BOSTON — A large part of the U.S. is preparing for the coldest December in years after the polar vortex made a comeback.


The polar vortex is located in the middle and upper tropospheres and extends into the stratosphere. It is in fact a band of low pressure Arctic air normally centered around the North Pole.


It is held in place by the jet stream, a river of wind that divides cold air from warm air. The jet stream bends around high or low pressure weather systems. When it does, the center of the vortex moves south into North America.


The polar vortex can strengthen or weaken. A strong polar vortex keeps the cold air closer to the North Pole, whereas a weak polar vortex spreads the cold air southward.


Two arctic blasts will make their ways into the U.S., with the first round of cold air already affecting the Plains and Upper Midwest. The second wave will plunge south of the Canadian border by this weekend. The most intense lobe of the polar vortex will cover New England by Friday, bringing ‘dangerously cold’ weather to the area.