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.

A Winter Weather Advisory is in effect until 6 a.m. Thursday.Widespread freezing rain and drizzle will continue to increase Wednesday evening, causing untreated roads, sidewalks, and driveways to become icy and slick.

Forecasters are warning of possibly significant ice accumulations in several mid-Atlantic states that could cause widespread power outages and disrupt travel

Storms spawned at least one tornado, sent creeks over their banks and caused flash flooding Thursday in portions of West Virginia and Kentucky, while a wintry mix coated trees and roads in ice and even dropped “thunder ice” in several states.