Atmospheric Rivers Trigger Heavy Snowmelt in the Western United States
In mountainous regions, both precipitation and snowpack control runoff patterns that are integral to water supply and flood risk. Previous studies examined the roles of precipitation and snowpack individually—but not together. Scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory found that interactions between snowpack and precipitation were critical to runoff in the high-elevation mountains of the western United States. The team also found that precipitation induced by atmospheric rivers (ARs)—long, narrow bands of intense atmospheric moisture transport across the subtropics—caused more snowmelt than non-AR precipitation. ARs that made landfall were responsible for up to 10‒20% of significant snowmelt events in the region, though they accounted for only 2% of the precipitation events involving snowpack (melt or accumulation).
By quantifying the contribution of snowpack to runoff and extreme flooding in mountainous regions in the western United States, researchers provided a unified view of the interactions between snowpack and precipitation. Their analysis revealed regions within the western United States where current water resources management practice can benefit from incorporating snowpack information and underscored the unique footprint of ARs in snowpack and snowmelt. Researchers can apply the framework developed in this study to other mountainous regions where snowpack affects land-surface hydrologic processes.
Precipitation directly contributes to runoff by pushing water through soil, and it can also modulate runoff through its impacts on snow accumulation or melt. To quantify the interaction between snowpack and precipitation across the western United States, researchers used a decades-long, high-resolution regional hydroclimate simulation at 6-kilometer grid spacing. The team compared the magnitude of snowpack and precipitation, which are two components of the surface water budgets, in all precipitation events over existing snowpack. Researchers then classified the precipitation events into four categories based on the roles of snowpack in runoff: light snowmelt, heavy snowmelt, light snowpack accumulation, and heavy snowpack accumulation. This framework showed that precipitation in mountainous regions was mostly correlated with snowpack accumulation. Snowmelt during precipitation was rare and limited to high-elevation areas, but such events were responsible for a considerable amount of runoff and were critical to flooding. Researchers also found that ARs drove significant snowmelt along with heavy precipitation, creating a hydrologic condition more conducive to flooding.
The findings highlight the importance of snowpack response to AR-induced precipitation in high-elevation areas. Given that current water management practice does not sufficiently consider snowpack dynamics, AR-induced snowmelt events pose greater challenges to infrastructure safety. Because mountain snowpack is projected to change under warming, the roles of snowpack in land-surface hydrologic processes warrant further investigation.