Avalanche forecast

Purcells

Dec 5 – 6, 2011Avalanche Canada
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Avalanche danger

Wed
Thu
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–3
    Stiff wind slabs in the alpine and open areas at treeline may be triggered by skiers and riders. It may be possible to remotely trigger a slide from a weak spot nearby.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible - Likely
    Size
    2–5
    The weak facetted layer near the ground may still be triggered by human activity. Areas with a lot of variability in the snow cover may be the most suspect. Triggering may occur from weak spots like rocks or clumps of small trees.

Confidence

Fair - Due to limited field observations

Weather Forecast

A fast moving cold front is forecast to produce some light precipitation combined with strong winds from the southwest early Tuesday. The freezing level should drop to valley bottom overnight and remain below treeline on Tuesday. However, warm air is forecast to stay in the alpine during the day. Wednesday and Thursday should be cooler and drier with freezing levels rising to 1500 metres during the day.

Avalanche Summary

There are a few reports of explosive controlled avalanches up to size 2.0 in pockets that are releasing near the ground on weak facetted crystals. We have dropped the danger rating in the alpine to moderate. It is still likely that you could trigger a windslab in steep terrain. Triggering a full depth release is getting less likely, but weak spots are sure to exist that may propagate a large avalanche. The snow pack is still relatively young and needs some time to mature.

Snowpack Summary

We suspect that surface hoar may be growing during the clear cold weather that has dominated over the past few days. Some steep south facing slopes may have developed a thin sun-crust. Windslabs continue to be a concern in the Purcells. Strong winds last week developed unstable slabs of transported snow on steep terrain in the alpine and at treeline. Snow depths are quite variable through the region. In the alpine there is 170- 250cm. At treeline there is anywhere from 130-150cm.The upper snowpack is tightening up forming a stronger mid-pack. A highlighted concern for the Purcells was the mid-November storms that created a heavy slab over a weak basal faceted base, resulting in a significant cycle of large avalanches running full depth.

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