Forecasters say one of the key factors driving the weather is an atmospheric river, which is a long band of water vapor that can transport moisture from the tropics to more northern areas.

Parts of the U.S. East Coast and Atlantic Canada are preparing for the impact of an atmospheric river, a weather phenomenon that is more commonly associated with the West Coast.

Places like western Maine could see freezing rain, downpours, unseasonably high temperatures and damaging winds — all in the span of a day.