Additional snow, wind, and warm temperatures will determine the hazard in the next few days.
Confidence
Fair - Timing of incoming weather systems is uncertain
Weather Forecast
Sunday: Moderate snowfall with 10-15cm of accumulation, freezing levels rising up to 1800m and strong southwesterly winds. Forecast models show a bit more precipitation in the south. Freezing levels expected to remain at or near valley bottoms with tree line temperatures around -8. Ridge top winds are expected to be moderate westerlies. Monday: Little snowfall expected with less than 5cm of accumulation at higher elevations, freezing levels will rise to around 1500m with strong westerly winds. Tuesday: Moderate to locally heavy snowfall as a Pacific frontal system moves into the interior. Strong westerly winds and freezing levels rising to 1800 metres in parts of the forecast area.
Avalanche Summary
Avalanche activity has been limited to a few loose sloughs from steep terrain. On Tuesday, a wind slab avalanche size 1.5 was easily triggered under the weight of a rider on a NE aspect. Also a large ( 2.5) natural avalanche was reported in the interior of the Purcells on an E aspect. The potential exists for large destructive avalanches with increased loading.
Snowpack Summary
Recent snow and winds are adding to the surface load. Snowpack depths at tree line currently vary from 70 - 110 cm with the highest variability in wind-exposed areas. A persistent weakness of buried surface hoar and facets, along with an associated crust, is down approximately 20-40cm . This layer has been producing variable results with snowpack tests, and may become reactive as a cohesive slab develops with warming temperatures and additional loads. At the base of the snowpack are weak facets and depth hoar. This deep persistent weakness may be stubborn to trigger, especially in deeper snowpack areas, but the sensitivity to triggers likely increases in shallower locations, especially on steep, convex slopes. At lower elevations the snow depth is below the threshold for avalanches.
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.