On the Dynamical Causes of Variability in the Rain-shadow Effect: a Case Study of the Washington Cascades

Washington 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 SNOTEL monitoring network, supplemented by stream gauge measurements. Two orthogonal indices are introduced as a basis set for representing variability in wintertime (DJF) Cascade precipitation. First, the Total Precipitation Index is a measure of region-wide precipitation and explains the majority of the variance in DJF 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. The strength of the winter-mean rain shadow is found to significantly correlate with ENSO. The analysis is supported by stream-flow 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 for warm frontal passages. This is consistent with known changes in storm tracks associated with ENSO, and a variety of mechanisms likely contribute.