Warming and new snow will increase the likelihood of triggering the buried weak layer where it is present. The uncertainty is best managed with conservative terrain choices.
Up to 15 cm new snow will be redistributed by moderate southwest wind and form fresh wind slabs. Previously, strong to extreme south and southwest wind have developed wind slabs in lee terrain features in the alpine and at treeline. Wind slabs might be especially reactive where they overlie a buried weak layer with surface hoar crystals.
The recent snow loaded a weak layer of surface hoar crystals, which is at a prime depth for human triggering. The layer exists at all elevation bands and has recently been very reactive between 1600 and 2200 m. Example terrain features to particularly treat as suspect include the lee side of ridges, openings in trees, cut blocks, and burns.
Rising temperatures will lead to wet loose avalanches at lower elevations.
TUESDAY NIGHT: Cloudy with snowfall, 5 to 10 cm new snow, light to moderate southwest wind, alpine temperature -5 C, freezing level 1100 m.
WEDNESDAY: Cloudy with snowfall, up to 5 cm new snow, light to moderate southwest wind, alpine temperature -1 C, freezing level rising to 1900 m.
THURSDAY: Mix of sun and cloud, up to 3 cm new snow, light to moderate westerly wind, alpine temperature +1 C, freezing level 2300 m.
FRIDAY: Sunny, light to moderate northwest wind, alpine temperature -7 C, freezing level 1000 m.
On Monday, skiers triggered slabs on the buried weak layer about 60-80 cm deep. Large wind slab avalanches up to size 3 released naturally. Explosives triggered wet loose avalanches up to size 2.
On Sunday, several avalanches released on the buried weak layer with surface hoar crystals and surprised riders. A few large avalanches (up to size 2.5) released naturally on that same layer. Check out some good example photos here. Small storm slab avalanches were reported on east and southeast facing slopes. Numerous loose wet and loose dry avalanches were triggered by the sun on steep slopes and reached large sizes. A deep persistent slab avalanche was likely triggered naturally by intense warming and sun and released on depth hoar or the November crust. It occurred just north of the region (see this MIN report).
On Saturday, numerous natural storm and wind slab avalanches up to size 2.5 at all elevations and aspects were reported. Explosives triggered several avalanches up to size 2.5. Skiers triggered a few slab avalanches up to size 2. A large (size 2) avalanche was triggered by skiers from a distance, which likely released on the buried weak layer (see this MIN report). A few small slab avalanches (up to size 1.5) released on that same layer naturally.
The recent storm brought 20 to 40 cm with locally higher amounts. The snow has formed wind slabs in exposed terrain at higher elevations from strong and extreme south to southwest wind. A sun crust formed on sun-exposed slopes at all elevations.
The snow loaded a weak layer of surface hoar crystals 5 to 15 mm in size. The layer may be around 60 to 100 cm deep, which is a prime depth for human triggering. The layer is most prominent in areas sheltered from the wind. Example terrain features to treat as suspect include the lee side of protected ridges, openings in the trees, cut blocks, and burns. The layer may not exist on steep sun-exposed slopes, where a melt-freeze crust may be found instead.
The facet/crust layer that formed in early December is buried around 120 to 250 cm. The last reported avalanche was on January 31 from a large explosive, and before that on January 23. Although unlikely, the layer could be triggered from a large load like a cornice fall or a shallower slab avalanche could step down to this layer. Humans are unlikely to trigger it, with perhaps the exception in thin, shallow snowpack areas. Check out the forecaster blog for more information.
