Avalanche activity is decreasing, but a complex snowpack presents a low probability, high consequence scenario. A tricky snowpack is best managed with conservative terrain selection.
The upper snowpack is tricky right now. Recent storm snow has been most reactive where it overlies surface hoar. Along with a handful of suspect layers in the upper snowpack, expect to find a more reactive slab on wind-affected slopes at higher elevations, and anywhere a slab forms over crust or surface hoar layers.
The early December crust can be found down 100-200 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.
SUNDAY NIGHT: Cloudy, cooling overnight. Decreasing northwest wind, temperatures dropping to -10 C, freezing level to valley bottom.
MONDAY: Broken skies, clearing in the afternoon. Decreasing west wind, daytime high temperatures dropping to -6 C, and freezing level below 700 m.
TUESDAY: Increasing cloud. Light southwest wind. High temperature -8 C. Freezing level at valley bottom.
WEDNESDAY: Sunny with increasing cloud. Light southwest wind. High temperature -8 C.
On Saturday, explosives near the Trans Canada triggered very large avalanches (size 2.5-3.5) failing on the early December facet/crust layer. This included a historical size 4.5 avalanche which started at ridgeline, created new trim lines, and destroyed mature timber. Avalanche reports also included wind slabs reactive to skiers and explosives to size 2, and one natural size 3 wind slab avalanche triggered by a cornice.
Between Thursday and Friday, size 1-2.5 storm and wind slab avalanches failed naturally or were triggered by riders/machines in the North Columbia region, these averaged 20-40 cm deep, and in some cases a weak layer of surface hoar or crust was reported at the interface. In neighboring South Columbia Forecast Region, another round of very large (size 2.5-4) deep persistent avalanches failed naturally in the same timeframe, with rapid loading and warming temperatures being the likely trigger. In the same timeframe, An impressive report documents a size 4 avalanche in Rogers Pass here.
This past week numerous very large (size 2.5-3.5) 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. One notable pattern is that many of these avalanches released at relatively low elevations (1700 to 2000 m) in open clearings, burns, and slide paths. In many cases, they were triggered by smaller avalanches starting at higher elevations. 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 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.
The most recent snowfall covered a layer of surface hoar now down 20-30 cm, likely responsible in the most recent natural storm slab avalanches reported Jan 20-21. The upper snowpack (top 30-100cm) has variety of layers that may or may not be a problem. These layers include surface hoar and crusts buried during January snowfalls and facets buried at the start of January. While none of these upper layers are necessarily going to remain a long-lasting problem, there is a fair bit of uncertainty about their spatial distribution and whether they will remain reactive with the warm temperatures.
The primary weak layer that remains a widespread concern across the Columbia Mountains is a 100-200 cm deep crust/facet layer that formed in early December. There has been regular avalanche activity on this layer at all elevations for over a month, and we expect to see avalanche activity continue on this layer.
