After carefully studying woolly bear caterpillars, the thickness of fur on squirrels' tails and other natural signs, "Abe Weatherwise" late last year predicted in The Old Farmers'Almanac that the current winter would be a cold one. Jerome Namias, a meteorologist at California's Scripps Institution of Oceanography, had made a similar forecast. But even Namias is surprised at the subfreezing temperatures that have prevailed over most of the eastern half of the U.S. Says he: "I was a little too conservative. Our forecast was for the coldest winter in perhaps 20 years, but it now looks as if it is proving to be even colder than that. In many locations, 75-year-old records are going to be broken before the winter is over."
Namias and other meteorologists agree on the immediate reason for the bitterly cold weather. The high-level westerly winds—including the jet stream—that whistle through the upper atmosphere high above the U.S. have been circulating in an unusual pattern. Normally in winter these winds flow more directly across the country from west to east. This winter they are cutting across the Rockies much farther to the north than usual and then, as they head toward the East Coast, dipping much farther south than normal.
The product of that unusual pattern is this winter's wild weather. According to Namias, the jet stream has been picking up Pacific storms and guiding them across the U.S., "pepping up" each one as it crosses the country. The resulting heavy snowfalls that have accumulated on the ground in Eastern states further refrigerate Arctic air as it moves down from Canada. The snow covering also contributes to the dramatic difference between land and water temperatures, which in turn stimulates more storms along the East Coast—including the nor'easters that have been battering oil tankers. All the while, Western states have remained relatively unscathed. Says John Firor, executive director of the National Center for Atmospheric Research in Boulder, Colo.: "This year winter seems to have become stuck east of the Rockies."
What triggered these changes, however, remains unclear. Namias notes that water temperatures in the Pacific rose a few degrees higher than normal last fall off the west coast of North America, while dropping off in midocean. He believes that these temperature shifts influence the winds and determine the course of storms that work their way up to the jet-stream level. Harry Geise, a California meteorologist, blames the storms and frigid temperatures on a high-pressure zone of warm air hovering off the country's Pacific coast and sometimes shifting over land.