An 11-Year Climatology of Atmospheric River Conditions within Different Watersheds along California and Their Association with Floods and Flash Floods

Atmospheric Rivers (ARs) are long (>2000 km), narrow (<1000 km) corridors of enhanced vertically integrated water vapor (IWV) and enhanced IWV transport (IVT). The landfall of ARs along the U.S. West Coast have been linked to extreme precipitation and may lead to potentially dangerous floods and flash floods. The three objectives of this study are to (1) graphically illustrate the relationship between AR conditions and precipitation for an 11-year period (2004–2014) using analyses of IVT magnitude, orientation, and precipitation (2) link AR conditions to occurrences of floods and flash floods in watersheds in California, and (3) to create an analog forecasting tool that creates 72-h climatological precipitation distributions for 72-h forecast IVT magnitude and direction for various watersheds across California in order to potentially inform flood and flash flood risks. Results from objective (1) indicate a strong correlation between 72-h averaged IVT and 72-h precipitation, with the most significant rain events occurring when IVT magnitudes exceed 250 kg m–1 s–1 and IVT direction is parallel to the upslope terrain gradient. Results from objective (2) indicate that AR conditions are present on all days with flooding in northern California and on cool-season days with flash flooding in southern California, whereas warm-season days with flash flooding in southern California are primarily associated with the monsoon circulation Results from objective (3) indicate that historic precipitation distributions associated with landfalling ARs created from threshold values of IVT magnitude and directions may be used to inform model precipitation forecasts and flood/flash flood risk.