WASHINGTON – Cold and snow keep battering the Midwest and East, and even Atlanta was temporarily paralyzed. California has been bone dry. Alaska set heat records.

The wild winter somehow became even more wicked Thursday morning when the national average temperature plunged to a brutal 11 degrees, prompting a weather-weary nation to ask a simple question: Why?

The answer is the jet stream, the river of air that dictates our weather.

(MORE: Cities Sick of Snow This Winter)

"A persistent dip in the jet stream in the central and eastern states has led to many blasts of cold air east of the Rockies since the start of January," explains Chris Dolce, a meteorologist for weather.com.

"Conversely, the jet stream has been poking northward along the West Coast and the adjacent Pacific, leading to mostly dry and warm conditions this winter, though some significant rain and snow have returned this week."

Why is the jet stream doing this?

"We are having an unusual jet stream that's giving us crazy cold weather in the East and the ridiculously resilient ridge as it's called in California," said Weather Underground meteorology director Jeff Masters.

There are three different forces probably at work here, but scientists still need to do more research, said Derek Arndt, of the National Climatic Data Center in Asheville, N.C. One is just the random natural variability of daily weather.

Another is a mid-length weather feature called the Pacific Decadal Oscillation – think of it as a cousin of El Nino – that warms the northern Pacific and helps push the jet stream south.

And finally, a new and controversial theory is that a warmer Arctic region and shrinking summer sea ice from man-made global warming has shifted jet stream patterns, making it wavier and bringing more unpredictable weather.

Is it unusual for the weather pattern to last this long?

It doesn't happen often, but it's not that unusual either, said Bruce Terry, of the National Weather Service's Weather Prediction Center in College Park, Md.

So how cold was Thursday?

The national average temperature of 11 degrees is the coldest of this winter and will likely be the coldest of the season, according to calculations by Weather Bell Analytics meteorologist Ryan Maue. It was computed from temperatures at 7 a.m. EST in the Lower 48 states.

The lowest was -47 degrees at West Yellowstone, Mont., according to weather.com meteorologist Chris Dolce

Has this been a record winter?

No. Given the unusual heat in the West and the cold in the East, they almost balance each other, Masters and Arndt said. So when the final monthly statistics come out, January in the U.S. won't be near record cold.

"When you compare it to the 20th century, it was still cold, but not dramatically cold," Arndt said.

Was this just a U.S. thing?

No. Parts of South America and Australia have had much warmer than normal weather. Parts of Europe have been cold and stormy, others record warm. For much of January, Greenland was 8 degrees warmer than normal.

"I'm sick of it," said the weather service's Terry.

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