The Influence of Landfalling Atmospheric Rivers on U.S. West Coast Precipitation during February 2014

Atmospheric Rivers (ARs) are long narrow corridors of high vertically integrated water vapor (IWV) and high integrated water vapor transport (IVT). The regions of high IWV and IVT are typically <1000 km wide and >2000 km long. ARs represent the majority of extratropical moisture transport and contribute to extreme precipitation and flooding along the U.S. West Coast. Several ARs made landfall along the U.S. West Coast during February 2014 and contributed a significant percentage of water year precipitation during an anomalously dry winter. This study investigates the role that landfalling ARs played in producing precipitation throughout the western United States during February 2014 as well as their influence on widespread drought after a long dry period. NCEP GFS model analyses and NCEP Stage IV precipitation data were used to monitor the lifetimes of ARs and their effect on precipitation production, respectively. In total, twelve ARs made landfall along the U.S. West Coast and produced locally up to 900 mm of precipitation during the month. Strong correlations between daily average IWV/IVT and daily precipitation were found. The analyses indicate that IVT vector orientation with respect to orography plays a large role in precipitation production. If IVT vectors were oriented roughly perpendicular to orography then significant topographic enhancement of precipitation occurred, while little to no precipitation occurred during events where IVT vectors were oriented parallel to orography. Further analysis shall demonstrate that these twelve ARs contain different moisture source regions with respective differences in the water vapor budget along each AR prior to landfall.