Waves of intense cold will send temperatures plunging as much as 20F (11C) below normal across the central and eastern US this month, triggering ice storms that could down power lines and snarl travel from the Midwest to the Mid-Atlantic.

Bitter temperatures are forecast to hit swathes of the U.S. this weekend as a polar vortex—the area of cold air that circulates around the Arctic—expands, with the National Weather Service predicting that the cold snap could last well into January.

The polar vortex is an area of cold air that circulates around the Arctic. That area can shift and expand, bringing lower temperatures further south than usual. Modelling by the independent forecaster Atmospheric and Environmental Research shows the polar vortex expanding over the US in the first two weeks of the new year.