Avalanche forecast

Northwest Coastal

Dec 18 – 19, 2012Avalanche Canada
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Avalanche danger

Thu
Fri
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–5
    A persistent weak layer of surface hoar, buried 60-80 cm below the surface, has been reported from around Shames. The chance of avalanche activity on this layer will increase as the slab above it gets stiffer.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible - Likely
    Size
    1–4
    New snow will likely be sensitive to human triggers, particularly in wind-exposed terrain. Natural avalanches are possible, particularly on northwest through east aspects.

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    3–7
    Deeply buried weaknesses exist. Large triggers, such as cornice falls, could release deeper avalanches on steep, smooth terrain, especially in thinner snowpack areas.

Confidence

Fair - Due to limited field observations for the entire period

Weather Forecast

Wednesday: Moderate snowfall with 5-15cm of accumulation, strong to extreme southerly winds, and freezing levels remaining at or near valley bottoms. Thursday: Light snow with only a few centimetres of accumulation, moderate southeasterly winds and freezing levels in valley bottoms. Friday: Mostly cloudy with isolated light flurries, continued moderate southeasterly winds, and freezing levels remaining in valley bottoms.

Avalanche Summary

Reports from Monday include more evidence of the natural avalanche cycle from Friday, which included 30-75cm deep storm slab avalanches up to Size 2.0 on all aspects, but was primarily soft wind slab avalanches up to Size 2.0 on steep east through northeast aspects in the alpine.

Snowpack Summary

Up to 65cm of recent storm snow is settling rapidly. Recent compression tests gave easy to moderate sudden planar shears down 20cm and 35cm, as well as moderate resistant shears down 70cm, all on recent storm snow interfaces. A buried surface hoar layer, reported to be 5 to 10mm thick, recently produced sudden snowpack test results down 60-80cm in the Shames backcountry, and may exist in other areas too. Avalanche professionals continue to monitor a crust down near the ground. It seems this problem is confined to  alpine areas and likely needs a large trigger and/or a shallow snow pack feature to trigger. Cornices are also really big and forecast strong winds and cooling temperatures could make them especially heavy and brittle.

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