The term “Polar Vortex” came into vogue a few winters ago and it still gets tossed around every winter, mostly because it sounds scary. Most of the time, however, the term is misused and misunderstood. The Polar Vortex is an upper level atmospheric circulation surrounding the polar region. This circulation has two basic phases, and the periodic change between the two phases in called the Arctic Oscillation, which can greatly influence our weather, particularly in winter.
When the AO is positive, the circulation is relatively circular and strong. Very cold air remains locked up in the polar region. When the AO is negative, the Polar Vortex becomes weak and wobbly. Bulges in this weaker circulation allow Arctic air to dump southward, often into the Northern Plains. When the negative phase of the Arctic Oscillation dominates a winter, it tends to be a cold one. Unfortunately, the AO phase is difficult to predict more than a week or two in advance.