New snow and warming are influencing a complex snowpack: storm and wind slabs sit above three weak layers that are reactive at all elevations. Perform cautious route-finding without overhead exposure and avoid alpine terrain as the snowpack adjusts.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
WEDNESDAY: Cloudy with snowfall, accumulation 15-25 cm, light to moderate southerly winds, alpine temperature -6 C, freezing level near 1100 m.THURSDAY: Cloudy with light snowfall, accumulation 5-10 cm, light to moderate southerly winds, alpine temperature near -9 C, freezing level near 600 m.FRIDAY: Partly cloudy, light southwesterly winds, alpine temperature near -10 C, freezing level below valley bottom.
Avalanche Summary
On Monday, many small to large (size 1 to 3) storm slab avalanches were triggered naturally and by explosives at and above treeline on all aspects, 40 to 60 cm deep. Numerous large to very large (size 2 to 3.5) persistent slabs were also triggered naturally, by skiers, and explosives at all elevation bands and aspects.These sorts of avalanches have been a consistent trend since last Wednesday, with small to large storm slabs observed at all elevations and large persistent slabs most often observed at treeline and alpine elevation bands. These avalanches can be triggered naturally and by skiers and snowmobilers. They have been releasing on all three persistent weak layers described in the Snowpack Discussion. Expect to see more avalanches with the changing weather and where the weak layers are preserved, such as in sheltered locations in the alpine and treeline elevations and in open cutblocks and gullies below treeline. Triggering such weak layers will likely produce large, destructive avalanches with high consequences.
Snowpack Summary
The current snowpack is complex, with three active weak layers that we are monitoring.50-80 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 mid-January surface hoar is 5 to 20 mm in size and was reported at treeline elevations and possibly higher. The recent storm snow fell with strong south winds, producing wind slabs in lee features at treeline and alpine elevations and in open areas below treeline.Deeper in the snowpack, the early-January persistent weak layer is 80 to 100 cm below the surface. It is composed of surface hoar on sheltered slopes as well as sun crust on steep solar aspects and is found at all elevation bands. Snowpack tests show sudden fracture characters with moderate loads and high propagation potential, and signs of instability such as whumpfs and cracking. Another weak layer buried mid-December consisting of a facet/surface hoar/crust combination is buried 100 to 150 cm deep. It is most problematic at and below tree line.A rain crust buried in November is generally 150 to 200 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.