Genesis Locations of the Costliest Atmospheric Rivers Impacting the Western United States

Atmospheric rivers (ARs) are responsible for the vast majority (similar to 88%) of flood damage in the Western U.S., an annual average of USD$1.1 billion. Here, using historical flood insurance data, we investigate the genesis characteristics of damaging ARs in the Western U.S.. ARs exceeding USD$20 million in damage (90th percentile), are shown to travel further across the Pacific Ocean, with median genesis locations 8 degrees -27 degrees further westward compared to typical ARs. Identifying regions of preferential genesis of damaging ARs elicit important implications for AR observation campaigns, highlighting distant regions not currently considered for AR reconnaissance. The genesis of damaging ARs is associated with elevated upper-level zonal wind speeds along with deeper cyclonic anomalies, most pronounced for Washington and Oregon ARs. Linking AR dynamics and lifecycle characteristics to economic damage provides an opportunity for impact-based forecasting of ARs prior to landfall, supporting efforts to mitigate extreme flood damages. Plain Language Summary We examine the economic impact of atmospheric rivers (ARs), corridors of elevated atmospheric moisture vapor transport, on the Western U.S.. Atmospheric rivers are responsible for the vast majority of flood damage which is experienced in the Western U.S. causing mean annual damages of USD$1.1 billion, 88% of the total annual flood damage. We use 40 years of historic flood insurance claims paired with a database of all ARs that make landfall on the U.S. West Coast to show, for the first time, the relationship between atmospheric river dynamics and the economic impact for the Western. U.S.. Our results show that damaging ARs tend to originate further from the U.S. coastline than all other, less damaging ARs. We also demonstrate the atmospheric conditions favorable for the initiation of damaging ARs with a particular focus on ARs that travel across the entire Pacific Ocean prior to landfall. Clear implications arise for atmospheric river data collection, identifying important regions which are not current examined where damaging ARs generate. The results presented aid in understanding the cause of damaging floods in the Western U.S. with the aim of supporting atmospheric river forecasts by linking atmospheric conditions to historic damage. Key Points Historical flood damages from the Western U.S. are paired with an atmospheric river lifecycle tracking algorithm Damaging atmospheric rivers (ARs) tend to have genesis locations further from the coastline, travel further and have a higher moisture flux The genesis of damaging ARs is associated with distinct dipole pressure anomalies paired with an elevated zonal jet stream