High winds and heavy snow turned much of the Midwest into a frosty mess on Friday. Roads and sidewalks across the region became treacherous, forcing some highways to close and grounding thousands of flights.

A mass of frigid Arctic air trailed behind the storm, biting at fingers, reddening cheeks and threatening a dangerously cold holiday weekend.

The storm affected nearly 30 million Americans, according to the National Weather Service, which said that more than a foot of snow could fall in parts of the Midwest by Saturday morning. Officials across the region delivered a drumbeat of similar messages: Avoid the storm. Keep warm. Stay home if you can.

“I’m from the Midwest and used to cold winters, but this snow and freezing temps are intense,” said Caleb Tracey, 26, who went for a walk in Lincoln, Neb., on Friday despite the heavy snowfall there. “I’m keeping today’s walk very short.”