The polar vortex is consists of two layers: one, the polar jet stream flowing at the height of an airplane and normally contained at the pole by the second layer, a higher frigid system called the stratospheric polar vortex. But when the stratospheric polar vortex is warmer than usual, the polar jet stream may go nuts, hurling frigid weather in random directions.

The image below shows global temperatures relative to the past four decades for this time. The warm winter we're enjoying explains why plums are blossoming so early here and why now is not a good time to visit North America. (Though my California surfing friends tell me the water is toasty and the air balmy.) - William