After battering the country's southwest and central areas, the storm system began to hit the heavily populated mid-Atlantic and northeastern states as a frigid air mass settled in across the nation.

"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 weather service said.

Snowfall was reported across the central United States, including Kansas, Oklahoma and Missouri, where some spots already recorded eight inches (20 centimeters) on the ground by Saturday night, the NWS said.

Shoppers stripped supermarket shelves as the National Weather Service (NWS) forecast huge snowfall in some areas and possibly "catastrophic" ice accumulations.

Transportation Secretary Sean Duffy warned that up to 240 million Americans could be affected by the storm.

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

"Take this storm seriously, folks," the NWS said on X, predicting an "astonishingly long swath" of snowfall from Colorado to Maine.

More than 4,000 flights in and out of the United States were canceled on Saturday alone, with nearly as many delayed, according to tracker FlightAware.

In Dallas, which has typically mild temperatures in January, freezing rain pelted the Texas city and the mercury plummeted to 21F (-6C).

State officials vowed that the Texas power grid was in better shape than five years ago, when it failed during a deadly winter storm and left millions without power.

Nearly 160,000 US customers were without power as of Saturday night, according to tracking site poweroutage.us, with around 50,000 reported in both Texas and neighboring Louisiana.

Speaking at the Washington headquarters of the US Federal Emergency Management Agency (FEMA), Department of Homeland Security chief Kristi Noem urged Americans impacted by the weather to "be smart, stay home if possible, take care of your family members, check on your neighbors, and continue to work with your local officials."

US President Donald Trump, who was riding out the storm at the White House, said on his Truth Social platform: "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 climate 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.

- 'Five or six minutes' -

New York Governor Kathy Hochul warned residents to stay inside amid frigid conditions: "Five or six minutes outside could literally be dangerous for your health."

She stressed precautions like protecting pipes, using heaters safely, and checking on vulnerable neighbors.

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.