Spatial coherence of extreme precipitation events in the Northwestern United States

The complexity of impacts resulting from extreme precipitation events varies with the spatial extent of precipitation extremes. Characteristics of precipitation extremes, defined by the top 5% of 3-day accumulated precipitation, including their spatial coherence and relationships to two contrasting synoptic phenomena, were examined at stations across the Northwestern United States. The spatial coherence of precipitation extremes generally decayed with distance between stations. However, distinct geographic variability in region-wide coherence of precipitation extremes was present with overall higher coherence west of the Cascades, and lower coherence in the lee of the Cascades and Northern Rocky Mountains. Illustrative patterns of coherence were also seen with respect to atmospheric circulation regimes; atmospheric rivers favoured broadly coherent extremes occurring primarily west of the Cascades during fall and winter, while closed lows favoured more isolated precipitation extremes in the lee of the Northern Rocky Mountains and Cascades, occurring primarily during spring. Geographic variability in spatial coherence, direction of maximum coherence and seasonality of extremes suggest that extreme precipitation events result from the interaction of atmospheric circulation and complex topography.