There is great interest in ascertaining the degree of climate change necessary to induce substantial changes in snow accumulation and ablation processes in mountain headwater catchments. Therefore, the response of mountain snow hydrology to changes in air temperature and precipitation was examined by simulating a perturbed climate in Reynolds Mountain East (RME), a headwater catchment with a cool mountain climate in Idaho, USA. The cold regions hydrological model was used to calculate snow accumulation, wind redistribution by blowing snow, interception by forest canopies, sublimation and melt for 25 seasons in RME. The uncalibrated simulations of the highly redistributed snow water equivalent compared well to measurements. Results showed that with concomitant occurrence of warming (5 °C) and precipitation change (±20%) in RME, the peak seasonal snow accumulation decreased by 84–90%, snowmelt decreased 51–79%, rainfall to total precipitation ratio increased from 30% to 78%, and overwinter blowing snow transport and sublimation losses from intercepted snow, the snow surface and blowing snow decreased dramatically. Warming causes an increase in inter‐water year snowcover variability but a decrease in spatial snow accumulation variability. When warming exceeded 1 °C and a precipitation increased by less than 20%, the peak snow accumulation declined dramatically. The results contrast with those from further north along the North American Cordillera in Yukon, Canada, where the impacts of similar warming on alpine snow can be partly compensated for by concomitant increases in precipitation of less than 20%. Copyright © 2015 John Wiley & Sons, Ltd.