Wind slabs remain triggerable in the alpine. Watch for wind-loaded pockets especially around ridgecrest and in extreme terrain.
New snow on the way Saturday night!
Wind slabs may remain reactive to human triggering in exposed terrain at higher elevations. Ongoing wind has continued to add to these slabs where there is still loose snow available for transport. Expect them to be most reactive in steep, unsupported terrain features or places where the slab overlies a lingering weak layer.
The early December crust can be found down 100-180 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 has decreased, there is ongoing concern for cornice falls or smaller avalanches stepping down to this layer. Extra caution is recommended around thin snowpack areas and reloaded bed surfaces.
Friday night: Partly cloudy, light SW wind, treeline temperature around -12 °C.
Saturday: Increasing cloud cover, light to moderate SW wind, treeline high around -5 °C.
Sunday: Snowfall 10-20 cm, moderate SW wind, treeline high around -5 °C.
Monday: Snowfall 5-15 cm overnight then clearing, light NW wind, treeline high around -10 °C.
The theme over the past week has been large cornice failures and cornice-triggered wind slabs size 2-3 on north to east aspects in the alpine.
Deep persistent slab activity on the early December facet/crust layer has slowed down during the dry spell. Evidence of an older persistent slab avalanche was reported on Wednesday in the Selkirks, on a west aspect at 2300 m. On Monday, explosive control triggered a deep persistent size 3 avalanche in the Monsahees, with a fracture line 120 cm deep. Between January 20 and 23, numerous very large (size 2.5-4.5) deep persistent slab avalanches occurred on this layer, typically 100-150 cm deep.
The snow surface is highly variable and consists of a crust at lower elevations and on solar slopes extending into the alpine, wind affected surfaces and wind slabs in exposed terrain, and settling powder in sheltered areas on shady aspects. Large feathery surface hoar crystals have been reported growing widespread in sheltered areas, most prominently at elevations near the top of the valley fog layer. This is a layer to watch out for once it gets buried.
There are three prominent layers in the upper snowpack which are mainly dormant now but could still create a problem during the next storm or warming event. The storm on Jan 20 covered a layer of surface hoar which is now down 20-30 cm. A layer of facets and surface hoar from early January is now down around 40-60 cm. A thick layer of facets from the cold drought end of December is now down close to 1 m.
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 100-180 cm, but as deep as 250 cm in wind-loaded terrain. 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.
