Characteristics of Large Snowfall Events in the Montane Western U.S. as Examined Using SNOTEL Data

Daily snow water equivalent (SWE) records from the SNOTEL archive are used to assess spatio- temporal characteristics of large snowfall events over the montane western U.S. The largest mean annual (leading) events are found in the Pacific Northwest and Sierra Nevada. The mean leading event, lasting up to 72 hours, typically accounts for 10-23% of total seasonal snowfall water equivalent, with the largest contribution in the Arizona/New Mexico sector. For most of the west, snowfall events in the top quartile or decile of station distributions for each year are most common in winter, but those for the Rocky Mountain states and Utah are most common in late winter or spring. Colorado also shows a secondary peak in large events during November. Large mid-winter snowfall events in the marine sectors, Idaho and Arizona/New Mexico are spatially coherent in the sense that when observed at one station, they tend to occur at surrounding stations. Large events are less spatially coherent for drier inland regions. When seasonal snowfall is anomalously positive, there tends to be an increase in the number of snow days as well as a shift in the distributions towards larger event sizes. Opposing relationships are observed for negative seasonal snowfall anomalies. These findings are in accord with recent studies using lower elevation data demonstrating that the probability of extreme precipitation events is altered during El-Nino or La-Nina conditions.