Glide avalanche response to an extreme rain-on-snow event, Snoqualmie Pass, Washington, USA

Rain-on-snow events trigger immediate and delayed avalanches as liquid water penetrates the snowpack. We present results from an extreme rain-on-snow event that triggered a glide avalanches near Snoqualmie Pass, Washington, U.S.A., located in the Cascade Mountains. Snoqualmie pass recorded 463 cm of snowfall from 13-December 2008 through 06-January 2009. Temperatures were cold with period of below normal snow water equivalencies. This period of snowfall and snowpack development was followed by a strong southwesterly flow of tropical origin that resulted in an extreme rain-on-snow event. Sensors at the study plot recorded 300 mm of precipitation over a 60-hour period. Flooding, slush flows, landslides, and avalanches resulted from this massive influx of precipitation. Snow heights decreased rapidly over the period with settlement rates approaching 8 cm/h. Liquid water infiltrated and flowed through the snow pack within a few hours of the the arrival of rain, yet many of the major snowpack failures occured 12 to 30 or more hours after the onset of rain and water outflow. A glide avalanche was recorded approximately 30 hours after the onset of rain and the establishment of drainage through the snowpack. Increasing glide rates correlate with periods of rapid snow settlement. During these periods, glide rates approached nearly 200 mm/h. Although glide and settlement rates increased during periods of intense precipitation, glide failure occurred some eight hours after peak precipitation and outflow