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 found around 40 to 80 cm deep. The layer exists at all elevation bands and has recently been very reactive between 1600 and 2300 m. Example terrain features to particularly treat as suspect include the lee side of ridges, openings in trees, cut blocks, and burns.
MONDAY NIGHT: Mostly clear with some clouds, trace of new snow, light to moderate west wind, alpine temperature -4 C, freezing level 500 m.
TUESDAY: Mostly cloudy with some sunny periods, up to 3 cm new snow, light southwest wind, alpine temperature -4 C, freezing level 1400 m.
WEDNESDAY: Cloudy, up to 5 cm new snow, light northwest wind, alpine temperature 0 C, freezing level rising to 2200 m.
THURSDAY: Mostly sunny, trace of new snow, light west wind, alpine temperature +2 C, freezing level 2700 m.
On Sunday, skiers triggered large avalanches that released on the buried weak layer with surface hoar crystals. Check out some good example photos here. Some avalanches on this layer were triggered remotely by riders. Several large and one very large (size 3.5) avalanche released naturally on the buried weak layer. The avalanches released at all elevation bands and all aspects. Explosives triggered numerous storm slabs up to size 2 below treeline on southeast aspects. Many large wet loose released naturally on sun-exposed slopes. Several large wind slab avalanches released in the alpine and at treeline on east, south and west aspects.
On Saturday, several large natural storm and wind slabs up to size 3 were observed on all aspects in the alpine. Explosives triggered avalanches up to size 2.5. Riders triggered many small wind slabs up to size 1.5 at treeline as well as a large (size 3) wind slab in the north of the region (see this MIN report). Many avalanches released on the weak layer that was buried at the end of January.
On Friday, a natural avalanche cycle of storm slabs up to size 2 was observed, as well as a large size 3 avalanche on an east aspect. Numerous small rider triggered avalanches and a large (size 2) storm slab triggered by a skier were reported. Explosives triggered numerous large (size 2) avalanches. A deep persistent slab avalanche was triggered by a machine and released on the early December facet/crust layer described in the Snowpack Summary.
The recent storm brought 10 to 30 cm snow. The snow has formed wind slabs in exposed terrain at higher elevations with 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 40 to 80 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 100 to 240 cm. The last reported avalanche was on February 4 in the southwest of the region. 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.
