Hugh Pickens writes writes: "This year the temperature in Minneapolis didn't fall to zero degrees until January 12, the daytime high in Rapid City, South Dakota hit a record-setting 71 on January 5, and just a couple of days before New Year's, visitors to Park City, Utah, skied on man-made snow and dined al fresco — without their parkas — as Eryn Brown reports that a combination of factors has trapped winter's cold air over Canada and Alaska, making for unseasonably warm weather in the Lower 48 and creating disappointment for outdoor enthusiasts who revelled in last year’s snow. "The talk across the whole country has been, 'Where has winter been?'" says Dale Eck, who runs the global forecast center at the Weather Channel in Atlanta. The culprit is a mercurial weather pattern called the Arctic oscillation. André Viau, a climatologist at the University of Ottawa, describes the polar jet stream as similar to a ribbon that snakes across the continent, at the intersection of the colder air in the north and the warmer air that’s farther south. When the Artic oscillation is weak, or negative, the ribbon buckles, allowing colder Arctic air to penetrate farther south. This season, however, the oscillation has been almost exclusively positive. Strong polar winds have pulled the ribbon of the jet stream so taut, “it’s been almost straight,” says Viau, preventing Arctic air from escaping southward. Last week, the strength of the oscillation finally relaxed, moving first to neutral and then to negative, allowing dense Arctic air to flow south and bring the first bone-chilling temperatures of the winter south. But even with the recent dip, it’s possible this winter will go on record as one of our warmest says Canadian climatologist David Phillips. "Memories of this winter will be quite something: short and mild.”"
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