Doppler Radar Analysis of a Snake River Plain Convergence Event

Abstract

A convergence zone periodically forms in the Snake River plain (SRP) of eastern Idaho as a result of terrain-induced boundary layer flow under synoptic northwesterly flow at low and midlevels. Complex terrain in central and eastern Idaho is critical to the formation of this mesoscale feature, which is characterized by wind shift lines accompanied by cumulus or stratocumulus bands. These cloud bands routinely produce virga or light rain or light snow and, occasionally, result in moderate to heavy precipitation. Doppler weather radar analysis of a convergence event that produced heavy postfrontal snowfall in the lower SRP on 26 November 1995 is presented. This event was selected for study because significant snowfall resulted from a persistent and well-defined boundary layer convergence zone. The high-resolution (40 km) Eta Model data available from the National Centers for Environmental Prediction did not sufficiently resolve the mesoscale processes that initiated this event. The Eta Model indicated postfrontal clearing in the SRP, generally northwest winds, and light precipitation amounts limited to higher terrain east of the SRP. The Springfield, Idaho, WSR-88D provided crucial real-time data to the forecaster who identified the convergence zone and issued short-term forecasts and a snow advisory for the affected area during this event. WSR-88D radial velocity and local mesonet data defined the region of convergence, and composite reflectivity data showed the development, persistence, and dissipation of snow within the convergence region.