Have a safe and fun holiday.
Confidence
Fair - Intensity of incoming weather systems is uncertain
Weather Forecast
On Monday, expect flurries or light snow with light to moderate NW winds. Freezing levels are expected to gradually drop to near valley floor by Tuesday. A second storm system is due to arrive some time on Tuesday, bringing snow (around 5-10cm).
Avalanche Summary
On Saturday, explosives triggered size 2-2.5 slabs at the northern end of the Purcells. These are suspected to have failed on the lower layer of surface hoar, and in isolated spots stepped down to the early November crust. Smaller slabs were also failing on the most recently buried layer of surface hoar. A size 2.5 naturally-triggered slab was observed at 2600m on an east aspect on Friday.
Snowpack Summary
Recent storm snow sits above two layers of surface hoar in the upper snowpack. The more recent of these sits on a crust below about 2100m, and on settled snow at higher elevations. A hard rain crust with facets from early November is buried over 1 m down and is still reactive to light loads in some locations.
Problems
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
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.