"Weather forecasters are warning of dangerously cold conditions in parts of the nation," the agency said in a press release on Tuesday night. "Some places could see wind chill readings as low as 60 below zero."
It added that "due to this arctic outbreak and concerns for the safety of USPS employees, the Postal Service is suspending delivery" on Wednesday in several locations, including Detroit, Chicago, and several parts of Pennsylvania, Indiana, Nebraska and Ohio.
It's unclear when deliveries will resume in those areas.
More than 220 million Americans will be forced to contend with below-freezing temperatures this week. The temperature in Chicago on Wednesday morning was about 29 degrees Celsius (minus 20 in Fahrenheit) below zero, according to the US National Weather Service, with the windchill extending even more into the negatives.
"It's cold, period," the NWS's Chicago office said, adding that it's rare to see temperatures so low.
In many places, it's simply too cold for people to be outside safely. The NWS, as well as other weather and medical officials, has warned that the frigid wind can cause hypothermia and frostbite in minutes.
"You're talking about frostbite and hypothermia issues very quickly, like in a matter of minutes, maybe seconds," Brian Hurley, a meteorologist with the Weather Prediction Center, told The Associated Press.
Thousands of flights have been cancelled in Chicago and other airports, and schools were closed in Chicago and parts of eastern Iowa on Wednesday, in addition to closures in Michigan, Wisconsin, and Minnesota.
Although it has no formal status, the US postal service has long been associated with the saying: "Neither snow nor rain nor heat nor gloom of night stays these couriers from the swift completion of their appointed rounds".
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