Incoming weather is not conducive to natural avalanche activity, but human-triggered avalanches are possible. Uncertainty remains about this difficult-to-forecast deep persistent slab problem.
Expect to find lingering wind slabs on wind-affected slopes at higher elevations, and anywhere a slab forms over a crust or surface hoar layers. Fresh slabs have developed further down slope where strong winds have transported loose snow. Identify features of concerns such as steep, convex and unsupported slopes, where triggering is more prone.
The early December crust can be found down 100-150 cm below the surface, with weak sugary crystals (facets) on top of it. Large avalanches have been observed at all elevations including some unusual areas such as the middle of slide paths, open trees, and burns. Although the likelihood of triggering an avalanche is decreasing, there is ongoing concern for smaller avalanches stepping down to this layer. Cautious of thin snowpack areas and reloaded bed surfaces.
An upper ridge over the province is giving dry conditions for the next couple of days, although some low clouds will persist in interior valleys. It will gradually start shifting east by Friday morning, opening the door for a change in the weather pattern for the weekend.
TUESDAY NIGHT: Cloudy with clear periods / Light west wind gusting 40 km/h / Low temperature -8C / Freezing level at valley bottom.
WEDNESDAY: Cloudy with sunny periods / Light northwest wind gusting 40 km/h / High temperature -5 C / Freezing level around 1000 m.
THURSDAY: Mix of sun and clouds / Moderate west wind / High temperature -3C / Possible temperature inversion / Freezing level at valley bottom.
FRIDAY: Cloudy with sunny periods / Light southwest wind gusting 40 km/h / High temperature -2C / Possible temperature inversion / Freezing level around 1000 m.
On Monday, wind slab avalanches on cross-loaded lee slopes produced natural avalanches (up to size 2). A large glide slab (size 2.5) released near Revelstoke on south alpine slope due to strong solar input. Explosive controls triggered a deep persistent size 3 avalanche in the Monsahees, with a fracture line of 120 cm deep.
Around Silverton area, a natural size 3 avalanche was reported Sunday on steep alpine feature. The failure was down to 100-150 cm the December facet/crust layer and ran full path. See the MIN report here.
The past week, numerous very large (size 2.5-4.5) deep persistent slab avalanches occurred in both the Selkirks and Monashees. These avalanches failed on the early December facet/crust layer, typically 100-150 cm deep. This layer is likely to plague our snowpack for the remainder of the season and will be a low probability but extremely high consequence result if triggered.
A variety of snow surfaces can be found: crusts at lower elevations and on solar slopes into the alpine, wind press and wind slabs in open and exposed terrain, and settling powder in sheltered areas. Surface hoar growth has also been reported at the surface and within the upper snowpack in sheltered areas.
The most recent snowfall covered a layer of surface hoar now down 20-30 cm, likely responsible for the most recent natural storm slab avalanches reported Jan 20-21. The top 50-100 cm of the snowpack has a variety of little layers that may or may not be a problem. These layers include surface hoar and crusts from January snowfalls and facets buried at the start of January.
The primary weak layer that remains a widespread concern across the Columbia Mountains is a deeply buried crust/facet layer that formed in early December. It is typically down 100-150 cm, but as deep as 200 cm in wind-loaded terrain. There has been regular avalanche activity on this layer at all elevations for over a month, but we are now seeing decreasing evidence since the last warm event.
