Observed Climate–Snowpack Relationships in California and their Implications for the Future

A study of the California Sierra Nevada snowpack has been conducted using snow station observations and reanalysis surface temperature data. First-of-the-month snow water equivalent (“SWE”) measurements were combined from two data sets to provide sufficient data from 1930 to 2007. The monthly snapshots are used to calculate peak snow mass timing for each snow season. Since 1930, there has been an overall trend towards earlier snow mass peak timing by 0.4 days per decade. The trend towards earlier timing also occurs at most individual stations. The majority of stations have experienced simultaneous reductions in April 1st SWE. Reductions in April 1st SWE may therefore be due to earlier snowmelt rather than reductions in total snowfall. Analysis of individual years and stations reveals that warm March temperatures are associated with earlier snow mass peak timing for all spatial and temporal scales included in the data set. The influence is particularly pronounced for low accumulation years indicating the importance of albedo feedback for the melting of shallow snow. Regional mean March temperatures have increased at a rate of 0.4◦C per decade since 1948, and the robustness of the March temperature influence on peak timing suggests the trend towards earlier peak timing is attributable to the March temperature trend. Given scenarios of warming in California, we can expect to see acceleration in the peak timing trend; this will reduce the warm season storage capacity of the California snowpack.