A 10-Year Climatology of Atmospheric River Conditions within Different Watersheds along the U.S. West Coast and Their Association with Flash Floods

Atmospheric Rivers (ARs) are long (>2000 km), narrow (<1000 km) corridors of strong vertically integrated water vapor transport (IVT) and integrated water vapor (IWV). The landfall of ARs along the U.S. West Coast have been linked to extreme precipitation and flooding in regions of complex topography, and as a result may lead to potentially dangerous flash floods. The twin objectives of this study are to (1) graphically illustrate the relationship between AR conditions and precipitation for a 10-year period (2005–2014) using analyses of IVT magnitude, orientation, and precipitation, and (2) link AR condtions to occurrences of flash floods in watersheds across California, Oregon, and Washington. The IVT magnitude and orientation are computed from the NCEP Climate Forecaste System Reanalysis (CFSR) and the Global Forecast System (GFS) model analyses, whereas precipitation and flash flood data are obtained from the NCEP Stage-IV quantitative precipitation estimate and the NCDC Strom Events Database, respectively. Result from both objectives indicate that extreme precipitation and flash floods occur across different watersheds during landfalling AR events with a preferential IVT vector orientation that is parallel to the upslope terrain gradient and IVT magnitudes that generally exceed 250 kg m–1 s–1 .