Rapid warming at the snowiest locations in the western USA: spatial patterns and mechanisms

We analyzed hourly temperature records over 30 years for 486 snow telemetry (SNOTEL) stations at the highest elevations in 13 states across the western USA. Trends determined using robust linear modeling (resilient to outliers) across the region are 0.88 +/- 0.09 C/decade, which is about three times the magnitude of the regional warming trend and six times the trend in global mean temperatures. Temperature trends across SNOTEL sites are not significantly correlated with elevation (r^2 = 0.03). Trends at most sites are strongest in fall and winter, and weakest in spring and summer. The much faster warming at very snowy sites in the western USA relative to the global mean might be related to "alpine amplification" through snow-albedo feedback. To evaluate this hypothesis we analyzed trends in the number of days with significant snow cover at the same 486 sites for which temperature trends were obtained. High-elevation warming was most rapid in Arizona, New Mexico, and southern Colorado, where trends at 23 sites exceeded 1.25 C/decade. These sites also showed strong trends toward fewer days with snow cover of more than 5 days per decade. Increasing dust deposition has been documented on mountain snowpack in that region, probably contributing to shorter snow seasons and warmer temperatures. Warming trends are typically weakest in the Pacific Northwest, northern Utah and southwest Wyoming where many stations have warmed less than 0.5 C/decade. Many of these sites have significantly more days with significant snow cover which may slow warming. Trends in both temperature and snow-covered days elsewhere in the Rocky Mountains are intermediate. Rapid warming trends at the snowiest sites in the western USA have important implications for projections of water supply in the region in coming decades.