Forecasters say over half the U.S. population could be affected by snow, ice or bitter cold as a winter storm sweeps across the county.

A potentially historic, massive winter storm will slam more than half of the United States today, moving east as it brings heavy snow, widespread ice accumulation and dangerous cold.

Meteorologists and public officials said the storm’s southern flank could endure a rare and damaging ice event capable of bringing down trees and power lines and causing prolonged power outages, while...