Avalanche Forecast

Issued: Dec 8th, 2020 4:00PM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is low. Known problems include Wind Slabs and Deep Persistent Slabs.

Avalanche Canada swerner, Avalanche Canada

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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.

Weather Forecast

A general cooling and drying trend through the forecast period.

Wednesday: Mix of sun and cloud. Alpine temperatures near -6 and freezing levels 400 m. Ridgetop wind is generally light from the southwest with some strong gusts.

Thursday: Overcast with flurries. Alpine temperatures near -7 and freezing levels dropping to 500 m. Ridgetop wind moderate from the southeast.

Friday: Cloudy with sunny periods. Alpine temperatures -7 with freezing levels down the valley bottom. Ridgetop wind light from the southwest.

Avalanche Summary

No recent avalanche observations reported.

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

Have you been out playing in the mountai8ns? If so, we'd love to hear from you! You don't necessarily have to submit a technical report as a photo can say it all! Please share your observations on the Mountain Information Network (MIN). Thanks for submitting!

Snowpack Summary

10-20 cm of new snow fell at upper elevations and may sit above a newly formed crust from the warmer storm this past weekend. 

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. 

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. 

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 (check out this MIN report for a great visual). Lower elevation terrain may still be below the threshold for avalanches

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

An icon showing 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

Possible - Likely

Expected Size

1 - 2

Deep Persistent Slabs

An icon showing 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

Possible

Expected Size

2 - 3

Valid until: Dec 9th, 2020 4:00PM