Atmospheric rivers drive flood damages in the western United States

Abstract

Atmospheric rivers (ARs) are extratropical storms that produce extreme precipitation on the west coasts of the world's major landmasses. In the United States, ARs cause significant flooding, yet their economic impacts have not been quantified. Here, using 40 years of data from the National Flood Insurance Program, we show that ARs are the primary drivers of flood damages in the western United States. Using a recently developed AR scale, which varies from category 1 to 5, we find that flood damages increase exponentially with AR intensity and duration: Each increase in category corresponds to a roughly 10-fold increase in damages. Category 4 and 5 ARs cause median damages in the tens and hundreds of millions of dollars, respectively. Rising population, increased development, and climate change are expected to worsen the risk of AR-driven flood damage in future decades.

Fig. 1. An AR on 9 January 1995 caused substantial damages on the west coast of the United States.

(A) 4 National Center of Environmental Prediction–National Center for Atmospheric Research (NCEP-NCAR) precipitable water preceded peak damages in California. (B) Maximum coastal IVT (given here over the entire west coast), generated peak precipitation over Sonoma County (see Materials and Methods), streamflow at the U.S. Geological Survey Guerneville gauge, and total insured losses in Sonoma County. (C) Flood insurance claims along the lower reach of the Russian River in Sonoma County are marked as red dots; the 100-year flood plain is indicated in blue.

Fig. 2. From 1978 to 2017, ARs accounted for 84.2% of all insured flood losses in the 11 western states across all seasons.

In many areas in coastal northern California and the Pacific Northwest, ARs accounted for over 95% of insured flood losses.

Fig. 3. Flood damages increase exponentially with AR category.

(A) The Ralph et al. AR scale (20) classifies ARs into five categories depending on IVT and duration. For example, an AR with a peak IVT of 800 kg m⁻¹ s⁻¹ and a duration of 78 hours is classified as a category 4 or extreme AR. (B) NDJFM flood damages increase exponentially with AR category: Vertical scale is logarithmic; black bars are medians; boxes are 25th and 75th percentiles, and whiskers are 5th and 95th percentiles; dots are extrema; numbers in parentheses are the number of NDJFM events in each category. Note that the total number of ARs in the Gershunov AR catalog (1603) is greater than the number of ARs over the sample period using the Ralph et al. AR scale (1134), which classifies events with a duration of less than 24 hours as non-ARs.