A Calm Start to Winter
2 PM EST, December 16, 2011
By WeatherBug Meteorologist, John Bateman
So far, the U.S. has kicked off meteorological winter relatively unscathed. In fact, the worst "winter" weather so far has been from two storms: the East Coast October snowstorm, and the recent Santa Ana wind event in the West. Even large Arctic outbreaks have been relatively absent, though parts of the Western U.S. have been cooler than average. What`s the reason behind this, and what`s to come as we move toward the holidays?
A moderate La-Nina is still in charge of our weather this season, which typically brings a cooler, wetter West and milder East. Last year was a La-Nina year as well, but it was much colder and snowier along the East Coast, mainly because of a pattern that tends to trump La-Nina along the Eastern Seaboard called the North Atlantic Oscillation. The NAO is a shifting Arctic weather pattern that can either bring frigid air southward to the eastern U.S., or keep it bottled up in eastern Canada. Last year, it was in a negative pattern which tends to bring colder and snowier weather to the Mid-Atlantic and Northeast. When it`s in a positive pattern, the East tends to be milder and wetter. So far this season it has been mainly positive, and the forecast for the next couple of weeks tends to continue this trend.
Also, the jet stream, the "river" of fast-moving air thousands of feet high in the atmosphere that separates cold and warm air masses and steers storms, has been relatively flat, with few ridges and troughs. Lately the jet stream has been flowing in a pattern more reminiscent of fall than winter, allowing the East to stay mild. What`s more, any persistent stormy pattern has been mainly secluded to the Pacific Northwest. Even the typically wintry lake-effect snowbelt cities have seen well-below-average snowfall this season. This has given a large swath of the eastern two-thirds of the U.S. a relatively headache-free winter... so far.
What`s to come? Well, it appears that through Christmas, no major weather pattern changes are expected. For the beginning and middle of next week, a system will slide across the nation's midsection, but as of now, it appears it will be mainly a rain-maker for much of the country. Temperatures still look to be running above average in the East and a little cooler in the West. Also long-range jet stream forecasts don`t show any appreciable storms coming between this weekend and the holidays. This may not bode well for people hoping for a white Christmas, other than those who typically see snowflakes flying or a blanket of white on the ground such as the Northern Plains, western mountains, and the Great Lakes cities.
As for 2012, the trends for January still seem to support a relatively mild eastern-third of the U.S., with cooler weather in the West. As always, however, that remains to be seen.
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