The polar vortex is a semi- permanent circulation that only exists in the upper levels of the atmosphere, that is usually seen near the north and south poles. You can't see the polar vortex and it doesn't extend down to the ground, but it does effect the weather at the surface.
*Image courtesy of the NWS
The image below shows the typical pattern of the polar vortex, moving from west to east. This happens when the jet stream is strong, and keeps the cold arctic air contained to the polar regions. The stronger the winds in the jet, the more the flow is from west to east.
But, when the jet winds weaken the jet will buckle and loose some of its west to east flow.
When the jet buckles it allows the cold air, which it normally contains near the poles, to get displaced south of its normal position. This usually means unseasonably cold temperatures at the surface. There is no temperature threshold associated with polar vortex displacement, and temperatures at the surface depend on a lot of different factors like the time of year, cloud, and snow cover.
During the 2013-14 winter, there were many times when part of the polar vortex was displaced into the Midwest. This meant long lasting and brutal periods of very cold temperatures, and quite a bit of snow. But, while the Midwest was experiencing the cold, Alaska and the western part of the United States were abnormally warm and dry.
It is not a new phenomenon for portions of the polar vortex to break away from its normal placement occasionally, and it doesn't always happen in the Midwest. Also, not every cold snap is caused by the displacement of the polar vortex. And every time portions of the polar vortex is displaced, it doesn't mean there will be sub-zero temperatures or even snow!
-KC
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