An ancient story tells of Damocles and a sword.
Damocles was a nobleman who exclaimed how fortunate his king was. The annoyed king offered to allow the nobleman to sit on his throne. Damocles scrambled onto the throne and found himself surrounded by luxury. Then he looked up. Gulp!
Overhead was a sword, held by the single hair of a horse's tail! With power came great danger and worry. Damocles left the throne and crept away.
The Sword of Damocles by Richard Westall. 1812
Look north. Hanging over the US is the coldest Arctic air on record. What keeps it from sweeping south is a single wind—the Arctic Oscillation.
This year, the Arctic has been abnormally cold. Last summer, Arctic sea-ice increased by 60%. Less than fifty days had temperatures above freezing, compared to the normal 90. That was summer. Now it is winter and time for Arctic air to move south. We already saw the air form an “Arctic Vortex” over the Midwest and all 50 states experienced below freezing weather on the same day.
LEFT – Positive Arctic Oscillation RIGHT − Negative Arctic Oscillation.
What keeps the air north are the circumpolar winds. Scientists measure these winds and call this the Arctic Oscillation (AO). When the AO is positive, it means the winds are so strong that the cold air is trapped. When the AO is negative, it means the winds are weak and Arctic air can break through and surge south.
Most of November and December the AO was positive. Even when the weather got nippy, it was normal cold. In the beginning of January, the AO was negative and the US was swept by bitter cold.
That’s all it takes. The wind gets a little weaker. Most experts expected the AO to get negative again and it is happening now (week of January 20). Some think it will get positive by the end of the month.
Experts predict the AO will go negative later this month and the Arctic cold will return. Source: NOAA’s Climate Prediction Center
So how is a snowstorm like a sword? When you have a winter like this, those sharp cold temperatures are dangling just to the north and it won’t take much to make them drop south. It’s enough to make one want to creep away like Democles and find a cozy tropical island!
Evelyn Browning Garriss, historical climatologist, is a longtime writer for The Old Farmer's Almanac. She is also editor of The Browning World Climate Bulletin  and has advised farmers, businesses, and investors worldwide on upcoming climate events and their economic and social impact for the past 21 years.