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.

An Arctic outbreak featuring punishing cold weather is beginning across much of the U.S. at the same time that Europe prepares for plunging temperatures. Why it matters: About three weeks of cold east of the Rocky Mountains — potentially accompanied by major winter storms — will boost natural gas prices,