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.