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Climate warming threatens snowmelt‐derived water supplies in the western US (WUS) by reducing snowfall and snowmelt runoff, yet future rates of these declines remain highly uncertain in an evolving climate. Here, we analyze historical data, land surface model warming experiments, and climate projections across three major WUS river basins. We find that runoff loss become less sensitive to warming as snowpack shrinks, stemming from reduced snowmelt‐radiation feedback, a consequence of smaller snow‐cover changes and shifts in snowmelt timing to lower‐energy periods. Near‐linear projected warming with time (IPCC SSP245) exhibit a stable, possibly decelerating decline in runoff ratios. Although decelerating runoff declines do not eliminate broader water‐management challenges under continued warming, our findings complement the view that snowmelt‐radiation feedback drives runoff decline by highlighting the negative feedback from a shrinking snowpack on runoff warming sensitivity. Our findings should facilitate more comprehensive future water supply assessments in snow‐affected regions. Plain Language Summary Studying runoff sensitivity to warming helps us understand how water availability will change as global temperatures rise. Current understanding of runoff sensitivity mainly relies on long‐term static quantifications, which may overlook evolving sensitivities and future uncertainties. Our study shows that as snow‐cover shrinks in a warming world, runoff becomes less sensitive to further temperature increases. Specifically, in warmer conditions, snowmelt shifts to colder months with less sunlight, reducing the energy available for evaporation, while diminished snow cover limits albedo changes and further lowers energy absorption. These combined effects reduce (further) runoff declines, revealing a negative feedback that challenges static theories that predicting a constant acceleration of runoff loss with warming. Our findings highlight this nonlinearity, offering a more realistic and comprehensive view of water supply uncertainty in snow‐affected regions as the climate warms. Key Points Sensitivities of runoff, snow, and evapotranspiration to warming decrease as temperatures warm for three major western US river basins Responses of runoff loss to warming are generally weaker in subbasins with less snow Decelerated snow cover shrinkage and earlier melt timing weaken snowmelt‐radiation feedback, decreasing runoff sensitivity to warming

Atmospheric rivers (ARs) play vital roles in the western United States and related regions globally, not only producing heavy precipitation and flooding, but also providing beneficial water supply. This paper introduces a scale for the intensity and impacts of ARs. Its utility may be greatest where ARs are the most impactful storm type and hurricanes, nor’easters, and tornadoes are nearly nonexistent. Two parameters dominate the hydrologic outcomes and impacts of ARs: vertically integrated water vapor transport (IVT) and AR duration [i.e., the duration of at least minimal AR conditions (IVT ≥ 250 kg m−1 s−1)]. The scale uses an observed or predicted time series of IVT at a given geographic location and is based on the maximum IVT and AR duration at that point during an AR event. AR categories 1–5 are defined by thresholds for maximum IVT (3-h average) of 250, 500, 750, 1,000, and 1,250 kg m−1 s−1, and by IVT exceeding 250 kg m−1 s−1 continuously for 24–48 h. If the AR event duration is less than 24 h, it is downgraded by one category. If it is longer than 48 h, it is upgraded one category. The scale recognizes that weak ARs are often mostly beneficial because they can enhance water supply and snowpack, while stronger ARs can become mostly hazardous, for example, if they strike an area with antecedent conditions that enhance vulnerability, such as burn scars or wet conditions. Extended durations can enhance impacts. Short durations can mitigate impacts.

Streamflow in high-mountain Asia is influenced by meltwater from snow and glaciers, and determining impacts of climate change on the region’s cryosphere is essential to understand future water supply. Past and future changes in seasonal snow are of particular interest, as specifics at the scale of the full region are largely unknown. Here we combine models with observations to show that regional snowmelt is a more important contributor to streamflow than glacier melt, that snowmelt magnitude and timing changed considerably during 1979–2019 and that future snow meltwater supply may decrease drastically. The expected changes are strongly dependent on the degree of climate change, however, and large variations exist among river basins. The projected response of snowmelt to climate change indicates that to sustain the important seasonal buffering role of the snowpacks in high-mountain Asia, it is imperative to limit future climate change.High-mountain Asia streamflow is strongly impacted by snow and glacier melt. A regional model, combined with observations and climate projections, shows snowmelt decreased during 1979–2019 and was more dominant than glacier melt, and projections suggest declines that vary by river basin.

