Influence of Recent Major Atmospheric River Events on Snowpack in Western Washington

Mid-latitude, east Pacific cyclones, with attached atmospheric rivers (ARs), are important in producing extreme, basin-wide precipitation events along the west coast of the U.S. and Canada. ARs are narrow corridors, less than about 1,000 km wide, of concentrated low-level water vapor, extending more than about 2,000 km long (Bao et al., 2006). After making landfall, ARs often produce abundant orographic rainfall when intersecting mountainous terrain. They are a major component in linking weather with climate, as they can produce a relatively high amount of the annual winter precipitation in a few days. ARs play an important role in water management, flood control, and water supply, as well as public safety. Studies indicate ARs generally increase snowpack through California, but in western Washington their influence generally causes minor decreases in snow water equivalent due to generally lower terrain and relatively higher AR snow levels. This paper explains the nature of recent major AR weather events (WY 2004 - 2009) and examines their effect on the snowpack in western Washington.