A recent avalanche triggered on the mid December persistent weak layer indicates deeper instabilities remain active.
Confidence
Good - Due to the quality of field observations
Weather Forecast
Significant overnight cooling and another re-freeze of the snowpack is expected tonight. Friday should bring a mix of sun and cloud and freezing levels rising between 1100-1350m. Saturday is expected to bring change as a system moves in from the coast. Overcast with moderate westerly winds in the alpine, a high of -6 at treeline and a possibility of very light precipitation starting in the early morning. Sunday will be overcast with the possibility of very light accumulations throughout the day.
Avalanche Summary
A large size 2 avalanche was accidentally triggered on an east aspect in the alpine by a sledder near Racehorse Pass (Crowsnest) on Wednesday on the December persistent weak layer. Over the past 4 days there have been numerous loose wet and wet slab avalanches up to size 3 releasing naturally, mostly on solar affected aspects. The forecast cooling trend should result in a decrease of natural activity; however solar aspects on Friday may still be a concern during the warmest parts of the day.
Snowpack Summary
Previous rain and the recent daily temperature fluctuations over the last few days have created a melt freeze cycle in the upper parts of the snowpack. These warm conditions have decimated the lower elevation snowpack and the threshold for avalanches has risen to approximately 1600m, potentially higher on solar aspects up to 1800m. In most areas of the region a new crust has formed on the surface of the snowpack right up to the lower alpine and on all solar aspects. The early January crust is buried 10-20cm and exists up to 1850m. The mid-December crust/facet/surface hoar layer is down 40-80cm and remains reactive in isolated areas. This layer has proven easier to trigger from thin, shallow, rocky areas and on slopes currently affected by sun and warming. There have been some recent near misses with this layer creating some large avalanches over the last couple weeks. The forecast cooling trend should bring with it a general increase in snow stability in the upper snowpack, and decreased avalanche hazard.
Problems
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.