From Texas to New England, the monster storm brings hazardous conditions, prompting warnings to stay off roads.

Nearly a million customers across the United States are without electricity and more than 10,000 flights have been cancelled as a monster winter storm threatens to paralyse a large part of the country with heavy snowfall and freezing rain.

The storm is forecast to sweep the eastern two-thirds ⁠of the nation on Sunday and into the week, plummeting temperatures to below freezing and causing “dangerous travel and infrastructure impacts” to linger for several days, the National Weather Service (NWS) said.

As ​of 10:47 am EST (15:47 GMT) on Sunday, more than 850,000 customers were without electricity, according to PowerOutage.us, with ‍at least 290,000 in Tennessee and over 100,000 each in Mississippi, Texas and Louisiana. Other states affected included Kentucky, Georgia, Virginia and Alabama.

Heavy snow was forecast from the Ohio Valley to the Northeast, while “catastrophic ice accumulation” threatened from the Lower Mississippi Valley to the Mid-Atlantic and Southeast.

“It is a unique storm in the sense that it is so widespread,” said NWS meteorologist Allison Santorelli, adding that about 213 million people were under some sort of winter weather warning.

“It was affecting areas all the way from New Mexico, Texas, all the way into New England, so we are talking like a 2,000-mile [3,220km] spread.”

Calling the storm “historic”, US President Donald Trump on Saturday approved federal emergency disaster declarations as nearly 20 states and the District of Columbia declared weather emergencies.

“We will continue to monitor, and stay in touch with all States in the path of this storm. Stay Safe, and Stay Warm,” Trump wrote in a post on Truth Social.

More than 10,000 flights were cancelled on Sunday and another 8,000 have been delayed, according to the flight tracker FlightAware.com. Major US airlines warned passengers ⁠to stay alert for abrupt flight changes and cancellations.

The Federal Emergency Management Agency pre-positioned commodities, staff and search-and-rescue teams in numerous states, Department of Homeland Security Secretary Kristi Noem said, warning Americans to take precautions.

“It’s going to ‍be very, very ⁠cold. So we would encourage everybody to stock up on fuel, stock up on food, and we will get through this together,” Noem said. “We have utility crews that are working to restore that as quick as possible.”

The Department of Energy on Sunday issued ‌an emergency order to authorise grid operator PJM Interconnection to run “specified resources” in the mid-Atlantic region, regardless of limits due to state laws or environmental permits.

The NWS warned that heavy ice could cause “long-duration power outages, extensive tree damage, and extremely dangerous or impassable travel conditions”, including in many states less accustomed to intense winter weather.

Authorities warned of life-threatening cold that could last a week post-storm, especially in the Northern Plains and Upper Midwest, where wind chill lows were forecast to dip to extremes under -50F (-45C). Such temperatures can cause frostbite within minutes.

The massive storm system is the result of a stretched polar vortex, an Arctic region of cold, low-pressure air that normally forms a relatively compact, circular system but sometimes morphs into a more oval shape, sending cold air spilling across a large region, in this case, North America.

Scientists say the increasing frequency of such disruptions of the polar vortex may be linked to climate change.