Snow, wind, warming and sun-exposure will keep storm slabs touchy, and coupled with the potential for cornice triggers, the likelihood of massive deep persistent slabs is expected to increase.
Confidence
Moderate - Freezing levels are uncertain on Monday
Weather Forecast
SATURDAY: A mix of sun and cloud with localized periods of intense sun-exposure and isolated light flurries. Increasing cloud with light snow starting in the afternoon. Light southerly winds becoming moderate westerlies. Freezing levels rising as high as 1500m for southern areas, but remaining near valley bottoms in the north.SUNDAY: Mainly cloudy with 5-10cm of fresh snow by the morning, moderate westerly winds and freezing levels again rising to 1500m for southern areas, but remaining near valley bottoms in the north. MONDAY: A mix of sun and cloud with isolated light flurries, light to moderate southwesterly winds and freezing levels rising above 2000m in the afternoon.
Avalanche Summary
Reports from Thursday include two 50-150cm thick Size 2-2.5 skier remote triggered deep persistent slab avalanches, failing on facets and depth hoar above the November crust with impressive propagation across alpine wind loaded feature. Subsequent explosives control produced another 40-200cm thick Size 2.5 deep persistent slab avalanche with 5m deep deposit. Elsewhere, skiers remotely triggered a 50cm thick Size 3 wind slab avalanche and extensive explosives control produced slab avalanches up to Size 3 with large full depth full path avalanches in thin snowpack areas.
Snowpack Summary
Around 15-20cm of fresh snow has added to the 40-80cm of settled recent storm snow is bonding poorly to weak faceted snow and small surface hoar on sheltered shady slopes, and/or a thin crust on southerly aspects. Southerly winds have formed touchy slabs at all elevations with multiple weaknesses within and under this recent storm snow. The persistent weakness buried mid-February is lurking down 70-120 cm and composed of a thick rain crust as high as about 2000 m, sun crusts on steep southerly aspects, and spotty surface hoar on shaded aspects. This layer has produced easy results in recent snowpack tests and has proven especially reactive on steep southerly aspects. Several deeper persistent weaknesses also remain a concern, including surface hoar buried early-February (around a metre deep), and mid-January (well over a metre deep primarily in the northern Purcells). The november crust and basal facets are still sensitive in shallow, rocky start zones.
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.
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.