Forecast new snow and wind continue to build storm slabs at all elevations. Deep pockets of wind transported snow may be easy to trigger at higher elevations.
Confidence
Moderate - Wind effect is extremely variable
Weather Forecast
Another pulse of Pacific moisture should bring 5-10 cm by Monday morning combined with moderate southwest winds and freezing levels down to about 300 metres. Light winds and periods of convective flurries during the day Monday. Light snow on Tuesday with westerly winds and freezing levels briefly rising to 500 metres. Wednesday continued light snow and moderate southwest winds.
Avalanche Summary
Skiers triggerd a thin ( 20 cm deep) storm slab on a poorly supported east aspect in the Coquihalla that suggests that wider fracture propagations are possible.
Snowpack Summary
Up to 20 cm overnight in the south of the region and closer to 15 in the north. Variable winds across the region, with reports of strong southwest forming soft slabs in the south, and some social media accounts of unconsolidated snow and light winds in the north around Steep Creek. I suspect storm slabs and weak new cornices developed at higher elevations across the region. There is now 20-40 cm above the surface hoar layer that was reported to be buried on December 17th (151217 SH). There may be a thin sun crust on southerly aspects in the Coquihalla beneath the recent storm snow. Incremental loading like this, with consistent cool temperatures can make it difficult to forecast when the storm slab will become cohesive and possibly reactive above any weak layers or combination of smooth sliding surface and weak layer. There is not a lot of load yet if you look at the mm of water equivalent; only about 25 mm for the 40-50 cm that has fallen, but it is still developing with each new snow fall.
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.