Avalanche forecast

Purcells

Dec 3 – 4, 2013Avalanche Canada
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Avalanche danger

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

Large variation in storm snow totals occurred across the forecast region. The current danger rating is largely being driven by conditions in the south. If you have been out in the mountains, please send your observations to [email protected].

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely - Possible
    Size
    4–5
    The load from the new snow is testing the deeply buried October crust weak layer.  The additional load of a rider or a surface avalanche may be all that is require to tip the scale.  These deeper layers can produce large and destructive avalanches.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    3–5
    The storm slab problem will be heightened in the southern portion of the forecast region where 70cm of new snow fell. To complicate the scenario, a shift in wind pattern has northerly winds depositing wind slabs on south facing slopes.

Confidence

Fair - Due to the number of field observations

Weather Forecast

The arctic ridge of high pressure dominates the interior of British Columbia and will persist for the forecast period. Mainly clear, cold and dry conditions will prevail. Expect periods of moderate northerly winds and alpine temperatures to be chilly, -15C to -25C by Friday.

Avalanche Summary

Explosive work today produced numerous avalanches to size 2 on north aspects in the alpine. In some areas (particularly the south), significant storm snow now overlies well preserved weaknesses which may be primed for rider triggering.

Snowpack Summary

The snow distribution varies greatly across the region but average depth of snow at treeline is approximately 60-110cm.The southern portion of the region received substantially more snow (40-70cm) than the north in the past storm. The storm slab is sitting on top of various weak layers; surface hoar on sheltered, shaded slopes and a melt-freeze crusts on south facing slopes. In the south Purcells, there have been reports of preserved surface hoar crystals overlying a melt-freeze crust below treeline around 1500m. In this location, 70cm overlies this weak layer which produced very easy results in stability tests. Reports are sparse, but it is likely this same layer exists in sheltered locations in the alpine and at treeline. Digging deeper, the lower snowpack is fundamentally weak, composed of an early season crust (more prevalent on north aspects) and weak facetted crystals. The weight of new snow may be currently 'testing' these deeper layers and the additional load of a rider (particularly in shallow snowpack areas) may just be enough to tip the scale. If so, persistent slabs have the potential to produce large, destructive avalanches.

Regions covered