Several recent studies have reported that large portions of the mid- and high-latitude land areas of the northern hemisphere (including much of the United States) have experienced increasing precipitation over the last half of the 20th century. From a relatively small number of studies of snowfall trends, the characteristics of northern hemisphere wintertime precipitation over the period have been found to vary over space and time. Over the United States, some specific findings from these studies include: increasing snowfall in parts of the great plains and northeast; increasing inter-annual variability in nationally-averaged snowfall; and, as recently reported by the IPCC, a reduction in a directly related variable-spring snow cover area-in the latter half of the 20th century.

In our current study, daily snowfall records for 1948-2001 from several hundred stations in the United States Historical Climatology Network (lying north of 35 degrees north latitude) are examined in several ways. We analyzed records over the whole year and also over an October through May "snow season" to study trends and variations in variables such as number of snow days, total snowfall, average magnitude of snowfall events, near-surface air temperature on snow days, length of snow season, and the contribution of snowfall liquid water equivalent to total annual precipitation. We've found evidence of decreasing snowfall in the Pacific Northwest (PNW) and over an area centered roughly on Ohio. We also found a swath of increasing snowfall across the central plains. Snowfall events were broken down into class intervals to determine which event sizes were responsible for the decreasing/increasing trends. As found in other precipitation studies, our results also show that extreme events significantly influence precipitation climatology. The decreases in snowfall in the PNW are influenced by changes in the number and magnitude of extreme events. Changes in other variables such as temperature and length of snow season are also thought to play a part in the PNW snowfall decreases.