Relationships between 700 hPa height anomalies and 1 April snowpack accumulations in the western USA
Relationships between atmospheric circulation and the temporal and spatial distributions of snowpack accumulations in the western USA are examined. Winter mean 700 hPa height anomalies, representing the average atmospheric circulation during the snow season, are compared with snowpack measurements made on or about 1 April at 311 snowcourse 7stations in the western USA during the winters of 1947–1948 through 1986–1987. Correlation and anomaly pattern analysis are used to identify relations between atmospheric circulation and the temporal and spatial distributions 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 the snowpack accumulations. In general, above-average snowpack accumulations are associated with negative 700 hPa height anomalies over the eastern North Pacific Ocean and the western USA. These anomalies are indicative of anomalous cyclonic circulation, which produces an anomalous westerly flow of moist air from the eastern North Pacific Ocean into the western USA and increases winter precipitation and snowpack accumulations. Below-average snowpack accumulations at most of the snowcourse stations are associated with positive 700 hPa height anomalies over the western USA. These positive anomalies indicate anomalous anticyclonic circulation which prevents the intrusion of moist air from the eastern North Pacific Ocean into the western USA, increases subsidence, and decreases winter precipitation. Five winter mean 700 hPa height anomaly patterns also were identified that explain the spatial variability in snowpack accumulations.