Avalanche forecast

Purcells

Nov 29 – 30, 2011Avalanche Canada
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Archived · expired 15 years ago

Avalanche danger

Thu
Fri
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–4
    Changing winds have created new wind slabs in the alpine and at treeline on all aspects. Wind-slabs may cause avalanches to step down to the weak layer near the ground.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–6
    Where it still exists, weak basal layers in the snowpack are suspect to fail under stress of the strong slab above. It is capable producing large full depth avalanches. Large avalanches may continue to be a concern for human triggers.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–3
    Recent storm snow has created a storm slab that is likely to be triggered by backcountry riders. Avalanches started in the storm snow may step down to the weak layers near the ground.

Confidence

Fair - Due to limited field observations

Weather Forecast

Light-moderate snow amounts will end sometime Tuesday evening. A trough over the interior will move through to the east Wednesday morning bring a ridge of high pressure which will produce clear skys over the region into Thursday and Friday. Freezing levels could rise up to 1500m during the day, then lower back to valley bottom at night.

Avalanche Summary

In the Dogtooth range isolated natural avalanche activity wasobserved. These avalanches occurred from steep, rocky terrain at daysend Sunday, up to size 1.5 due to rapid loading. I suspect this mayhave been the norm through the region as the freezing level was at1800 meters and a surge of new snow and wind hammered through. Thismay be a good time to wait out the storm, let the new snow and thesnowpack settle, and make observations from your local s mountainswhen the visibility improves.

Snowpack Summary

Snow depths in the alpine are between 170 & 250cm. I suspect Sunday's warm, moist storm combined with strong southwesterly winds created stiff wind slabs on lee slopes, and some storm slab instabilities at upper elevations A highlighted concern for the Purcells was the mid-November storms that created a heavy slab over a weak basal faceted base. The Purcells saw a significant cycle of large avalanches running full depth as a result. (The Lizard Range also saw very similar results all failing on the weak basal layers.) . While the upper snowpack is likely tightening up, an avalanche triggered in the upper layers or initiated from a shallower spot in the snowpack could step down to weak basal facets at the ground creating large & unmanageable avalanches with very high consequences.

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