Atmospheric rivers are responsible for most flooding on the West Coast of the U.S., but they also bring much-needed moisture to the region. The size of these storms doesn’t always translate to flood risk, however, as other factors on the ground play important roles. Now, a new study helps untangle the other drivers of flooding to help communities and water managers better prepare. The research, published June 4 in the Journal of Hydrometeorology, analyzed more than 43,000 atmospheric river storms across 122 watersheds on the West Coast between 1980 and 2023. The researchers found that one of the primary driving forces of flooding is wet soils that can’t absorb more water when a storm hits. They showed that flood peaks were 2–4.5 times higher, on average, when soils were already wet. These findings can help explain why some atmospheric river storms cause catastrophic flooding while others of comparable intensity do not.