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BBC forecaster Susan Powell looks in detail at the storm path
Nearly 1.5 million people are without power across several states as a powerful Arctic winter storm sweeps through the US and Canada.
Large parts of both countries are under winter weather alerts that stretch from coast-to-coast and as far south as the US-Mexico border.
The storm has brought damaging winds and freezing temperatures that can quickly lead to frostbite.
At least six storm-related deaths have been reported in the US.
Major airports have cancelled thousands of flights as the storm intensifies.
As of Friday morning, more than 1,400,000 people from Texas to Maine had no electricity as intense winds damaged power lines.
Power outages have also been reported in Canada, affecting 317,900 people in the provinces of Quebec and Ontario.
The US National Weather Service said that over 200 million people – or roughly 60% of the US population – are under some form of winter weather advisory.
Much of Canada, from British Columbia to Newfoundland, is also under extreme cold and winter storm warnings. Several school boards in Ontario, including Toronto, have cancelled classes.
This storm is set to bring the iciest Christmas in decades, say forecasters.
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Watch: Blizzard and bitter cold in North and South Dakota
The National Weather Service (NWS) said temperatures of -50F (-45C) and even -70F were possible.
In the US, the bad weather has been linked to several deaths. Three fatal car crashes have been reported in the state of Kansas due to poor road conditions.
At least three others have died in Kentucky, Governor Andy Beshear said on Friday – two were killed in car accidents and another person described as "housing insecure" has died.
In Fort Worth, Texas, a man who was thought to be homeless died after being found outside in the bitter cold.
The Arctic air mass is projected to bring strong wind gusts and temperature of 15F (-9.4C) to El Paso, Texas, where newly arrived undocumented migrants are sleeping rough on city streets.
Meteorologists have described the winter storm as a bomb cyclone, a term given to an explosive storm that intensifies rapidly, with its central air pressure dropping by at least 24 millibars in 24 hours.
"This is not like a snow day, when you were a kid, this is serious stuff," President Joe Biden said in a White House briefing on Thursday.
The NWS has described the storm, which began in the Pacific Northwest on Tuesday, as a "once-in-a-generation" winter weather event, saying that more than 100 daily cold temperature records could be tied or broken over the next few days.
Even Florida, a famously warm state, is projected to see its coldest Christmas in 30 years. Here's what other regions have seen so far:
Several US states, including New York, West Virginia, Kentucky, Georgia, North Carolina and Oklahoma have declared a state of emergency, while Wisconsin declared an "energy emergency".
Some interstate highways have closed due to dangerous driving conditions, including in Western New York and South Dakota.
The storm has also brought flooding to parts of the US east coast on Friday, shutting down rail services on the Long Island Rail Road, while Lake Ontario on the US-Canada border may see waves up to 37 feet.
Ohio governor, Mike DeWine, called the weather a "unique and dangerous situation", particularly as people travel to be with loved ones over Christmas.
The storm has already wreaked havoc from Colorado to Kansas to Wyoming, and north in Minnesota.
On Wednesday, Wyoming Highway Patrol received 787 calls for help and recorded 104 crashes in just 12 hours.
In Ontario, Canada, police reported around 100 collisions due to freezing rain, heavy snow and ice pellets that have hit the region since Friday morning.
More than 6,900 flights in the US have been cancelled on Thursday and Friday, according to the flight-tracking site FlightAware.
In Canada, WestJet alone has cancelled 266 flights on Friday.
In anticipation of travel disruptions, major airlines including United, Delta and American have offered to waive fees for travellers who wish to reschedule their flights.
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