Nearly 100 million people in Canada and the US brace for some of the coldest air on earth, as a record-breaking deep freeze hits North America.
The frigid blast could bring "once-in-a-generation" wind chills that cause frostbite in less than 10 minutes, the National Weather Service has warned.
Residents from Manitoba to Maine are being urged to limit their time outdoors through Friday and Saturday.
At least 11 people have died in the bad weather in the US south since Monday.
There were eight fatalities in Texas, two in Oklahoma and one in Arkansas.
The expected drop in temperatures is attributed to a powerful Arctic front that stretches from the Canadian maritime provinces to the core of the US.
About a dozen records are expected to be broken by Friday afternoon in several US states, where a total of 82 million people will face temperatures of -17 Celsius (0 F) or lower.
In Maine, for example, parts of the state are expecting the lowest temperatures recorded since 1971. In the city of Portland, wind chill is expected to reach -41 F (-40.5 C).
In nearby Burlington, Vermont, Friday's highest temperature is expected to reach just -20 C (-5 F).
Boston, which is anticipating sub-zero wind chills, is currently under a cold emergency. Public schools have been closed in the city, as well as in nearby Worcester.
Temperatures in New York City and other major cities are also expected to bottom out in the single digits Fahrenheit (around -13 C to -17 C) by Saturday, although forecasters predict they will rebound by the end of the weekend.
Parts of Canada are expecting temperatures anywhere between -38 C to - 50 C (-36.4 F to -58 F). An extreme cold advisory issued by Environment Canada on Friday morning has blanketed the Maritimes, most of Quebec and all of Ontario, spilling into Manitoba.
In Ottawa, Canada's capital city, the extreme cold has forced the closure of a local ski hills and outdoor ice skating trails. In Toronto, the wind chill plunged the temperature to - 29 C (-20 F) on Friday.
The brutal winter weather follows this week's deadly ice storm in parts of Texas, where temperatures have begun to climb above freezing, and ice was expected to melt on Friday.
More than 309,000 people were still without power as of Friday morning in Texas, Arkansas, and Mississippi, according to poweroutage.us.
Emergency crews in Texas have responded to hundreds of collision calls due to icy road conditions since Monday, some of which have been fatal.
In the neighbouring state of Oklahoma, in Custer County, a 35-year-old driver was killed on Thursday morning in a six-car pileup on icy roads, Oklahoma Highway Patrol said, according to local media outlets.
Dallas Police Department have asked people to make sure their tyres are inflated, to slow down on icy roads and to avoid using cruise control.
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