Forecasting for dry and wet avalanches during mixed rain and snow storm events

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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.

Additional publication details

Publication type Conference Paper
Publication Subtype Conference Paper
Title Forecasting for dry and wet avalanches during mixed rain and snow storm events
Year Published 2018
Language English
Contributing office(s) Northern Rocky Mountain Science Center
Description 5 p.
Larger Work Type Book
Larger Work Subtype Conference publication
Larger Work Title Proceedings of the International Snow Science Workshop
First page 1040
Last page 1044
Conference Title International Snow Science Workshop
Conference Location Innsbruck, Austria