Recent storm snow has the potential to form large avalanches and step down to buried weak layers. Conservative decision-making is essential!
Confidence
Moderate - Intensity of incoming weather systems is uncertain
Weather Forecast
SATURDAY NIGHT: Cloudy with light snowfall, accumulation 5 to 10 cm, cooling overnight.SUNDAY: Cloudy with light snowfall, accumulation 5 to 10 cm, light southwest winds, alpine temperature -24 C.MONDAY: Cloudy with light snowfall, accumulation 5 cm, light southwest winds, alpine temperature -24 C.TUESDAY: Partly cloudy skies, light northeast winds, alpine temperature -16 C.
Avalanche Summary
Many large storm slab avalanches were triggered naturally, by skiers, and by explosives on Saturday. On Friday, skiers triggered small slabs around 10 to 40 cm deep, between 1600 m and 1900 m, likely on the weak layer described in the Snowpack Summary.
Snowpack Summary
Somewhere around 60 mm of precipitation fell on Friday with a freezing level around 1600 m. Above 1600 m, expect to find around 30 to 60 cm of new snow. This snow overlies feathery surface hoar crystals in shaded and sheltered areas, previously wind-affected snow in exposed terrain, and a melt-freeze crust on southerly aspects. Below 1600 m, the precipitation fell as rain and froze into a melt-freeze crust. Any new snow that falls on Saturday night into Sunday will accumulate onto these surfaces.The mid-January layer of surface hoar or a crust is buried around 50 to 100 cm. The surface hoar is found on shaded and sheltered slopes and is most prominent between 1600 m and 1900 m. The melt-freeze crust is found on south aspects at all elevations.The remainder of the snowpack is generally well-settled. Thin snowpack areas, such as in the east of the region, may find weak and sugary faceted grains near the base of the snowpack.
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.