Temporal evolution of liquid water content and propagation saw test results during the transition from a dry to wet snowpack

Abstract

Wet slab avalanches are challenging to predict and may become more common in a warming climate. Despite their destructive potential, the mechanical processes governing wet slab avalanche release remain poorly understood. This study aims to address this knowledge gap by providing field-based insights into wet snow processes. Specifically, we examined the temporal evolution of propagation saw test (PST) results and liquid water content (LWC) measurements during the first critical melt event of the 2022-2023 season at our study site in the Madison Mountains of southwest Montana, USA. From April 10 to April 12, 2023, we completed 67 PSTs, 39 LWC profiles, and 6 snow profiles. During the first two days of sampling, increasing weak layer LWC and decreasing cut lengths marked a transition from low to consistently high propagation propensity. Elevated median LWC values above 6% for the weak layer and 10% for the slab characterized this high-propensity period. Near the beginning of Day 3, slab fractures signaled a shift to low propensity, and we no longer observed a clear relationship between weak layer LWC and cut length. Propagation mode shifted 19 hours before changes in cut length were observed, suggesting that while cut length may help signal the onset of high propagation propensity, propagation mode may help pinpoint the subsequent decline. We also observed temporal delays between changes in LWC and subsequent shifts in snowpack properties related to propagation propensity. Our results align with prior research that demonstrates that real-time LWC measurements alone may not reliably capture propagation propensity or be able to predict the peak or decline of wet slab avalanche activity. Integrating temporal LWC measurements with mechanical snowpack properties could improve predictive models. The temporal patterns and relationships we observed in our study show potential to improve our understanding of wet snow avalanches. Future work should validate the PST in wet snow, incorporate particle tracking velocimetry, and examine the temporal evolution of wet snowpack properties under laboratory conditions. A hybrid approach combining field and laboratory research offers a path toward advancing wet slab avalanche forecasting.