Climate change will impact western USA water supplies by shifting precipitation from snow to rain and driving snowmelt earlier in the season. However, changes at the regional-to-mountain scale is still a major topic of interest. This study addresses the impacts of climate change on mountain snowpack by assessing historical and projected variable-resolution (VR) climate simulations in the community earth system model (VR-CESM) forced by prescribed sea-surface temperatures along with widely used regional downscaling techniques, the coupled model intercomparison projects phase 5 bias corrected and statistically downscaled (CMIP5-BCSD) and the North American regional climate change assessment program (NARCCAP). The multi-model RCP8.5 scenario analysis of winter season SWE for western USA mountains indicates by 2040-2065 mean SWE could decrease −19% (NARCCAP) to −38% (VR-CESM), with an ensemble median change of −27%. Contrary to CMIP5-BCSD and NARCCAP, VR-CESM highlights a more pessimistic outcome for western USA mountain snowpack in latter-parts of the 21st century. This is related to temperature changes altering the snow-albedo feedback, snowpack storage, and precipitation phase, but may indicate that VR-CESM resolves more physically consistent elevational effects lacking in statistically downscaled datasets and teleconnections that are not captured in limited area models. Overall, VR-CESM projects by 2075–2100 that average western USA mountain snowfall decreases by −30%, snow cover by −44%, SWE by −69%, and average surface temperature increase of +5.0 ∘C. This places pressure on western USA states to preemptively invest in climate adaptation measures such as alternative water storage, water use efficiency, and reassess reservoir storage operations.

Climate warming induces shifts from snow to rain in cold regions 1 , altering snowpack dynamics with consequent impacts on streamflow that raise challenges to many aspects of ecosystem services 2 , 3 – 4 . A straightforward conceptual model states that as the fraction of precipitation falling as snow (snowfall fraction) declines, less solid water is stored over the winter and both snowmelt and streamflow shift earlier in season. Yet the responses of streamflow patterns to shifts in snowfall fraction remain uncertain 5 , 6 , 7 , 8 – 9 . Here we show that as snowfall fraction declines, the timing of the centre of streamflow mass may be advanced or delayed. Our results, based on analysis of 1950–2020 streamflow measurements across 3,049 snow-affected catchments over the Northern Hemisphere, show that mean snowfall fraction modulates the seasonal response to reductions in snowfall fraction. Specifically, temporal changes in streamflow timing with declining snowfall fraction reveal a gradient from earlier streamflow in snow-rich catchments to delayed streamflow in less snowy catchments. Furthermore, interannual variability of streamflow timing and seasonal variation increase as snowfall fraction decreases across both space and time. Our findings revise the ‘less snow equals earlier streamflow’ heuristic and instead point towards a complex evolution of seasonal streamflow regimes in a snow-dwindling world. Analysis of streamflow measurements from 1950 to 2020 across 3,049 snow-affected catchments over the Northern Hemisphere shows that seasonal streamflow occurs earlier in snow-heavy catchments but later in less snowy regions.

