A: A vigorous cold air outbreak is usually a midwinter, not late fall, phenomenon.

As with most unusual events in the atmosphere, a cold air outbreak is a commingling of circumstances — some common and some not — brought to extreme by perfect timing.

Because the sun has long since set for the winter at the North Pole, the endless night is a breeding ground for very cold air masses. As we head into mid-November, the darkness has crept southward into northern Canada, bringing with it an even more proximate source of very cold air. That circumstance is the same every year.

The high altitude flow of air around the hemisphere is most often predominantly west to east. Occasionally, large north-south meanders develop in this flow, bringing warm tropical air toward the pole in the northward directed flow and cold, polar air toward the equator in the southward directed flow.

If a strong ridge of high pressure develops at high altitude over the west coast of the United States and Canada, the eastern side of such a ridge is characterized by southward flow that can usher a mass of cold air from high latitude central Canada into the central United States.