Flooding in Western Washington: The Connection to Atmospheric Rivers**.

This study utilizes multiple decades of daily streamflow data gathered in four major watersheds in western Washington to determine the meteorological conditions most likely to cause flooding in those watersheds. Two are located in the Olympic Mountains and the other two in the western Cascades; and each has uniquely different topographic characteristics. The flood analysis is based on the maximum daily flow observed during each water year (WY) at each site [i.e., the annual peak daily flow (APDF)], with an initial emphasis on the 12 most recent water years between WY1998 and 2009, and then focusing on a 30-year interval between WY1980 and 2009. The shorter time period coincides with relatively complete passive microwave satellite coverage of integrated water vapor (IWV) over the Pacific basin. The combination of IWV imagery and streamflow data highlights a close link between landfalling atmospheric rivers (ARs) and APDFs (i.e., 46 of the 48 APDFs occurred with landfalling ARs). To complement this approach, the three-decade time series of APDFs, which correspond to the availability of the North American Regional Reanalysis (NARR) dataset, are examined. The APDFs occur most often, and are typically largest in magnitude, from November to January. The NARR is used to assess the composite meteorological conditions associated with the 10 largest APDFs at each site during this 30-year period. Heavy precipitation fell during the top 10 APDFs, and anomalously high composite NARR melting levels averaged ~1.9 km MSL, which is primarily above the four basins of interest. Hence, on average, mostly rain rather than snow fell within these basins, leading to enhanced runoff. The flooding on the four watersheds shared common meteorological attributes, including the presence of landfalling ARs with anomalous warmth, strong low-level water vapor fluxes, and weak static stability. There were also key differences that modulated the orographic control of precipitation. Notably, two watersheds experienced their top 10 APDFs when the low-level flow was southwesterly, while the other two basins had their largest APDFs with west-southwesterly flow. These differences arose because of the region's complex topography, basin orientations, and related rain shadowing. [ABSTRACT FROM AUTHOR]