Hydrologic and land surface models require spatiotemporally complete and accurate hydrometeorological forcings. In mountainous regions, hydrometeorological forcings are often obtained as the output of coupled land‐atmosphere models, like the Weather Research and Forecasting (WRF) model, configured to run at spatial scales that permit orographic convection (e.g., ≤${\\le} $ 4 km). Models like WRF, however, require physical parameterizations, the selection of which significantly influences model predictions of precipitation, temperature, and radiant fluxes used as input to hydrologic and land surface models. Here we investigate the impact of two critical aspects of WRF configurations, namely the selection of the cloud microphysics parameterization and lateral boundary conditions, on modeled hydrometeorological forcings and associated snow conditions in a mountainous region of the western United States. We conducted eight experiments with WRF configured at convection‐permitting scales using two reanalysis data sets as lateral boundary conditions (ERA5 and CFSRv2) and four alternative cloud microphysics schemes. These experiments reveal that the choice of lateral boundary conditions and cloud microphysics schemes imposes substantial variability in simulated surface hydrometeorological conditions, with precipitation and radiation emerging as key factors. When compared to the accumulated precipitation average over the Snow Telemetry (SNOTEL) stations, the relative bias in precipitation across experiments ranges from −18.15% to +15.48%. These biases impact the land surface model, leading to discrepancies in modeled snow. The relative bias in snow water equivalent compared to the SNOTEL average ranges from −39.84% to 10.72%, while for snow depth, it falls between −37.72% and 0.32%. Further comparisons of annual snow fraction and snow disappearance date (SDD) with Moderate Resolution Imaging Spectroradiometer (MODIS) reveal a consistent overestimation at higher elevations, with snow persisting beyond the MODIS SDD. These findings highlight the critical role of model configuration in improving hydrometeorological forcings and enhancing hydrologic predictions in complex terrain.

Years-long global storm-resolving model (GSRM) simulations of the present-day and warmed climates are conducted with 3.25-km horizontal grid spacing. We focus on wintertime precipitation in the coastal western United States, a notably water-sensitive region with complex topography. The model generates more realistic orographic precipitation compared with a coarser-resolution model having the same dynamical core. In response to uniform sea surface warming, the increase in extreme precipitation rates together with the better resolved orographic precipitation lead to persistence of snowpack in parts of the coastal western United States, modulated by shifts in the larger-scale circulations. In contrast, snowpack in the coarser-resolution model is largely eliminated in the warmed climate. Whether snowpack persists influences the regional surface energy budget due to the snow-albedo feedback. The results highlight the value of consistently representing both local orographic circulations and the large-scale circulation responses to warming, which is provided by a GSRM but not by either conventional climate models or regional downscaling approaches.

Snowpack stores cold-season precipitation to meet warm-season water demand. Climate change threatens to disturb this balance by altering the fraction of precipitation falling as snow and the timing of snowmelt, which may have profound effects on food production in basins where irrigated agriculture relies heavily on snowmelt runoff. Here, we analyse global patterns of snowmelt and agricultural water uses to identify regions and crops that are most dependent on snowmelt water resources. We find hotspots primarily in high-mountain Asia (the Tibetan Plateau), Central Asia, western Russia, western US and the southern Andes. Using projections of sub-annual runoff under warming scenarios, we identify the basins most at risk from changing snowmelt patterns, where up to 40% of irrigation demand must be met by new alternative water supplies under a 4 °C warming scenario. Our results highlight basins and crops where adaptation of water management and agricultural systems may be especially critical in a changing climate.Snowmelt runoff is an important source of water for irrigating agricultural crops in high-mountain Asia, Central Asia, western Russia, western US and the southern Andes. Climate change places water resources in these basins at risk, indicating the need to adapt water management.

Western United States wildfire increases have been generally attributed to warming temperatures, either through effects on winter snowpack or summer evaporation. However, near-surface air temperature and evaporative demand are strongly influenced by moisture availability and these interactions and their role in regulating fire activity have never been fully explored. Here we show that previously unnoted declines in summer precipitation from 1979 to 2016 across 31–45% of the forested areas in the western United States are strongly associated with burned area variations. The number of wetting rain days (WRD; days with precipitation ≥2.54 mm) during the fire season partially regulated the temperature and subsequent vapor pressure deficit (VPD) previously implicated as a primary driver of annual wildfire area burned. We use path analysis to decompose the relative influence of declining snowpack, rising temperatures, and declining precipitation on observed fire activity increases. After accounting for interactions, the net effect of WRD anomalies on wildfire area burned was more than 2.5 times greater than the net effect of VPD, and both the WRD and VPD effects were substantially greater than the influence of winter snowpack. These results suggest that precipitation during the fire season exerts the strongest control on burned area either directly through its wetting effects or indirectly through feedbacks to VPD. If these trends persist, decreases in summer precipitation and the associated summertime aridity increases would lead to more burned area across the western United States with farreaching ecological and socioeconomic impacts.