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 avalanche near Snoqualmie Pass, Washington, USA. Snoqualmie Pass recorded 463 cm of snowfall from 13 December 2008 to 6 January 2009. This period of snowfall was followed by a strong southwesterly tropical flow that resulted in an extreme rain-on-snow event. Sensors at Snoqualmie Pass recorded 285 mm of precipitation over a 52 hour period. Flooding, slush flows, landslides and avalanches resulted from the influx of precipitation. Snow heights decreased rapidly over the period, with settlement rates approaching 80 mm h(-1). Liquid water infiltrated and flowed through the snowpack within a few hours of the arrival of rain, yet many of the major avalanches occurred 12-30 or more hours after the onset of rain and water outflow. A glide avalanche occurred similar to 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. Here glide rates approached 670 mm h(-1). Although glide and settlement rates increased during periods of intense precipitation, glide failure occurred 8 hours after peak precipitation and outflow.