Extreme Wet Spells in the Upper Colorado River Basin during the Cool Season

Abstract Multiday periods of precipitation resulting in extreme accumulations can make substantial contributions to cool-season precipitation and snowfall in the upper Colorado River basin (UCRB) and, thereby, significantly impact regional water supply. This study conducts a climatological (1981–2023) investigation of these periods, termed extreme wet spells (EWSs), over the UCRB during the cool season (October–April). These spells tend to be long lasting (>5 days) and occur most frequently at high elevations in the basin. EWSs can account for appreciable portions (up to 20%–30%) of seasonal precipitation and snowfall accumulations in the UCRB, with EWS-related precipitation explaining 65% of the interannual variance in the basin-integrated cool-season precipitation totals. Examination of relationships to four large-scale North Pacific–North America weather regimes reveals that EWSs predominantly occur in two regimes characterized by an anomalous ridge upstream over the eastern Pacific. Composite analyses conditioned on regime type reveal four distinct synoptic-scale patterns associated with EWSs, each featuring an upper-level trough over the western United States. In three of the regimes, this trough tends to be associated with anomalous water vapor transport within atmospheric rivers extending into the UCRB from the eastern Pacific. Compared with nonextreme wet spells, EWSs are characterized by more persistent synoptic-scale conditions and by a greater tendency to involve the occurrence of multiple upper-level troughs and atmospheric rivers in rapid succession over the western United States. The persistent conditions during EWSs are related to a tendency for these spells to involve atmospheric blocking. Significance Statement This study investigates extreme wet spells—periods of precipitation that result in exceptional accumulations—over the upper Colorado River basin during the cool season. These spells contribute substantially to cool-season precipitation totals and snowfall accumulations in the basin, especially at high-altitude locations, and, thus, exert a strong influence on the regional hydroclimate. Extreme wet spells typically last for >5 days and tend to occur in persistent weather patterns that feature the successive passage of multiple storm systems, often accompanied by inland moisture transport from the Pacific, over the western United States. The findings of this study may have utility for prediction and could provide context for future research on precipitation processes and predictability in the upper Colorado River basin.