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Avalanche Forecast

Archived

Dec 7th, 2020–Dec 8th, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Northwest Inland.

Fresh 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.

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

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 Monday night 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

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.