A Climatology of Mean Monthly Snowfall for the Conterminous United States: Temporal and Spatial Patterns.

Mean monthly snowfall data for 216 stations across the conterminous United States were analyzed to produce a climatology that identifies statistical, spatial and intraseasonal aspects. Geographic variations in the length of the snowfall season are characterized using two statistics: the number of months of snow and the Snow Concentration Index (SCI).The annual distribution of mean monthly snowfall is also examined using harmonic analysis. Snowfall across the conterminous United States generally peaks in February; earlier snowfall maxima are found in the Great Lakes area and in the Pacific Northwest, whereas late February or March maxima occur in the western High Plains. Stations with relatively high amounts of variance explained by the second harmonic indicate 1) areas with a short snowfall season such as the southeastern United States, or 2) areas with a long snowfall season that have a tendency toward a bimodal distribution.A climatology of the changing monthly patterns of snowfall is identified through the mapping of station deviations from a national composite. This procedure produces contiguous regions that can be related to seasonal changes in the extent and positioning of the circumpolar vortex. The maps reveal that positive snowfall deviations predominate 1) in autumn in the northern and western Great Plains and the Rocky Mountains (with a full-latitude trough over the central United States and associated upslope precipitation); 2) in early winter in the Great Lakes (associated with lake-effect storms); 3) in late winter over the southern and western states (with a fully expanded circumpolar vortex); and 4) in spring in the western states (linked to seasonal changes in preferred locations of cyclogenesis and associated storm tracks).