US braces for 'extremely dangerous' winter storm

More than 160 million Americans are expected to face an unusually brutal winter storm from Friday, with heavy snows and freezing rains forecast.

The storm is expected to sweep through much of the United States, leaving "extremely dangerous" conditions in its wake as it tracks eastwards from the High Plains and Rockies, the National Weather Service (NWS) said.

It warned that the Arctic blast will bring sub-zero temperatures and wind chills, which "pose a life-threatening risk of hypothermia and frostbite to exposed skin".

US transportation officials, including airport authorities in several major cities, have warned of weekend travel disruption, delays and cancellations.

The winter storm is forecast to move slowly across the US, blanketing cities including Memphis, Nashville, Washington DC, Baltimore, Philadelphia and New York with snow.

Heavy snow is predicted through the Southern Rockies and Plains, into the Mid-Atlantic and reaching the Northeast.

According to the NWS Probabilistic Precipitation Portal, the regions that could see more than a foot of snow stretch from Colorado to West Virginia to Boston.

Numerous low temperature records are likely to be broken, according to the NWS, with the coldest wind chills potentially falling below -50F (-46C) across the Northern Plains.

A much wider area of the south-eastern US is also forecast to experience freezing temperatures.

Governors in several US states, including Arkansas, Georgia, Texas, North Carolina, South Carolina, have declared states of emergency, allowing emergency officials - including National Guard troops - to begin mobilising response efforts.

Some airlines have offered passengers the option to change their flights, in some cases without incurring extra fees, due to concerns about cancellations.

Motorists should avoid driving across the weekend, with travel expected to be "nearly impossible during the peak of the storm", NWS meteorologists Rich Otto and Tony Fracasso told the BBC's US news partner CBS.

In Canada, freezing temperatures are already gripping the country, with snowfall expected in eastern and Atlantic regions on Monday, meteorologist Geoff Coulson told CBC earlier in the week.