Human triggered avalanches that release on the buried weak layer keep surprising skiers and riders. The uncertainty on where and if this layer can be triggered is best managed with conservative terrain choices.
Strong south and southwest wind have formed wind slabs in lee terrain features at higher elevations. Wind slabs are especially touchy 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.
MONDAY NIGHT: Clear with cloudy periods, up to 3 cm new snow, light to moderate west wind, alpine temperature -6 C, freezing level 600 m.
TUESDAY: Cloudy, up to 5 cm new snow, light to moderate southwest wind, alpine temperature -5 C, freezing level 1400 m.
WEDNESDAY: Cloudy with snowfall, 10 to 15 cm new snow, light to moderate west wind, alpine temperature -1 C, freezing level rising to 2000 m.
THURSDAY: Sunny with cloudy periods, up to 3 cm new snow, light to moderate west wind, alpine temperature +2 C, freezing level 2400 m.
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.
On Friday, a natural avalanche cycle of small slabs up to size 1.5 was observed, as well as a few large (size 2.5 to 3) storm slab avalanches on northeast and south aspects at treeline and large (up to size 2.5) wind slab avalanches on north aspects in the alpine. Skiers triggered small storm/wind slab avalanches, and explosives triggered numerous large (up to size 2.5) avalanches.
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 south to southwest wind and storm slabs in wind sheltered areas.
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.
