IStock Photo 6670389 © Catharina van den Dikkenberg

(To see the odds your area will have a white Christmas, check out our interactive map.)

It’s the stuff of pop songs and holiday cards, but if you have your heart set on a white Christmas—defined as at least one inch of snow on the ground on December 25th—your chances are best in the mountain West, upper Midwest, or Northeast (New York state and New England). According to the National Weather Service, the odds of snow at Christmas are at least 1 in 2 (50%) throughout most of those areas.

Not surprisingly, some of the best odds are at far northern points like Fairbanks, Alaska (1 in 1); Minneapolis, Minnesota (1 in 1.37—73%), and Caribou, Maine (1 in 1.03—97%). But topography and weather patterns make snowy weather common in some places at lower latitudes. Average temperatures fall as altitude increases, so Denver—the “Mile-High City” at the foot of the Rockies—has 1 in 2 odds of a white Christmas. So does Des Moines, Iowa, in the central Great Plains, where Arctic air masses often sweep down from Canada.

Cities east and south of the Great Lakes often receive heavy lake-effect snow, which forms when cold air masses move over lake waters and pick up moisture. This extra water vapor then cools when it reaches land and falls as snow. Buffalo, New York—1 in 1.75 (57%) odds of a white Christmas—and Cleveland, Ohio (1 in 2 odds) are both known for lake-effect snowstorms.

Not fond of shoveling? Head south or to the coast. The odds of a white Christmas fall sharply below the Mason-Dixon line, from 1 in 14.29 in Memphis, Tennessee, to 1 in 33.33 in Huntsville, Alabama. They are also relatively low in coastal cities like Seattle (1 in 14.29) and New York (1 in 10), because ocean waters cool more slowly than land in winter, resulting in moderate temperatures along the shore. But surprises can happen anywhere: parts of southern Texas got 1 to 13 inches of snow on Christmas in 2004, and Seattle had a white Christmas last year.

Many changes in US snowfall patterns are triggered by El Niño and La Niña, global weather cycles that start with water temperature changes in the Pacific Ocean. According to the National Weather Service, El Niño is currently in effect and will probably strengthen through winter 2009-2010 in the Northern Hemisphere. El Niño cycles tend to produce above-average temperatures and below-average snowfall in the West and Plains states, but can increase snowfall along the East Coast.

However, snowstorms develop within a few days, so these long-range patterns and forecasts don’t say much about the odds on December 25th. White Christmas forecasters would do well to recall Mark Twain’s observation that “Climate is what we expect, weather is what we get.”