Bulletin d’avalanche

Northwest Coastal

Dec 9 – 10, 2020Avalanche Canada
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Archivé · expiré 6 years ago

Danger d’avalanche

Fri
Sat
Alpin
3 · Marqué
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Points saillants

Natural avalanche activity will likely decrease with a cooler and drier weather pattern. However, conditions may still be primed for human triggered avalanches.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely - Possible
    Taille
    2.5–3.5

    Large to very large avalanches over the past week provide evidence that a buried crust formed in early November combined with weak sugary snow at the ground are reactive persistent and deep persistent weak layers. Observations suggest that this problematic snowpack structure may be more prevalent in colder, shallower areas North and in the East in the region. Keep in mind that storm slab avalanches and cornice falls have the potential to step-down to these deeper layers, producing large, destructive avalanches.

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible - Likely
    Taille
    1–2.5

    Natural storm slab avalanche activity will likely decrease, however; they could remain reactive to human triggers. These will be more sensitive to triggering and larger in size in areas where strong winds have transported the snow into deeper slabs. Natural triggers from heavy snowfall, wind, and cornice falls are a concern.

Confidence

Moderate - Confidence is due to a stable weather pattern with little change expected.

Weather Forecast

A general cooling and drying trend through the forecast period.

Wednesday: Mainly cloudy. Alpine temperatures near -3 and freezing levels 700 m. Ridgetop wind is generally light from the southwest.

Thursday: Overcast. Alpine temperatures near -5 and freezing levels dropping to 500 m. Ridgetop wind moderate from the East.

Friday: Cloudy with sunny periods. Alpine temperatures -9 with freezing levels down the valley bottom. Ridgetop wind light from the southwest.

Avalanche Summary

On Tuesday a widespread natural avalanche cycle and to size 3 and reports of a size 4 in the Bear Pass area. These larger slab avalanches are said to be mostly storm instabilities with some active persistent slab avalanches. As conditions start to get cooler and dryer the natural avalanche activity will likely decrease, however; uncertainty sits with the persistent slab problem which should remain on your radar.

Have you been out riding in the mountains? We would love to hear about it and it doesn't need to be technical! Photos can be extremely helpful, please share your observations on the Mountain Information Network (MIN). Thanks!!

Snowpack Summary

As of Tuesday morning, weekend storm totals ranged from 55-110mm of water with above freezing temperatures up to 1900 m and now dropping significantly. At summit elevations and in areas further north in the region, precipitation fell as snow and some operators are reporting up 90 cm throughout this stormy period. Strong winds from the south have rapidly loaded areas receiving snow with touchy storm slabs. More snow and strong southwest wind will continue to build these storm slabs over the next few days. Cornices may also become overloaded and weak. Below the freezing level, the snowpack is saturated at this point but may start to freeze into a crust as the freezing level drops.

The intense periods of precipitation has the potential to bring buried weak layers to their breaking point. Two early season weak layers are still on our radar: a facet/crust combination that formed in early November and a layer of facets near the ground. In the Ningunsaw area, basal facets have recently produced very large deep persistent slab avalanches. The early November crust also been reactive in the Bear Pass area within the last week. The extent of this problematic snowpack structure in the region is not well-known, but it likely exists in colder, shallower snowpack areas.

Snowpack depths are highly variable across aspects and elevations as a product of wind scouring, above-freezing temperatures, and rain. Snow has melted fast at lower elevations, and snowpack depths have seen rapid settlement.

Terrain and Travel

  • Keep in mind that human triggering potential persists as natural avalanching tapers off.
  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.

Régions couvertes