Shoppers stripped supermarket shelves as the National Weather Service forecast huge snowfall in some areas and possibly “catastrophic” ice accumulations from freezing rain.

Transportation Secretary Sean Duffy warned that up to 240 million Americans could be affected by the storm. At least 16 states and the US capital Washington, declared states of emergency.

“Take this storm seriously, folks,” the National Weather Service said on X, predicting an “astonishingly long swath” of snowfall from New Mexico to Maine.

More than 3,400 flights in and out of the United States were cancelled on Saturday alone, and more than 1,100 others were delayed, according to tracker FlightAware.

In Dallas, freezing rain pelted the city, and temperatures plummeted to 21F (-6C).

Houston Mayor John Whitmire urged residents of the country’s fourth most populous city to hunker down by late Saturday: “Be where you need to be for the next 72 hours.”

Warming centres were set to open on Saturday for those in need.

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.

Snow also hit Oklahoma and Arkansas, where some spots already recorded six inches (15 centimetres) on the ground, the NWS said.

After battering the country’s southwest and central areas, the storm system was expected to hit the heavily populated mid-Atlantic and northeastern states before a frigid air mass settles in.

“The snow/sleet impacts will linger well into next week with rounds of re-freezing that keep surfaces icy and dangerous to both drive and walk on for the foreseeable future,” the weather service said.

The US federal government preemptively announced offices would be closed on Monday.

“We will continue to monitor, and stay in touch with all States in the path of this storm. Stay Safe, and Stay Warm!” US President Donald Trump, who was riding out the storm in Washington, said on his Truth Social platform.

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 National Weather Service 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.

In New York state, Democratic 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 neighbours.

Professional and collegiate basketball games have been rescheduled as a result of the 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.