Polar vortex

Source In January 2014, arctic air poured in. Temperatures in Boston dropped from a high of 56F Boston to as low as 15F, means 41 degree drop. From LA Timas:
"A huge mass of arctic air has slipped away from the North Pole, settled over the U.S. and made everything really, really cold, meteorologists told The Times. You may know the phenomenon by a phrase you've seen in headlines lately: the ominous-sounding "polar vortex."" (...) The National Weather Service's Heitkamp likened the effect to a large mass of cold building up over the North Pole and getting "flushed" south.