RELATIONSHIPS BETWEEN 700 HPA HEIGHT ANOMALIES AND 1 APRIL SNOWPACK ACCUMULATIONS IN THE WESTERN USA

Relationships between atmospheric circulation and the temporal and spa tial distributions of snowpack accumulations in the western USA are ex amined. Winter mean 700 hPa height anomalies, representing the average atmospheric circulation during the snow season, are compared with sno wpack measurements made on or about 1 April at 311 snowcourse stations in the western USA during the winters of 1947-1948 through 1986-1987. Correlation and anomaly pattern analysis are used to identify relatio ns between atmospheric circulation and the temporal and spatial distri butions of snowpack accumulations, and to quantify the degree to which the temporal and spatial variability in snowpack accumulations can be attributed to variations in atmospheric circulation. Results indicate that winter mean 700 hPa height anomalies account for a statistically significant portion of both the temporal and spatial variability in t he snowpack accumulations. In general, above-average snowpack accumula tions are associated with negative 700 hPa height anomalies over the e astern North Pacific Ocean and the western USA. These anomalies are in dicative of anomalous cyclonic circulation, which produces an anomalou s westerly flow of moist air from the eastern North Pacific Ocean into the western USA and increases winter precipitation and snowpack accum ulations. Below-average snowpack accumulations at most of the snowcour se stations are associated with positive 700 hPa height anomalies over the western USA. These positive anomalies indicate anomalous anticycl onic circulation which prevents the intrusion of moist air from the ea stern North Pacific Ocean into the western USA, increases subsidence, and decreases winter precipitation. Five winter mean 700 hPa height an omaly patterns also were identified that explain the spatial variabili ty in snowpack accumulations.