Lingering 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.
Weather Forecast
A ridge of high pressure should keep the region dry through Friday. On Wednesday, mostly sunny conditions expected with freezing levels around 1000m and light-to-moderate alpine wind. On Thursday, a mix of sun and cloud is expected with freezing levels rising to around 2000m and moderate-to-strong alpine wind from the SW-W. On Friday, mostly sunny conditions are expected with freezing levels over 2000m and moderate-to-strong alpine winds from the SW-W.
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. On Sunday, several size 1 storm slabs and wind slabs were ski cut. A natural size 2 was reported in the Waterton area. Natural avalanche activity is generally not expected on Wednesday but remains possible in isolated areas such as steep sun exposed slopes. Human-triggering is possible or likely in wind loaded areas and steep terrain features, especially where surface hoar underlies the recent storm snow.
Snowpack Summary
10-20cm of new snow fell over the weekend. Strong winds have redistributed the new snow in exposed terrain and wind slabs have formed in lee features at alpine and isolated areas at treeline. 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.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.