Why Extreme Cold Has Hit U.S.

What's going on? Blame the so-called lake effect, the polar vortex, a kink in the jet stream—and an old super typhoon.

The lake-effect snow comes from west-southwesterly winds of a storm aligning neatly with the lengths of Lakes Erie and Ontario, which sit between the American Midwest and Canada. That alignment is "allowing the air to pick up large amounts of moisture," writes Jeff Masters, director of meteorology at Weather Underground, on his blog.

Lake-effect snow tends to result from cold, dry winter winds sweeping from Canada across unfrozen lakes. As they cross open water, the winds pick up moisture and become warmer. When that moist air reaches the eastern sides of the lakes, it's forced to rise abruptly over the land and over the colder air above the land.

The results are exceptional snowfalls in bands along the southeastern sides of the lakes, often accompanied by the unusual winter phenomena of thunder and lightning. But even by those standards, this week's snow stands out. "I can't remember and I don't think anyone else can remember this much snow falling in this short a period," Erie County, New York, executive Mark Poloncarz told CNN.

The most dramatic effect normally extends up to 70 miles (113 kilometers) inland from the lakes, but bands of light snow and flurries can extend hundreds of miles inland.

Now about that extreme cold...

Temperatures in much of the country have dipped 15° to 35°F (8 to 19°C) below average as a blast of Arctic air from the polar vortex—which normally circles the North Pole—has drifted southward over the eastern two-thirds of the United States. Cold temperatures this week reached south all the way into the Florida Panhandle, where Pensacola hit 28°F (-2°C).

Masters also blames what he calls "an unusually extreme jet stream pattern," which is seeing a collision of high and low pressure. The meteorologist describes a "sharp ridge of high pressure along the U.S. West Coast and a deep trough of low pressure diving to the south over the central United States."

The juxtaposition has allowed cold air to spill out of the Arctic, behind the trough, and into the central U.S.

The extreme early chill is also due in part to the impact of Super Typhoon Nuri, which warmed up waters west of Alaska ten days ago and "caused a ripple effect on the jet stream," Masters wrote.

http://news.nationalgeographic.com/news/2014/11/141119-snow-cold-weather-polar-vortex-weather-jet-stream-science/