The polar vortex is a large area of cold aloft that typically spins directly above the poles, hence the name, “polar” vortex. The polar vortex is at its strongest when it is circulating west to east within the Arctic Circle.
What keeps the polar vortex sustained in the poles is a strong jet stream. Large pressure differences keep the jet stream straighter and cold air trapped north.
The jet stream within the polar vortex can weaken if a plunge of warm air from down south flows into the Arctic Circle. The weakened jet stream allows cold air to plunge southward into the United States. When this happens, really cold arctic plunges affect the country, typically the northern half. Sub-zero temperatures and strong wind chills typically occur as a result.