Item: Forecasting for dry and wet avalanches during mixed rain and snow storm
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Title: Forecasting for dry and wet avalanches during mixed rain and snow storm
Proceedings: International Snow Science Workshop Proceedings 2018, Innsbruck, Austria
Authors:
- Scott Savage [ U.S.D.A. Forest Service Sawtooth Avalanche Center, Ketchum, Idaho, USA ]
- Erich Peitzsch [ U.S. Geological Survey Northern Rocky Mountain Science Center, West Glacier, Montana, USA ] [ Snow and Avalanche Lab, Department of Earth Sciences, Montana State University, Bozeman, Montana, USA ]
- Simon Trautman [ U.S.D.A. Forest Service National Avalanche Center, Bozeman, Montana, USA ]
- Ben VandenBos [ U.S.D.A. Forest Service Sawtooth Avalanche Center, Ketchum, Idaho, USA ]
Date: 2018-10-07
Abstract: Natural wet slab avalanches release when rain or melt water decreases snowpack strength, and natural dry slab avalanches release when an increased load overcomes snowpack strength. This study investigates avalanche activity resulting from mixed rain and snow falling on a faceted snowpack. This scenario produced an extensive slab avalanche cycle in March 2018 in the mountains near Ketchum, Idaho, when a 24 hour storm deposited 50 to 65 mm of water. We investigate the contributions of the pre-existing snowpack structure and weather to avalanching, and suggest possible mechanisms for the observed slab avalanche activity. At upper elevations, expected widespread, 0.5 to 3 m deep, dry slab avalanche activity occurred on many aspects. However, at middle elevations (2300 m to 2700 m) near the fluctuating rain-snow line, a low frequency return period avalanche cycle occurred in a much smaller geographical area, and was concentrated around north-northwest aspects. This differs significantly from avalanches above this elevation that spanned all aspects. This scenario illustrates the challenges forecasting and communicating these events. In our experience, some avalanche cycles exist in a continuum of avalanche types that are not easily sorted into simple "wet" and "dry" categories. We discuss challenges in using current advisory and bulletin communication tools. Furthermore, it is possible that a changing climate will increase the frequency of mixed rain-snow events in areas with traditionally drier and colder climates. We believe the avalanche community will benefit from the refinement and development of tools and techniques to describe and forecast this challenging problem.
Object ID: ISSW2018_O11.3.pdf
Language of Article: English
Presenter(s):
Keywords: Wet Avalanche, Rain-On-Snow, Conceptual Model for Avalanche Hazard
Page Number(s): 1040-1044
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