Winds slabs remain a concern in the alpine and exposed areas at treeline. Persistent slab avalanches may still be possible in some areas with a heavy trigger or thin-spot triggering.
Confidence
Fair - Due to the number of field observations
Weather Forecast
A mix of sun and cloud is expected for Thursday and Friday with freezing levels around 1500m and moderate-to-strong alpine winds from the SW. Unsettled conditions are forecast for Saturday with a mix of sun and cloud, light flurries, strong SW winds in the alpine, and freezing levels climbing to over 2000m.
Avalanche Summary
On Tuesday, explosives triggered 3 persistent slabs up to size 2. These released on the mid-Dec layer down 40-60cm. They were all on east aspects around 2000m elevation. On Monday, explosives triggered a size 2.5 deep persistent slab on the early-Nov layer near the ground. This was on an east aspect at 1700m in the Harvey Pass area. Natural avalanche activity is generally not expected on Thursday. Human-triggering remains a concern, especially in wind loaded areas and steep unsupported terrain features.
Snowpack Summary
10-20cm of snow fell last weekend. Moderate-to-strong SW wind has and will continue to redistribute snow in the alpine and will continue to load leeward features. The storm snow buried a layer of surface hoar and/or a crust that exists in many places up to 1900m. At higher elevations the new slow fell on widely wind affected surfaces. The mid-December crust layer is down 40-80cm. In many places this crust is overlaid by facets and/or surface hoar. In areas where the overlying slab is thick and cohesive, large avalanches remain possible on this interface. Closer to the ground a crust/facet interface that formed in November is generally dormant.
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.