Wind slabs may remain reactive to human triggering in exposed high elevation terrain. There is ongoing concern for the early-December deep persistent problem and we are now in a period of low probability, high consequence with this layer.
Wind slabs may remain reactive to human triggering in exposed terrain at higher elevations. Ongoing periods of strong wind have 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.
The ridge of high pressure brings two more days of dry and sunny conditions before the next storm system arrives Saturday night bringing snowfall for Sunday.
Thursday Night: Mainly clear with valley cloud, light SW wind, treeline temperature around -8 °C.
Friday: Sunny with valley cloud, light SW wind, treeline high around -3 °C.
Saturday: Increasing cloud cover, light to moderate SW wind, treeline high around -3 °C.
Saturday night and Sunday: Snowfall 10-20 cm, moderate SW wind, treeline high around -3 °C.
On Wednesday, a cornice fall triggered a size 2 storm slab on a N aspect at 2000 m. An older persistent slab avalanche was reported on a W aspect at 2300 m and some older cornice falls up to size 2.5 were reported on a NE aspect at 2300 m. On Tuesday, a variety of natural wind/storm slabs up to size 2 and cornices up to size 3 were reported but most of this natural activity had occurred on Monday or Sunday. A cat triggered a cornice fall which triggered a size 1.5 slab on the slope below at 2200 m on a N aspect. On Monday, natural wind slabs to size 2 were reported and explosive control triggered a deep persistent size 3 avalanche in the Monsahees, with a fracture line of 120 cm deep.
Between January 20 and 23, numerous very large (size 2.5-4.5) deep persistent slab avalanches occurred. These avalanches failed on the early December facet/crust 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 polar aspects. Ongoing surface hoar growth up to 12 mm has also been reported to be widespread in sheltered areas which is expected to be most prominent at elevations near the top of the valley fog layer.
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.
