DECISION-MAKING AND AVALANCHE MITIGATION DURING A RAPID ONSET WET SLAB AVALANCHE CYCLE

Although all avalanche types are difficult to forecast and mitigate, wet slab avalanches are particularly problematic. The dynamics of wet slabs are poorly understood and large wet slab cycles are relatively rare in comparison to dry slab cycles. We provide a case study of a historic wet slab cycle at Crystal Mountain Ski Area in Washington State in March of 2014. This cycle resulted in numerous large natural and explosive-triggered avalanches, including one particularly notable slide that destroyed a 34- year old chairlift and had an alpha angle of only 20o. The avalanches occurred after a brief period of warming followed by rain. Though the amount of precipitation (1.4 inches or 36 mm) on the preceding day was not atypical for this location, we believe the existing snow structure was a key contributing factor to the avalanches. The snowpack contained numerous crusts and layers of facets near the base due to a lack of early-season snowfall, and a deep and relatively homogenous slab formed by a 10-day storm that dropped 12' (3.6 m) of new snow. Water penetration through the slab progressed much more rapidly than expected, perhaps due to the homogeneous nature of the slab. We discuss the snowpack evolution, contributory weather factors, operational challenges, mitigation efforts (including whether or not to use mitigation), and difficult decision-making that allowed the risks of this avalanche cycle to be managed without loss of life. A thin margin for error existed in this event, and we hope other practitioners can learn from our case study.