As near as I can tell, polar vortex refers to the persistent low pressure system which sits atop the Arctic. The cold air of the Arctic stays there, ecause of the circular weather pattern of this low pressure.
In past years, a lobe of the polar air might tear loose from the polar vortex and this packet of colder air would be flung southward. I remember these being described as “polar outbreaks.” The northern Midwest of the United States might experience a handful of these exceptionally cold air masses in any given winter.
From what i understand, when the US is said to be experiencing “a polar vortex”, it means that the actual Polar Vortex has thrown a tentacle all the way south. The situation is different from a polar outbreak, in that it is a continuous extension of the polar vortex and not a separate packet of air which spun away from the vortex.
“This is air that is circulating the Arctic,” Freedman said. "In the last couple of days, it’s sort of become lopsided — sort of like a figure skater that has extended their arms and then tripped.
"You know, when a figure skater pulls their arms in, they spin tighter and tighter and faster and faster. But when they put their arms out, they are a little bit slower and a little bit more wobbly and more prone to fall or stop skating at the end of their routine.
“What’s happening now is that a piece of it is down on the other side of the globe, but a piece of it kind of got lopsided and came down on top of us.”
So why did the PV wobble so much as to reach the United States? There are couple explanations. One is that the jet stream has an unusual pattern this year. In the Pacific, it is driving northward into Alaska (bypassing the west coast of the US) It then turns southward, and is diving deep into the US (instead of flowing across the US). This unusual jet stream pattern makes the polar vortex wobbly.
Another factor may be the melting of the Arctic sea ice. The warmer water which results may weaken the polar vortex, which slows down its spin, and as a result, it becomes more wobbly.
I may have some (or all!) of that wrong. But that is a best I can make of the situation.
Whatever the cause, I don’t like the result. :winter: