VARIABILITY AND TRENDS IN SNOWFALL AND FREEZING LEVEL IN THE WESTERN US

Several different studies of snow records from the National Weather Service cooperative observer network have shown recent trends in annual snowfall amount, extreme snowfall (both high years and low years), and in the rain/snow ratio in the western United States. The direction of these trends is associated with warming. A new web-based interactive tool, the North American Freezing Level Tracker, has recently been implemented to depict temporal variations in the height of the freezing level (and other temperture thresholds). The freezing level is important to water managers and hydrologists in mountainous terrain. This level has significant effects on accumulation, retention, and melting of snowpack, the main source of annual water supplies in many parts of the United States. The daily history of freezing levels has been assembled for a grid encompassing North America (in order to include Alaska) for a period extending from 1948, updated each day. The tool includes the ability to show time series of anomalies at a selected point for a given window of dates. In the western United States the freezing level has recently risen, especially in spring, and more recently in summer. Additional tools are in preparation to show maps of long-term averages, departures from these averages, and trends through time, for windows of dates over user-specified time frames. Products include line graphs, maps, and data downloads. Other feature under consideration include temperature surfaces other than 0 degrees C, and counts of exceedances above or below specified thresholds.