Item: PINPOINTING THE TIMING OF WEAK LAYER FAILURE: A CASE STUDY OF A LARGE, WIDESPREAD NATURAL AVALANCHE CYCLE IN THE BITTERROOT MOUNTAINS
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Title: PINPOINTING THE TIMING OF WEAK LAYER FAILURE: A CASE STUDY OF A LARGE, WIDESPREAD NATURAL AVALANCHE CYCLE IN THE BITTERROOT MOUNTAINS
Proceedings: Proceedings, International Snow Science Workshop, Whistler, BC, Canada, 2026
Authors:
- James Peabody [ West Central Montana Avalanche Center, Missoula, MT, USA ]
- Nathalie de Leeuw [ West Central Montana Avalanche Center, Missoula, MT, USA ]
- Ben VandenBos [ West Central Montana Avalanche Center, Missoula, MT, USA ]
Date: 2026-09-28
Abstract: In March 2026, a multi-day storm deposited over 180 mm of snow water equivalent (SWE) across the Bitterroot Mountains of Montana and Idaho. Prior to the storm, the snowpack contained multiple buried persistent weak layers, including a crust/facet layer, referred to as the January Drought Layer (JDL). In the days and weeks before the storm, cornice fall triggered large avalanches on the JDL, and forecasters anticipated a widespread natural cycle during the storm. A key forecasting challenge with persistent slab problems is predicting at what point during a storm cycle natural avalanches are most likely to occur. Previous studies have linked weather station data to persistent slab avalanche occurrence, however without knowledge of exactly when during the storm the avalanche released, it is difficult to determine exactly how much new load it took for avalanches to occur. The Bitterroot forecast zone receives few user observations, and it is unlikely observers would have been reporting the timing of avalanche occurrence during the storm. To estimate the timing of release within the storm, we measured SWE above the JDL at both the crown face and on the bed surface of a persistent slab avalanche that released during the storm. By correlating these measurements to continuous SWE data from a SNOTEL station approximately 6.5 km away, we are able to determine approximately when, and after how much load the avalanche occurred. We also used photos and mapping software to collect slope angle and aspect data from 23 additional avalanches within a 13 km radius that occurred during the same cycle. We can use this geospatial data to determine if these avalanches were similar in aspect and elevation to the avalanche where we collected the measurements. This data will allow us to look for patterns in avalanche release from this storm and determine if loading amounts before triggering may have been similar. This method has potential to improve forecasting of persistent slab cycles in intermountain snowpacks.
Object ID: ISSW2026_P2.61.pdf
DOI: https://doi.org/10.15788/1790099067
Language of Article: English
Presenter(s): James Peabody
Keywords: Timing of weak layer failure, avalanche forecasting, geospatial analysis, deep slab instability, case study
Page Number(s): 2330 - 2337
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