Summertime Rainfall Events in Eastern Washington and Oregon

Abstract

he temporal and spatial characteristics of summertime rainfall events in the Pacific Northwest are examined in relation to the prevailing regional 500-hPa geopotential height conditions, with focus on the forested slopes of eastern Washington and northeastern Oregon, where the absence/occurrence of events largely determines the start and end of the wildland fire season. The Daily U.S. Unified Precipitation dataset is used for specifying rainfall events (period 1949?2008). Events are defined as one or more consecutive days of rainfall exceeding 0.25 in. (0.65 mm), and occur on average two to three times per summer (July?September) in the focus region, east of the Cascade Mountain crest, with a minimum in frequency in late July. A relatively high percentage of the events in the northern portion of the domain of interest, and over the higher terrain, is associated with anomalous midtropospheric southwesterly flow; a high percentage of the events in the southern and lower elevation portions of the domain is associated with southeasterly flow anomalies. Southeasterly flow events are much more likely to be accompanied by lightning and a more localized rainfall distribution than southwesterly events. Southwesterly events mainly account for the late-July frequency minimum and produce more widespread/heavier precipitation on average. The forests of eastern Washington and Oregon receive a mix of southeasterly and southwesterly events. Results suggest that identifying flow types by (skillful) extended-range 500-hPa forecasts may provide a useful basis for predicting the associated aspects of the rainfall event distribution.