Snow and wind will drive the danger up for Friday, then warming and sun-exposure, coupled with the potential for cornice triggers, will increase the likelihood of massive deep persistent slabs for the weekend.
Confidence
Moderate - Freezing levels are uncertain on Saturday
Weather Forecast
FRIDAY: Cloudy with 10-15cm of fresh snow accompanied by moderate SW winds. Freezing levels as high as 1500m for southern areas, but remaining near valley bottoms in the north.SATURDAY: A mix of sun and cloud with localized periods of intense sun-exposure and isolated light flurries. Light to moderate SW winds. Freezing levels again rising to 1500m for southern areas, but remaining near valley bottoms in the north.SUNDAY: Mainly cloudy with 3-5cm of fresh snow, light to moderate westerly winds and freezing levels again rising to 1500m for southern areas, but remaining near valley bottoms in the north.
Avalanche Summary
Reports from Wednesday include a 100 cm deep natural Size 3.5 slab avalanche on a SW aspect and a large cornice failure that entrained quite a bit of mass. Explosives control produce wind slab avalanches up to Size 2.5 on north through east aspects. Reports from Tuesday include one natural Size 2.5 on a southeast facing alpine slope in response to intense cross-loading as well as numerous other natural dry loose and soft slab avalanches up to Size 1 on lee aspects. Explosives control produced wind slab and persistent slab avalanches up to Size 3. One of which was 100-300cm deep, failing on the deep persistent facet/crust weakness buried in November.
Snowpack Summary
Around 50-100cm of settled 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 now down 60-120 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 (80-140 cm deep), and mid-January (over a metre deep primarily in the northern Purcells ). Basal facets may still be reactive 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.