As natural storm slab releases taper off, human triggering potential will remain high - especially where winds have formed touchy slabs. Deeply buried weak layers remain a top concern in sheltered terrain at all elevations.
Weather Forecast
Wednesday night: Flurries bringing 5-10 cm of new snow. Light south winds.Thursday: Mainly cloudy with continuing scattered flurries and 2-4 cm of new snow. Light southwest winds. Alpine high temperatures around -8.Friday: Mainly cloudy with isolated flurries and a trace of new snow. Light southwest winds. Alpine high temperatures to -9.Saturday: Cloudy with scattered flurries and 3-6 cm of new snow. Light southeast winds. Alpine high temperatures to -8.
Avalanche Summary
Reports from Tuesday showed explosives control in the north of the region producing storm slabs to Size 2 as well as persistent slabs from Size 2-3.5. Further to the south, a couple of natural and skier triggered persistent slabs released to Size 2 and Size 3 on the early January and the deeper mid December weak layers. All of this activity was observed above 2200 metres and on all but southwest aspects.On Monday, small to large (size 1 to 2) storm and wind slabs were triggered by skiers, naturally, and by explosives on all aspects and elevation bands, generally 20 to 50 cm deep and often failing on the mid-January weak layer. Persistent slabs were also observed on the early-January weak layer, and were generally large (size 2 to 3), on polar aspects, at treeline and alpine elevations, and 40 to 100 cm deep. Similar storm and persistent slab avalanches have been reported across the region reported since last Wednesday. Expect to see continued storm slab avalanches in lee features and persistent slab avalanches where the weak layers are preserved.
Snowpack Summary
The current snowpack is complex, with three active weak layers that we are monitoring.20-60 cm of storm snow sits on a crust and/or surface hoar layer (mid-January). The crust is reportedly widespread, except for possibly at high elevations on north aspects. The surface hoar is 10 to 30 mm in size and at all elevation bands. The recent snow fell with strong south winds, producing wind slabs and cornices in lee features. Deeper in the snowpack, a persistent weak layer known as the early-January layer is buried 50 to 80 cm, present at all elevation bands, and composed of surface hoar on sheltered slopes and sun crust on steep solar aspects. Recent snowpack tests have shown sudden fracture characters with moderate loads and high propagation potential, as well as other signs of instability such as whumpfs and cracking. Another persistent weak layer that was buried mid-December is 60 to 100 cm deep and consists of a facet/surface hoar/crust combination. It is most problematic at and below tree line.A rain crust buried in November is 100 to 150 cm deep and is likely dormant for the time being.
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.