Forest fires increase vulnerability to midwinter rain-on-snow snowmelt in the western Oregon Cascades

Abstract Forest fires and rain-on-snow events in the seasonal snow zone of the Pacific Northwest are increasing in frequency and magnitude, yet the combined impacts of these events on snowpack and water resources remain poorly understood. We show that forest fires doubled midwinter snowmelt proportions in 2023 and 2024 compared to unburned reference sites in the western Oregon Cascades. Data from snow monitoring and micrometeorological stations installed across an elevational gradient revealed increased snowpack vulnerability during midwinter rain-on-snow events, where we observed more snowmelt per rain-on-snow event in the burned forest, indicating higher vulnerability of these snowpacks to rapid melt, increasing downstream flood risk. The most vulnerable snowpacks exist at mid elevations, where rain-on-snow induced snowmelt accounted for 26% more of total annual melt at the burned compared to the unburned sites. Enhanced net snowpack energy, dominated by longwave radiation at lower and mid elevations, indicates minimal buffering capacity of these already vulnerable snowpacks in high biomass burned forests to increasingly common rain-on-snow events. These findings underscore critical challenges for water managers balancing flood preparation and snowmelt-dependent water storage in a warming climate.