Modified AR Scale for Flood Impacts

Atmospheric rivers (ARs) are key drivers of regional water supply and flood risk in subtropical and mid-latitude regions, generating interwoven beneficial and hazardous impacts. The original AR scale, developed for early-warning communication, ranks ARs from 1 (“primarily beneficial”) to 5 (“primarily hazardous”) based on atmospheric vapor transport. However, the AR scale does not account for physical processes on the land surface that can strongly influence flood response. Analyzing over 70,000 AR events across 142 catchments in California and central Chile, here we show that runoff efficiency, primarily controlled by antecedent soil moisture, is the dominant source of peak streamflow variability not explained by the AR scale. Based on this insight, we present a modified AR scale for flood impacts that incorporates antecedent moisture conditions. This modification doubles the scale’s correspondence with peak streamflow and increases the number of floods classified as hazardous by over 25%, raising AR flood detection rates to 87% in California and 72% in central Chile. These findings demonstrate that incorporating critical land surface conditions into hazard classification can enhance early-warning tools for communicating not just hazard presence, but likely impact.