accustomed to hard winters, closed its schools due to snow and dangerously low temperatures, reports said.

The Governor, however, left the decision about school cancellations for Tuesday up to individual school districts.

“We encourage you, as always, to be mindful about the dangers of even brief exposure to these dangerously low temperatures as you make your decisions,” a letter from education commissioner Brenda Cassellius to superintendents said.

Temperatures on Tuesday morning were expected to drop to double digits below zero again, with continued windchill advisories for the entire State until afternoon.

The weather phenomenon is called a polar vortex. It occurs when ice cold air streams southward from the Arctic Circle over Canada and the United States. Monday was the first day of the cold snap, which is expected to continue until the middle of the week and possibly get worse.

Meteorologists predict the Arctic cold would move eastward over New York and other States in the upper northeast.

The irony of current weather conditions over the continental U.S. is that Anchorage, Alaska, one of the country’s northern-most cities, was set to be warmer than the southern city of Atlanta, where a high of minus 3°C was expected.

Overall, around 149 million people — about half the U.S. population — were expected to be in the grip of the Arctic chill Monday and Tuesday, meteorologists said.

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