Keep the persistent slab problem in mind when you choose terrain this week. The likelihood of triggering it may be low, but the consequences could be high.
Confidence
Fair - Due to the number of field observations
Weather Forecast
Synopsis: The ridge of high pressure gives way to weak upper trough on Saturday bringing more cloud and unsettled conditions through Sunday. We could see flurries on the weekend but no significant accumulation. Ridge winds are generally light or moderate from the W-NW. The ridge should rebuild on Monday bringing drier conditions and more sunshine. The freezing level remains at valley bottom throughout the period.
Avalanche Summary
One observer north of Sparwood reported one natural size 1.5 slab avalanche and one remotely triggered size 2 avalanche on Wednesday, both sliding on the buried crust layer down 30-50 cm. On Tuesday, there was one report of a size 2.5 natural slab avalanche from Allison Peak in the Crowsnest Pass.
Snowpack Summary
Below treeline and on south-facing slopes there may be a melt-freeze crust overlying up to 20 cm of recent snow. At higher elevations the new snow has been blown into wind slabs in exposed lee terrain. The recent snow sits on hard wind slabs in exposed terrain, and low density faceted snow in sheltered areas. Up to 75 cm below the surface you will likely find a hard, thick crust which formed mid-December. This crust has overlying facets and surface hoar (up to 10 mm in sheltered locations at treeline and below). This layer seems variably reactive throughout the region. In areas where the overlying slab is thick and cohesive, large avalanches are possible at this interface. A crust/facet combo which formed in November seems to have gone dormant for the time being.
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.