As the storm dumped snow, sleet and freezing rain across the wide expanse, officials warned that an arctic air mass behind the system would see temperatures fall dangerously low for days, prolonging disruptions to daily life.

"The snow/sleet impacts will linger well into next week with rounds of re-freezing that keeps surfaces icy and dangerous to both drive and walk on for the foreseeable future," the National Weather Service (NWS) said.

The PowerOutage.com tracking site showed over 700,000 customers without electricity as of Sunday morning, mostly in the southern US where the storm began on Saturday.

Nearly 250,000 residential and commercial customers were without power in Tennessee, while Texas, Mississippi and Louisiana -- where such storms are uncommon -- each had over 100,000 outages.

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 Sunday from the south to the northeast, dumping snow on heavily populated cities across the US east coast.

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

Residents in Washington awoke to a blanket of several inches of snow on sidewalks and roads, with forecasters predicting a transition into sleet later in the day.

Federal offices have been preemptively closed for Monday.

Several major airports in Washington, Philadelphia, and New York had nearly all of flights canceled for the day.

Tracking site Flightaware.com showed over 10,000 flights canceled in the United States on Sunday adding to over 4,000 the day before.

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 of the polar vortex 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 on Truth Social.

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.