Last week, snow covered more than half of the continental United States, the highest this measure has reached by this date in a decade, according to government scientists.

As of Dec. 15, snow covered 53 percent of the Lower 48, the National Oceanic and Atmospheric Administration (NOAA) reported. That's a significantly higher portion than in recent years. In 2006, for example, snow covered just 12 percent of those states on the same date, according to Climate Central, a climate news Web site.

November and early December have also been quite chilly for much of the country, below the 20th-century average, Climate Central noted. "With the noteworthy exception of Alaska, nearly every state was affected by the unusually cold air at some point during the November-to-December timeframe," Climate Central reported.

What's going on?

An unusual configuration of the jet stream has caused record heat in Alaska and corresponding below-average temperatures in the Lower 48. This strange jet stream has brought warm air from the Pacific into Alaska, and cold Arctic air south into the continental United States, NOAA reported.

While the United States has had a cold start to winter, the rest of the world was warmer than average in November. "According to NASA, November was the warmest such month on record worldwide since reliable instrument temperature data began being collected in 1880," Climate Central reported.

In fact, the only areas of the globe that were cooler than usual in November were the central and eastern United States and part of Antarctica. And those few cold spots don't counteract the overall global warming trend: The top 10 hottest Novembers in NASA's data set have all occurred since 2001, according to Climate Central.

While the large snowpack in the continental United States was largely the result of typical fluctuation in weather unrelated to climate change, it's possible that snowfalls in the United States might increase in the future with global warming, said Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo.

"If you warm up the atmosphere, you can actually get heavier snowfalls in winter" because warmer air can hold more moisture, Trenberth said. "That's one of the ironic things about global warming."