Our Two-Part Winter
As the snow piles up toward record heights in Boston, San Francisco is going through an extraordinary dry spell -- this was the first January in 165 years in which the city recorded no rain at all. As another snowstorm keeps schoolkids home in the Southeast, flowers are blossoming in the Pacific Northwest, where temperatures in February have been above 60 degrees.
Yet this is not as contradictory as it sounds -- just the opposite. This kind of winter doesn't happen every year (obviously), but the divergence reflects a weather syndrome long familiar to forecasters, borne of climate conditions originating over the Pacific Ocean.
Scientists view this weather -- especially what's happened in February -- as the outgrowth of two so-called teleconnection patterns. One is the Pacific North American pattern, in which high pressure persists over Hawaii and the mountain West, while low pressure prevails over the Aleutian Islands and the Great Lakes. The jet stream spools around these pressure zones, rising northward over the West Coast (drawing warm air up) and dipping southward through the center of the U.S. (pulling arctic air down). Once in place, this pattern can last weeks. In the winter of 1976-77, a PNA kept going for four months, relentlessly holding the eastern half of the U.S. in the freezer, and keeping the West warm and dry.
This year, the PNA has been in place most of February. But it isn't the only weather phenomenon at work: A second pattern known as the East Pacific Oscillation has been in its negative phase, causing a north-south ridge of high pressure to build from Alaska down the West Coast. Working together, the two patterns have steered frigid air from Siberia eastward through the Arctic Circle and then south through central Canada and deep into the eastern U.S. Hence, this year's "Siberian Express" has replaced last year's "polar vortex."
The result has seemingly been wonderful for the West and terrible for the East -- but only if all you care about is the weather. Drought-weary Californians were hoping for a lot more precipitation and, ideally, a bigger-than-usual snowpack in the Sierra. Instead that snow is about 25 percent its normal volume. In the Cascades, the snowpack is less than 20 percent of normal.
The good news for everyone everywhere is that the pattern is about to change. By next week, the extraordinary cold from Texas to the Carolinas will end, forecasters say. And cooler, stormier weather will arrive in the West. California may still not get a ton of rain, but there will be increased snowfall in the Rockies and the interior West, where it's also needed, says Dan Leonard, senior energy meteorologist for the private forecasting company WSI. The northern Plains, Midwest and East Coast will remain colder than normal, alas, but perhaps not as cold as February.
Then in a few weeks, spring will arrive and winter will be gone -- though not forgotten, at least by meteorologists. "There will be a lot of interesting talks about this winter as a whole," said Leonard, because it came in two parts. December was warm in the East, and usually when that happens the whole season is mild. This year, winter turned sharply colder in January, Leonard said, and "everything just opened up big time in February." That didn't fit any pattern.
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