A winter storm that brought snow, sleet and freezing rain to a large swath of the Midwest swept into the mid-Atlantic and Northeast on Thursday, causing treacherous travel conditions, prompting school delays and closures and potential power outages for millions of Americans.

Of all the precipitation types, freezing rain is by most measures the worst because it can weigh heavily on trees and power lines.

Winter weather advisories for all 21 counties are in effect with ice and up to 3 inches of snow possible in spots.