Why an unusual phenomenon will cause a frigid December in the U.S.

"Despite the name, its occurrence does not imply there will be any sudden warming where people live. In fact, it's almost the opposite. A rapid warming and weakening of winds in the stratosphere can disturb the polar vortex, a ring of frigid, stormy air that typically sits high above the North Pole. Impacts can cascade downward through the atmosphere, sometimes causing very cold air to spill southward toward more populated areas, as the polar vortex becomes displaced or elongated."

"Sudden stratospheric warming events are natural phenomena, happening once every other winter on average. But major events in November are very uncommon, with only one occurring since the 1950s - and the one predicted this November may become the earliest on record. They more typically occur in January and February and not in November or December. That's because the polar vortex is still in its formative stages during late fall and early winter, making it less susceptible to warming events."

A sudden stratospheric warming (SSW) involves rapid warming and deceleration of winds in the stratosphere that can disturb the polar vortex. Disturbance of the polar vortex can displace or elongate the ring of frigid air over the North Pole and allow Arctic air to cascade downward and spill southward into populated regions. Major SSWs typically occur once every other winter and are rare in November, with most events happening in January or February. When thresholds for rapid temperature rise and wind slowdown are met, impacts can propagate downward over weeks, favoring below-average temperatures across many U.S. states from late November into December.

Read at Boston.com