Historical and National Perspectives on Extreme West Coast Precipitation Associated with Atmospheric Rivers during December 2010

Strong winter storms battered the U.S. West Coast from Western Washington to Southern California in December 2010, producing as much as 250–670 mm in mountainous areas. A common denominator among these events is that the synoptic weather patterns produced a series of strong atmospheric rivers (ARs) that transported large amounts of water vapor from over the Pacific Ocean to the U.S. West Coast. These ARs fueled the heavy rain and flooding, and provided beneficial increases in snowpack. For example, the Southern Sierra snowpack increased from 27% of 1 April normal snowpack on 16 December 2010 to 73% by 22 December—the first full day of winter. The season was well on its way to one of the deepest annual snowpacks ever recorded.

Just how “extreme” were these events relative to other atmospheric river cases in the region? More generally, how does West Coast AR-fed precipitation compare with extreme precipitation in other parts of the United States, such as from landfalling hurricanes and tropical storms? This report uses decades of Cooperative Observer (COOP) daily precipitation reports from more than 5,800 stations across the United States to address these questions and then summarizes the West Coast events and forecasts of December 2010.