On the Dynamical Causes of Variability in the Rain-Shadow Effect: A Case Study of the Washington Cascades

Abstract

ashington State?s Cascade Mountains exhibit a strong orographic rain shadow, with much wetter western slopes than eastern slopes due to prevailing westerly flow during the winter storm season. There is significant interannual variability in the magnitude of this rain-shadow effect, however, which has important consequences for water resource management, especially where water is a critically limited resource east of the crest. Here the influence of the large-scale circulation on the Cascade rain shadow is investigated using observations from the Snowfall Telemetry (SNOTEL) monitoring network, supplemented by stream gauge measurements. Two orthogonal indices are introduced as a basis set for representing variability in wintertime Cascade precipitation. First, the total precipitation index is a measure of regionwide precipitation and explains the majority of the variance in wintertime precipitation everywhere. Second, the rain-shadow index is a measure of the east?west precipitation gradient. It explains up to 31% of the variance west and east of the crest. A significant correlation is found between the winter-mean rain shadow and ENSO, with weak (strong) rain shadows associated with El Niño (La Niña). The analysis is supported by streamflow data from eastern and western watersheds. A preliminary review of individual storms suggests that the strongest rain shadows are associated with warm-sector events, while the weakest rain shadows occur during warm-frontal passages. This is consistent with known changes in storm tracks associated with ENSO, and a variety of mechanisms likely contribute.