The sensitivity of snowmelt-driven water supply to climate variability and change is difficult to assess in the mountain west, where strong climatic gradients coupled with complex topography are sampled by sparse ground measurements. We developed a model which ingests daily satellite imagery and meteorological data and is suitable for areas >1000 km2, yet captures spatial variability of snow accumulation and melt in steep mountain terrain. We applied the model for the years 2000-2008 to a 2900 km2 snowmelt-dominated watershed in NW Montana. We found that >25% of the basin's snow water equivalent (SWE) accumulates above the highest measurement station and >70% accumulates above the mean elevation of surrounding SNOTEL stations. Consequently, scaling point measurements of SWE to describe basin conditions could lead to significant misrepresentation of basin snow. Simulations imply that present-day temperature variability causes measures of snowmelt timing to vary by over 4 weeks from year-to-year. Temperature variability causes a larger spread in snowmelt timing in a warmer climate. On average, snowmelt timing occurs 3 weeks earlier in late 21st century projections, with about 25% of future conditions observed today.