(Bloomberg View) — As the snow piles up toward record heights inBoston, San Francisco is going through an extraordinary dry spell —this was the first January in 165 years in which the city recordedno rain at all. As another snowstorm keeps schoolkids home in theSoutheast, flowers are blossoming in the Pacific Northwest, wheretemperatures in February have been above 60 degrees.

Yet this is not as contradictory as it sounds — just theopposite. This kind of winter doesn't happen every year(obviously), but the divergence reflects a weather syndrome longfamiliar to forecasters, borne of climate conditions originatingover the Pacific Ocean.

Scientists view this weather — especially what's happened inFebruary — as the outgrowth of two so-called teleconnectionpatterns. One is the Pacific North American pattern, in which highpressure persists over Hawaii and the mountain West, while lowpressure prevails over the Aleutian Islands and the Great Lakes.The jet stream spools around these pressure zones, rising northwardover the West Coast (drawing warm air up) and dipping southwardthrough the center of the U.S. (pulling arctic air down). Once inplace, this pattern can last weeks. In the winter of 1976-77, a PNAkept going for four months, relentlessly holding the eastern halfof 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 itisn't the only weather phenomenon at work: A second pattern knownas the East Pacific Oscillation has been in its negative phase,causing a north-south ridge of high pressure to build from Alaskadown the West Coast. Working together, the two patterns havesteered frigid air from Siberia eastward through the Arctic Circleand then south through central Canada and deep into the easternU.S. Hence, this year's “Siberian Express” has replaced last year's“polar vortex.”

The result has seemingly been wonderful for the West andterrible for the East — but only if all you care about is theweather. Drought-weary Californians were hoping for a lot moreprecipitation and, ideally, a bigger-than-usual snowpack in theSierra. Instead that snow is about 25% its normal volume. In theCascades, the snowpack is less than 20% of normal.

The good news for everyone everywhere is that the pattern isabout to change. By next week, the extraordinary cold from Texas tothe Carolinas will end, forecasters say. And cooler, stormierweather will arrive in the West. California may still not get a tonof rain, but there will be increased snowfall in the Rockies andthe interior West, where it's also needed, says Dan Leonard, seniorenergy meteorologist for the private forecasting company WSI. Thenorthern Plains, Midwest and East Coast will remain colder thannormal, 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 bea lot of interesting talks about this winter as a whole,” saidLeonard, because it came in two parts. December was warm in theEast, and usually when that happens the whole season is mild. Thisyear, winter turned sharply colder in January, Leonard said,and “everything just opened up big time in February.” That didn'tfit any pattern.

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