Several snow rollers form as a winter storm threatens the United States

An unusually brutal winter storm is set to pummel more than 160 million Americans from Friday, as a stretched "polar vortex" sends a devastating blast of Arctic air, bringing heavy snows and freezing rains.Winter Storm Fern is forecast to engulf an area well over half the length of the continental United States, stretching from Texas and the Great Plains region to the mid-Atlantic and northeastern states.

Scientists say the increasing frequency of such disruptions of the polar vortex may be linked to climate change, though the debate is not yet settled and natural variability also plays a role.