Atmospheric Rivers and Insured Flood Losses in the Western United States

This research quantifies the economic impacts of flooding due to atmospheric river (AR) events in the western United States from 1978 to 2017, using National Flood Insurance Program (NFIP) claims and loss data, and National Weather Service data on total flood damages. ARs are responsible for much of the intra-seasonal, inter-annual and longer-scale variability in west coast hydrology and many of the precipitation extremes in the West. Comparable to hurricanes and tropical storms in the eastern United States, they also drive many of the region's foods. Analyses of the NFIP data confirm that AR-related floods cause significant economic damages and constitute the primary source of insurance claims and flood losses in the western coastal states. Approximately half of all AR events in the sample resulted in claims and losses. Total mean AR impacts were on the order of $28m per event, with extreme AR events causing damages in excess of $1b every three to seven years. Peak damages generally occurred the day after landfall; the most significant events lasted several days, with losses distributed over wide spatial areas covering multiple basins. AR events oriented southwest-to-northeast caused the greatest damages, with losses often occurring to the northeast of initial landfall. The spatial and temporal structure of flood damages depended significantly on integrated vapor transport (IVT), exposure to flood risk, and antecedent soil moisture conditions. These results stress the importance of understanding ARs to inform policies to mitigate flood losses and respond to future flood disaster scenarios in the western United States.