White Christmas? An Application of NOAA’s 1981–2010 Daily Normals

Abstract

re we going to have a white Christmas? That is a question that scientists at the National Oceanic and Atmospheric Administration (NOAA) receive each autumn from members of the media and general public. NOAA personnel typically respond by way of a press release and map depicting the climatological probability of observing snow on the ground on 25 December at stations across the contiguous United States. This map has become one of the most popular applications of NOAA?s 1981?2010 U.S. Climate Normals.The purpose of this paper is to expand upon the annual press release in two ways. First, the methodology for empirically calculating the probabilities of snow on the ground is documented. Second, additional maps describing the median snow depth on 25 December as well as the probability and amount of snowfall are presented. The results are consistent with a climatologist?s intuitive expectations. In the Sierras, Cascades, the leeward side of the Great Lakes, and northern New England, snow cover is a near certainty. In these regions, most precipitation falls as snow, and the probability of snowfall can exceed 25%. At higher elevations of the Rocky Mountains and at many locations between the northern Rockies and New England, snowfall is considerably less frequent on Christmas Day, yet the probability of snow on the ground exceeds 50%. For those who would like to escape the snow, the best places to be in late December are in Southern California, the lower elevations of the Southwest, and Florida.