This Winter's Weirdly Warm Weather Explained

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It felt more like March than January in many places last week, as more than 1,000 temperature records fell across the country during a winter that has been unusually warm and dry in many places.

Bellingham, Wash., for example, saw a high of 60 in the first week of the year, while the mercury soared to a balmy 44 in Fargo, 61 in New York City, 72 in parts of Colorado and 79 in Tucson. In some regions of the Midwest, temperatures are 40 degrees higher than average. And snow covers just 19 percent of the country at the moment, compared to a usual coverage of about 50 percent at this time of year.

"The whole lower 48 and much of southern Canada is feeling the effects of what I call 'Marchuary,'" said Paul Douglas, meteorologist and founder of Weather Nation, a weather outsourcing company in the Twin Cities, Minn. "It really is on the verge of being unprecedented meteorologically to be this warm for this long, this deep into winter."

Whether you are rejoicing the lack of cold or lamenting the lack of snow, you may be wondering: What's behind the weirdly warm weather?

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Several forces are at work, experts say. To begin with, La Niña conditions have pushed warm water toward Australia in the western Pacific, leaving ocean waters off the American West coast about 5 degrees F colder than usual. As a result, moisture levels are currently low in the atmosphere from California to Washington State.

To understand why, you can think of a La Niña-dominated Pacific like a cold bathtub, said Jeff Weber, a climatologist at the University Corp for Atmospheric Research in Boulder, Colo. Compared to a hotter and steamier vat, water is less likely to evaporate from a chilly ocean. And since weather patterns generally move west to east, very little rain or snow is falling from the jet stream right now. La Niña also pushes the jet stream northward, so that it flows near the border between Canada and the United States.

A widespread lack of snow cover explains the recent spell of high temperatures, Weber said. Snow normally acts to reflect the sun's energy, adding more moisture to the air and causing even cooler conditions. Without snow on the ground, though, exposed brown soils and green grasses are absorbing solar radiation, warming the ground, and feeding back into exceptionally warm temperatures from Michigan to California's Sierra Nevada mountains.

But last winter was a La Niña year, too, and conditions couldn't have been more different -- with massive and relentless snow storms pounding much of the West and Midwest. It turns out that, even though La Niña and El Niño get all the press, they are not the only drivers of seasonal weather patterns.

"A few months ago, just about everyone was predicting colder and snowier for northern tier states based on La Niña," Douglas said. "We are discovering that every La Niña is different. And there are larger forcings on the atmosphere that really transcend anything La Niña can do."

There are two forces that have made the difference between last year's relentless series of snowmaggedons and this year's January blooms. They are the Arctic Oscillation and the North Atlantic Oscillation, and they work together like gears to alter jet stream patterns across the U.S. 

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Last year, both the AO and NAO were in their negative phases, with low pressure near Iceland and high pressure off the coasts of Portugal and Spain. That "blocked" the Atlantic, Weber said, preventing weather systems from moving smoothly from west to east. Instead, kinks developed in the jet stream, causing weather patterns to meander. And that led to all sorts of extreme weather conditions, including moist air in the north, cold air in the south and lots of hurricanes.

This year, on the other hand, the AO and NAO are positive, a situation that favors a strong and uninterrupted flow of air from west to east over the northern half of the country. And since the air is predominantly dry as a result of La Niña, precipitation is simply not falling across states in the north and west.

That trend, Douglas said, may end soon.

"The strongly positive AO and NAO is finally showing signs of breaking down," he said, "meaning a return to more seasonable temperatures by the third week of January."

And come spring, La Niña is expected to weaken, at which point moisture could return.

For spring skiing, Weber said, Washington State, Montana and Idaho will likely be your best bets. So far, ski resorts in New Mexico and Arizona have received a fair amount of snow, while Colorado and Utah remain dry. In typical La Niña years, the south and east get the most precipitation.

As for what's ultimately beneath the weather rut we're in, climate change is a tempting target but global warming is not necessarily to blame. In fact, a warmer world would cause warmer oceans, Weber said -- and an opposite pattern from what we're seeing. Other theories include low solar activity and melting Arctic ice.

On the other hand, Douglas said, this unprecedented warm spell adds to what has been the most severe stretch of weather in American history since record keeping began in the early 1800s. There were 99 federal weather disasters in 2011, he said, a number exceeded only by 2010. And then along comes a bizarre winter like this.

"The last 18 months have been utterly amazing from the standpoint of meteorology," Douglas said. "It's going to be a wild ride."