Snow at the MSC.
“They might also have a sense that the jet stream is not just something over the United States — it circles the globe. The tropospheric polar vortex is nothing more than this jet stream making a complete loop around the hemisphere, plus all the cold air inside the loop.”
So is the polar vortex the jet stream, or is it the cold air inside the jet stream? “It’s both,” says Nielsen-Gammon. “It’s sort of a chicken and egg thing. Whenever you have deep cold air, you’ll have a jet stream along its edge, and whenever you have a jet stream, you’ll have cold air on one side. You can’t have one part of the polar vortex without the other.”
“In the Northern Hemisphere, whenever we’re north of the polar jet stream, we’re inside the polar vortex, and the weather is cold. This happens a lot in the winter, as the vortex expands and waves in the jet stream occasionally cause the jet stream to swing even farther south.”
While waves like that are common, the coldest weather is usually underneath the very center of the polar vortex, he points out. “Usually the center of the vortex is somewhere over the Arctic Ocean, or northern Canada, or Siberia. On rare occasions, though, it can temporarily swing down over the northern United States,” Nielsen-Gammon says.
That’s what happened in the winter of 2013-2014, Nielsen-Gammon adds. The center of the vortex made it all the way across the northern United States border, so cold air covered a large portion of the United States.
“This week will be different,” Nielsen-Gammon says. “The problem this time is that the jet stream is unusually wavy. Rather than forming a round vortex about the North Pole, it will look more like a squeezed water balloon, with one lobe extending into the northern United States and the other into Siberia.”
“What’s noteworthy about this cold snap arriving in a few days is that it could be the coldest air of 2016 and set temperature records in many locations,” he adds. “That’s because the traditionally cold months of last January and February were very mild across much of the country.”
Also, Nielsen-Gammon says, the cold air is arriving rather early in the season. “I would not be surprised to see many record lows occur in the central United States this week.”