That Crazy Winter Weather!

Changing winds cause deluges, droughts and misplaced snows

Even the Old Farmer's Almanac could not have foreseen it all. Barely a trace of snow in New England, yet mounds of it down in Dixie. Norfolk, Va., accumulated twice as much snow as Burlington, Vt, or Portland, Me., and about one-third more than Chicago. Florida too was taking its licks. In early March, temperatures plummeted below freezing, putting a squeeze on the citrus crop, and tornadoes cut across the southeast part of the state. Up North folks were trying to decide whether to pack away mufflers and mittens after spring-in-December readings of 16DEG C (60DEG F). The most startling weather occurred in California, where downpours of almost biblical proportions caused floods, mud slides and untold havoc.

This crazy-quilt winter weather was the result of erratic changes in the usual pattern of westerly winds--especially the high-altitude jet stream--that whip across the U.S. Part of a broader global feature known as the circumpolar vortex, the winds in winter usually follow a sharply undulating path round the Northern Hemisphere, like the bottom of a whirling crinoline skirt. Sweeping northeast over the Pacific, the winds pick up warmth and moisture. Heading down again from the cold north, they cause heavy rain and snowstorms from the Rockies through the heartland to New England.

Last December scientists at the National Weather Service noticed an unanticipated change in air flows. It was as if the skirt had stopped undulating: the curves in the prevailing winds flattened, and fewer chill breezes were blowing down from the north. High-level winds above the 40th parallel (near Philadelphia) were running at extra high speeds, while those to the south slackened. In effect, explains Donald Gilman, the service's chief long-range forecaster, the cold arctic air was blocked, almost as if it were being held back by a great fence, letting warmer, southern air dominate the weather. For the Northeast as well as other parts of the Northern Hemisphere, Gilman's fence meant a December of mild temperatures and little snowfall.

In late December meteorologists noticed another dramatic change. The upper-level westerlies south of the 35th parallel (from Southern California to North Carolina) had picked up speed while those north of it had slowed. The jet stream had edged to the south, along a path that was much less curved. Instead of sweeping north over the Pacific with a cargo of warm, moist air and dumping snow and rain on Alaska, Washington and Oregon, the westerlies now were aiming their punch directly at California. The result: Southern California was inundated for nine days by storms that brought a total of 32.39 cm (12.75 in.) of rain.

What caused the shift in winds? Meteorologist Jerome Namias of the Scripps Institution of Oceanography --who had correctly predicted a wetter-than-normal winter for California--believes that it was somehow related to an increase of 1DEG to 2DEG C (2DEG to 4DEG F) in ocean surface temperatures between California and Hawaii and unusually cold temperatures in waters in the central Pacific north of Hawaii. Though Namias cannot precisely explain the mechanism, he says that as the air passes over the water and warms up, the newly acquired heat influences the direction of the jet stream. Other scientists are not so sure. Climatologist Stephen Schneider of the National Center for Atmospheric Research in Boulder, Colo., points out that any number of factors could influence air flows, including solar flares, clouds of dust, snow on the ground and even the rising level of carbon dioxide in the atmosphere. Adds the Weather Service's Gilman: "The westerlies behave very much like the 'average' man--there are always some abnormalities."

Pondering this winter's abnormalities, meteorologists can only agree that while they are able to make accurate short-range forecasts--clear and cold today, rain tomorrow--the long haul still mystifies them (see ESSAY) Admits Schneider: "This business of ours is, in many ways, still more of an art than a science." After this winter, few Americans would care to argue with him.

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