Changes in winter snowfall/precipitation ratio in the contiguous United States

[1] The precipitation falling as rain or snow has different impact on regional water resources and their annual distribution. Shift from solid to liquid form of precipitation following the increase of the surface air temperatures could be quite important because such change could influence the timing of spring runoff and cause water shortage in summer. In this study, the ratio of snowfall to precipitation (S/P) for November - March in the contiguous United States is analyzed and temperature effects on the changes of S/P are examined for 1949 - 2005. Major results show that the S/P ratio has been decreasing strongly in the Pacific Northwest and the central United States. The S/P decreased slightly in the eastern United States. In the Pacific Northwest, the changes of S/P are attributed to decrease of both snowfall and precipitation with snowfall decreasing at a greater rate. In the central United States, decrease of the S/P ratio resulted primarily from the decrease of snowfall and increase of the winter precipitation. Averaged over the contiguous United States, the changes of S/P are mainly related to the changes of the snowfall and with little effect from changes of winter precipitation. Decreases of the S/P ratio are largest in March and least in January. The significant decreases of the S/P ratio are associated with large increase in mean winter wet-day temperatures in the western and central United States. Weak warming in the eastern United States concurred with weak and no change of S/P.