Wind slabs may be reactive to human triggers at upper elevations. Heads up! The winds changed on Thursday and reverse loading may build isolated wind slabs on opposite slopes for Friday.
Wind slab may exist and be reactive on lee and cross-loaded features in the alpine or in "alpine-like" features at treeline. Forecast East winds on Thursday may reverse load oposite slopes.
Very large avalanches over the past week provide evidence that faceted snow at the ground is a reactive deep persistent weak layer. Observations suggest that this problematic snowpack structure may be more prevalent in colder, shallower areas north and east in the region. Keep in mind that small avalanches and cornice falls have the potential to step-down to this deeper layer, producing large, destructive avalanches.
The cooling and drying trend continues with a mix of sun and cloud and isolated flurries Saturday and Sunday. Alpine temperatures will remain cool at -8 or less and freezing levels will remain in the valley bottom. Ridgetop winds will blow moderate from the east-southeast through the forecast period.
No new avalanche reports on Thursday.
On Wednesday, a bit more evidence of natural avalanche activity up to size 2 came forward from the last storm.
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Up to 20 cm of recent snow fell at upper elevations and may sit above a newly formed crust from December 6th. This new snow may have a poor bond to the underlying crust and be reactive to human triggering. Recent strong winds from the southwest may have sifted the new snow onto lee aspects and behind terrain features building isolated wind slabs.
In many treeline and below treeline areas, the combination of above freezing temperatures and rain saturated the entire snowpack. As a result, the snow has melted fast at lower elevations, and snowpack depths have seen rapid settlement. With the current cool conditions and freezing levels falling to the valley bottom I suspect a widespread melt-freeze crust may exist.
At the base of the snowpack sits the early November crust with basal facets below. Currently, hard shear snowpack test results exist on this potential weak interface and should continue to be tracked as the snowpack builds. In areas north and east in the region, the bottom of the snowpack reportedly consists of a crust from early November and weak facets near the ground. These basal facets have produced very large deep persistent slab avalanches near the Ningunsaw and Tatlatui Provincial Parks. Although the extent of this snowpack structure in the region is not well-known, it may be more prevalent in colder, shallower areas north and east in the region.
Snowpack depths vary substantially with aspect, elevation, and wind exposure. Below treeline the terrain may still be below the threshold for avalanches.
