Snow Droughts are Expanding While Deluges Shrink: The Importance of Spatial Extent in the Evaluation of Snow Extremes

An extensive body of literature has documented declines in western U.S. snow, including increases in snow drought (extreme low snow), but the spatial extent of snow extremes may also substantially impact the consequences of snow losses and has been relatively underexamined. Here, we use a gridded snow reanalysis across the western U.S. spanning 1982-2023 to examine drivers and trends in the spatial extents of snow anomalies. The largest snow drought extents occur in warm/dry conditions, while the largest deluge extents occur in cool/wet conditions. Like other drought types, severity generally varies inversely with spatial extent. El Niño conditions generate significantly larger snow drought extents than deluge (p < 0.05); the converse is true but not statistically significant for La Niña. Across the western U.S., the annual extent of snow drought in December has increased by 5.6% of the snow-receiving area per decade, while December deluge extent has decreased by 5.8% of area per decade. Trends in January-March are significant but smaller in magnitude. Trends also vary by extremity: the most extreme snow droughts are increasing significantly in February-May, while less extreme snow droughts are increasing significantly in December-January. Finally, significant winter declines in deluge extent are more widespread than drought increases, suggesting that declines in deluge merit further attention. These findings suggest that analyses focusing only on spring snow and drought, not deluge, may miss important trends in snow extremes. If observed trends continue, the increasingly widespread snow droughts and less widespread deluges documented here could exacerbate the impacts of snow loss for interconnected water, energy, and land management systems.