(Bloomberg View) — 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.