Avalanche Forecast
Issued: Dec 7th, 2020 4:00PM
The alpine rating is Wind Slabs and Deep Persistent Slabs.
, the treeline rating is , and the below treeline rating is Known problems includeFresh and reactive wind slabs may build on leeward slopes and behind terrain features in the alpine. Snowpack conditions can vary from one slope to another at all elevations, beware of changing conditions and investigate deeper layers before jumping into committing terrain.
Summary
Confidence
Low - Uncertainty is due to limitations in the field data. Uncertainty is due to field data and reports showing a wide variation in conditions throughout the region.
Weather Forecast
Monday Night: 10-15 cm expected overnight. Alpine temperatures near -3 and ridgetop wind strong from the southwest. Freezing levels 1200 m.
Tuesday: Alpine temperatures near -4. Ridgetop wind will be strong from the southwest. Freezing levels 1000 m.
Wednesday: Alpine temperatures near -7. Ridgetop wind will be strong southwest. Freezing levels 500 m.
Thursday: Some flurries up to 5 cm. Alpine temperatures near -10 with freezing levels falling to 500 m. Winds from the South will be strong.
Avalanche Summary
No recent avalanche observations reported on Monday. I suspect some natural avalanche activity will occur on Tuesday with continued snow and wind.
During the weekend storm, professional operators reported small to large (size 1-2) natural and explosive-triggered wind slabs on northwest, north, and northeast aspects in immediate lee features. During peak warming, a widespread cycle of small loose wet avalanches was observed up to 1800 m.
Areas near Ningunsaw continue to see large to very large deep persistent slab avalanches (size 2-3.5) releasing on weak snow at the ground. Outside of the region near Tatlatui Provincial Park on Friday, operators triggered two large (size 2) avalanches with explosives, also failing on basal facets. Shallow wind slabs or cornice falls may have the potential to step-down to this deeper layer and produce large, destructive avalanches. Open alpine slopes where the snow transitions from thick to thin are most suspect
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Snowpack Summary
10-20 cm of new snow fell at upper elevations Monday night and may sit above a newly formed crust from the warmer storm this past weekend.Â
In many treeline and below treeline areas, the combination of above freezing temperatures and rain saturated the entire snowpack. As a result, snow has melted fast at lower elevations, and snowpack depths have seen rapid settlement.Â
As the freezing level gradually drops below 1000 m, a widespread melt-freeze crust will be left behind across aspects and elevations. Ongoing southwest winds are expected to drift 5-15 cm of new snow into wind slabs on lee and cross-loaded features in the alpine or in "alpine-like" features at treeline. Where snow accumulates over the recently formed melt-freeze crust, shallow slabs may be possible to trigger.Â
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.Â
Snowpack depths vary substantially with aspect, elevation, and wind exposure (check out this MIN report for a great visual). Lower elevation terrain may still be below the threshold for avalanches.Â
Terrain and Travel
- Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
- Avoid freshly wind loaded terrain features.
- Use caution on large alpine slopes, especially around thin areas that may propagate to deeper instabilities.
Problems
Wind Slabs
As the freezing level gradually drops below 1000 m, new snow with moderate southwest winds are forecast to continue. Where snow accumulates, winds may drift the available snow into shallow wind slabs on lee and cross-loaded features in the alpine or in "alpine-like" features at treeline. Avoid areas that are prone to human-triggering, such as steep, rocky, or convex features near ridge crests and roll-overs.
Aspects: North, North East, East, West, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Deep Persistent Slabs
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.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 8th, 2020 4:00PM