More than a month before the official start of winter, residents of parts of the United States are being blanketed with snow and hit with frigid temperatures. But the infamous polar vortex that sent Americans shivering last January isn't the only weather system responsible for this unseasonably cold weather, experts say.
The early onset of winter has manifested itself in different ways across the country, dumping more than 2 feet (0.6 meters) of snow in parts of Michigan and sending temperatures plummeting in places like Colorado. Even states in the South haven't been spared. In San Antonio, Texas, residents are enduring temperatures that are a full 20 degrees colder than average for this time of year.
It's easy to pin the blame for this wintry blast on the so-called polar vortex, a swirling mass of frigid air typically found over the Arctic that occasionally ventures southward. But the vortex is just one of the culprits for this week's cold snap, said Tom Kines, a senior meteorologist with AccuWeather.
"It helped deliver the cold air, but the so-called polar vortex isn't going to be sitting over the U.S. for days and days," Kines told Live Science. In fact, the vortex is retreating northward into Canada right now, he said.
The reason the cold weather isn't heading back north with the polar vortex is simple: There's something in its way. That something is a jet stream, a river of wind that typically sits about 20,000 feet (6,100 m) or more above the Earth's surface and influences how air masses and weather systems are distributed.
"Not only does the jet stream guide weather systems across the country, it also separates warm air masses in the South from those cold air masses in the North," Kines said. "When the jet stream is north of a particular location, that region usually experiences mild weather because [the jet stream] is kind of blocking the cold air."
http://www.livescience.com/48742-heavy-snowfall-polar-vortex.html
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