Wet loose snow avalanching in southwestern Montana

Abstract

Wet loose snow avalanches are a significant hazard within many ski areas. Wet snow stability changes dramatically over short time periods which typically coincide with operating hours, and few quantitative tools exist for avalanche workers attempting to predict the onset of wet snow avalanching. Field work was conducted at two study sites in southwestern Montana during the springs of 2003, 2004, 2005, and 2006. The study is composed of three separate experiments. The first documents stratigraphic boundary conditions present during periods of wet loose instability. Results show that melt-water accumulation within the upper 15cm of the snowpack increases the likelihood of wet loose avalanche occurrence. The second focuses on the mean daily and minimum daily air temperatures, and how well each variable indicates wet loose avalanche activity.

Results are consistent with prior research and clearly show that temperature alone is not a good indicator. The third relates wet loose snow avalanching to surficial shear strength. A 250cm2 shear frame was used to make as many as 210 surficial shear strength measurements of melt-freeze snow per day. Changes occurred rapidly within the meltfreeze cycle as shown by highly significant changes in shear strength within half hour intervals. Most importantly, the data shows an apparent association between surficial shear strength and avalanche activity. When shear strength measurements dropped below 250 Pa wet loose avalanches were observed, and triggered, in the immediate vicinity of study slopes. Conversely, surficial stability on the study slope improved when shear strength values exceeded 250 Pa. This research provides insights into wet loose snow avalanching and the development of possible tools for better predicting wet loose snow avalanche occurrence.