Weather patterns throughout the world can fluctuate in time periods ranging from daily, weekly, decadal or even longer. One such atmospheric pattern that generally shifts slowly over time is a process called the Arctic Oscillation. This phenomenon refers to pressure differences between the Arctic and the middle latitudes.
This oscillation is considered negative when the Polar Regions have relatively higher pressure in relation to the mid-latitudes and positive when the opposite is true. When Polar Regions have lower pressure, the higher relative pressure in the mid-latitudes tend to push storms farther north, which in turn keeps the Arctic Air intrusions to a minimum. But when the Polar Regions have higher relative pressure in respect to the mid-latitudes, the Arctic Air is more easily displaced southward into the country.
These pattern changes can be measured and in recent weeks the Arctic Oscillation has been strongly negative, so much so, that current conditions have not been observed since the late 1970s.