Weather Insider: Explaining the polar vortex

The polar vortex is a persistent area of low pressure dozens of miles above the earth's poles (both northern and southern). Because the vortex is so high up in the atmosphere, and because it's over the poles, makes the air very very cold. Every now and then, the jetstream can interfere with and nudge the polar vortex. This can cause the vortex to wobble.

The jetstream can then pick up some of this frigid air and allow it to surge southward into the US, bringing arctic air to tens of millions of people. But why does Southern California not feel these temperatures? Well when we look at the graphic below, we see a ridge of high pressure bringing warm air northward. It's important to remember that the jetstream moves in waves, so if a trough surges southward over the US, warmer air will move north on either side.