Snow now in 49 states

But recently, the La Nina effect has been overshadowed by a negative North Atlantic Oscillation, an atmospheric pattern that brings cold and snow to the East and Southeast.

"If the North Atlantic Oscillation was what we call neutral or positive, we might have our lounge chairs out today," Robinson told LiveScience. [Read Q&A with Robinson About Wild Winter Weather]

Snow over time

Snow in so many states is "not an every-winter event," Robinson said, though the last time it snowed in all 50 states was last February. Before that, he said, you have to go back to the 1970s to find winters where snow fell across so much of the North American continent.

"The size of the United States is such that when the jet stream dips down in the east and brings a lot of cold air and snow to make it down into Georgia and South Carolina, it bulges up in the west," Robinson said. That usually keeps snowy conditions out of places like Texas while it's also snowing in the Deep South.

On his website, Robinson tracks daily divergence from average snowfall. The map for Jan. 11 shows a large swath of above-average areas.

Robinson's research hasn't turned up any long-term changes in winter-snow cover in the United States over the last 40 years. But spring-snow cover is decreasing, he said.

"Last winter, we had a record extent of snow cover over North America, and last spring was a record low extent [of snow cover]," Robinson said. "So we went from one extreme to another."

• Quiz: Global Weather Extremes
• The World's Weirdest Weather
• 101 Amazing Earth Facts

You can follow LiveScience Senior Writer Stephanie Pappas on Twitter @sipappas.

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