Item: A STUDY OF REPEATING PERSISTENT SLAB AVALANCHES ALONG THE PARK CITY RIDGELINE: THE EFFECTS OF REMOVING NEW SNOW ABOVE A WEAK LAYER
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Title: A STUDY OF REPEATING PERSISTENT SLAB AVALANCHES ALONG THE PARK CITY RIDGELINE: THE EFFECTS OF REMOVING NEW SNOW ABOVE A WEAK LAYER
Proceedings: Proceedings, International Snow Science Workshop, Whistler, BC, Canada, 2026
Authors:
- Michael Craig [ Canyons Village Snow Safety, Park City Mountain, Utah ]
Date: 2026-09-28
Abstract: Slide paths containing shallow snow depths can present a more unpredictable hazard. When a thick slab is unable to form above a persistent weak layer (PWL) to begin the healing process, it may be possible for weak layers to remain active through an entire season. This case study, from the Park City (PC) Ridgeline (Utah) during the 2024-25 season, documents how starting zones remained thin & weak due to repeated persistent slab avalanche release, as terrain adjacent to the "repeater" paths began to strengthen much earlier. The goal is to better understand and quantify how removing stronger snow above a PWL can lead to delayed strengthening and future reactivity. Field observations, snow profiles, weather data and avalanche activity are combined throughout three major cycles in upper elevation, northerly aspects (Intermountain snowpack). Early-season drought produced a shallow snowpack with continuous near surface faceting that became reactive during loading events in late December. Weak layers were then left exposed in starting zones with no slab above. Snow profile data suggests these weak layers were unable to heal during extended high-pressure periods and remained prone to reactivation with subsequent loading events. In contrast, adjacent terrain that did not avalanche maintained deeper and more uniform slab development, allowing faceted layers to strengthen much more quickly. Repeater avalanche paths required significant slab thickening without additional avalanche release to reduce temperature gradients and allow faceted grains to round and strengthen. Sintering of these weak layers occurred in mid-March and coincided with spring warming trends and two prolonged storm cycles with evenly-distributed precipitation over roughly 48- and 36-hour periods. When new snow is removed above a PWL, healing is delayed and the weak layer can remain reactive well after surrounding terrain has stabilized. RAC systems, hand charges and other explosives can create the "repeater avalanche path scenario" as they can strip the slab leaving behind exposed weak layers and a shallow snowpack with thin future trigger locations. Repeater avalanches (whether released naturally or artificially) can create dangerous hazard throughout the season, requiring the use of more explosives and necessitating longer closures during persistent slab avalanche cycles.
Object ID: ISSW2026_P1.11.pdf
DOI: https://doi.org/10.15788/1790098710
Language of Article: English
Presenter(s): Mike Craig
Keywords: SLAB FORMATION, SHALLOW SNOWPACK, REPEATER SLIDE-PATH, PERSISTENT WEAK LAYER
Page Number(s): 344 - 352
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