Biases of April 1 snow water equivalent records in the Sierra Nevada and their associations with large-scale climate indices

Do the April 1 snow course measurements underestimate the snow accumulation peak in the Sierra Nevada, thus possibly introducing biases in studies of trends and interannual variability that use the April 1 data? Although their period of record is many decades shorter, measurements of snow water equivalent (SWE) from automated snow pillows provide daily records throughout each year, enabling us to examine the timing and magnitude of the maximum SWE accumulation. Using generalized additive mixed models, on average, we find that the peak occurs earlier than April 1 at the lowest elevations and later than April 1 at the highest elevations and that the degree to which April 1 SWE under-measures maximum SWE is greatest at the lowest and highest elevations. In addition, our results show that the biases are associated with cold and warm phase interactions of certain climate indices.