Summertime rainfall events in eastern Oregon and Washington

Rainfall events in eastern Washington and northeastern Oregon during the summers (Jul-Aug-Sep) of 1948-2008 have been analyzed for their temporal and spatial characteristics, including those of the regional atmospheric circulation anomalies (500 hPa geopotential height anomaly patterns) at the times of the events. Defined here as days with rainfall exceeding 0.25”, these events occur on average about 2-3 times a summer, with a minimum in frequency in late July, at locations along the forested flanks of the terrain surrounding the lower elevation of the Columbia Basin, where, it is the authors' understanding, the occurrence or absence of such events, in a given year, can impact the duration of the “fire season”. Inspection of the regional atmospheric circulation anomaly conditions at the times of the events suggest that they can be grouped broadly into two main types; one distinguished by anomalous westerly flow and conditions in many respects similar to those previously known to bring heavy wintertime rainfall to the western flanks of the Washington and Oregon Cascades, and another characterized by a south-easterly to easterly component to the anomalous flow. A relatively high percentage of the events in the northern portion of the domain of interest, and over the higher terrain, are westerly flow-type; a high percentage of the events in the southern and lower elevation portions of the domain are easterly-flow type. Precipitation events in the latter category are much more likely to be accompanied by lightning and can show larger year-to-year changes in frequency than the westerly flow-type events, which show strong seasonal variations in frequency.