The storm dumped snow, sleet and freezing rain across swathes of the country from Texas to New England, with temperatures set to fall dangerously low this week.

New York City Mayor Zohran Mamdani said five people were found dead outside over the weekend, telling reporters "there is no more powerful reminder of the danger of extreme cold."

In Texas, authorities confirmed three deaths, including a 16-year-old girl killed in a sledding accident.

Two people died in Louisiana from hypothermia, the southern state's health department said.

Meanwhile, one person was killed and two others injured Saturday during a winter weather related collision in southeast Iowa, according to local State Patrol.

The PowerOutage.com tracking site showed more than 820,000 customers without electricity as of Monday, mostly in the US South where the storm intensified on Saturday.

In Tennessee, where a band of ice has downed power lines, more than 250,000 residential and commercial customers were without electricity, while Louisiana and Mississippi -- where such storms are less common -- each had over 100,000 outages as of Monday.

The outages are particularly dangerous as the South is being walloped by treacherous cold that the National Weather Service warns could set records.

Authorities from Texas to North Carolina and New York urged residents to stay home due to the perilous conditions.

"Stay off the roads unless absolutely necessary," Texas's Emergency Management Division posted on X.

The storm was moving Monday through the US northeast and southeast Canada, with some areas in Toronto seeing record-breaking snowfall.

At least 20 states and the US capital Washington have declared states of emergency.

Federal offices have been preemptively closed for Monday and several major airports in Washington, Philadelphia and New York had nearly all flights canceled for the day.

Tracking site Flightaware.com showed more than 19,000 flights into and out of the country had been scrapped since Saturday.

In Maine, a small aircraft carrying eight passengers crashed on takeoff Sunday evening at Bangor International Airport, the Federal Aviation Administration (FAA) reported.

It was not immediately clear if the crash was weather-related or the conditions of the passengers.

President Donald Trump, who was riding out the storm at the White House, said on his Truth Social platform Saturday: "We will continue to monitor, and stay in touch with all States in the path of this storm. Stay Safe, and Stay Warm!"

- Polar vortex -

The brutal 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 North America.

Scientists say the increasing frequency of such disruptions may be linked to climate change, though the debate is not settled and natural variability plays a role.

But Trump -- who scoffs at climate change science and has rolled back green energy policies -- questioned how the cold front fit into broader climatic shifts.

"WHATEVER HAPPENED TO GLOBAL WARMING???" the Republican leader posted.

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.