Item: FORECASTING CHALLENGES DURING A SEASON WITH UNPRECEDENTED AVALANCHE ACTIVITY: THE 2023-2024 WINTER IN SOUTH-WEST MONTANA
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Title: FORECASTING CHALLENGES DURING A SEASON WITH UNPRECEDENTED AVALANCHE ACTIVITY: THE 2023-2024 WINTER IN SOUTH-WEST MONTANA
Proceedings: Proceedings, International Snow Science Workshop, Whistler, BC, Canada, 2026
Authors:
- Alex Marienthal [ Gallatin National Forest Avalanche Center, Bozeman, MT ]
- Doug Chabot [ Latok, LLC, Bozeman, MT ]
- Ian Hoyer [ Gallatin National Forest Avalanche Center, Bozeman, MT ]
- Dave Zinn [ Gallatin National Forest Avalanche Center, Bozeman, MT ]
Date: 2026-09-28
Abstract: During the 2023-24 winter in southwest Montana, persistent weak layers formed in December and early January which caused widespread avalanche activity that lasted through spring. This activity reached levels of distribution and frequency never seen since the inception of the Gallatin National Forest Avalanche Center in 1990. We recorded 77 remote-triggered avalanches, some long after a storm, compared to an average of 13 over the previous five seasons. Our traditional applications of the avalanche danger scale were challenged, and we had to adjust our forecasting methods to best match our travel advice with the avalanche danger. In this paper, we summarize the weather associated with the formation of the season's surface hoar and facet layers, as well as the conditions that led to avalanche activity across all eight of our forecast regions for many months after those layers were buried. While many winters have had weak layers form during similar weather, the 2023-24 season differed from years that had less widespread avalanche activity of this type. In addition, we highlight the challenges of communicating this avalanche problem in public forecasts, and the approach we took to assign danger ratings when likelihood and descriptors of avalanche danger as stated in the North American Public Avalanche Danger Scale didn't give a clear answer. Assigning danger ratings was challenging. On many days a human-triggered avalanche seemed "likely" or "very likely", which warranted a Considerable or High rating. However, the definitions of those ratings also indicate the potential for natural avalanches (e.g., triggered by new snow or wind-loading), which was remarkably absent. After much internal discussion and debate, all four avalanche forecasters chose to focus on basing danger ratings mainly on travel advice, or best fit for likelihood of human-triggered avalanches, which aligned with more conservative and appropriate danger ratings. As a result, we often issued higher danger ratings on days when the perceived overall likelihood for avalanches, and associated danger rating, historically might have been lower. To provide accurate and relevant advice to backcountry users, we adjusted our ways of applying danger ratings. A danger rating is too simple of a tool to describe avalanche conditions in a winter that was anything but simple.
Object ID: ISSW2026_O9.7.pdf
DOI: https://doi.org/10.15788/1790098701
Language of Article: English
Presenter(s): Alex Marienthal
Keywords: Avalanche Forecasting; Danger Rating; NAPADS; Persistent Weak Layer; Surface Hoar
Page Number(s): 2310 - 2317
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