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Backcountry Advisory

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Backcountry Advisory

As of Nov. 28

The cool, unusually dry November was great for prolonging the fall mountain bike season, but it has given the mountain snowpack a bit of a shaky start. The initial precipitation of the winter formed a layer of laminated crusts, 40-60 cm. thick that seems to be well bonded to the ground. The next storm deposited a layer of snow and was followed by the week of sunshine. By the end of the week, this layer was transformed into loose facets overlain by surface hoar crystals. Now that our more normal November storms are finally starting to prevail, the avalanching that everyone expected is beginning to happen.

The sliding layer is variable. Some of the occurrences are going on a weak temperature crust at the top of the facetted layer (temperatures reached 6.0C at 1,850 m. on Nov. 21). Others are failing 2-4 cm into the facets. On one occasion, the crown line stepped down to the summer firn. (Whistler Bowl on Whistler Mtn.) The crown lines are averaging 40-60 cm in depth, but the occurrence in Whistler Bowl maxxed out at 140 cm. We have seen a variety of triggers – extensive naturalling in the backcountry, skiers/boarders remotely (up to 300 m. distant) and skiers/boarders on the slab. On Whistler and Blackcomb, some explosive testing has been carried out.

What should we anticipate for the future? As less snow covered slopes continue to receive a load, they too will likely fail. Unfortunately, the avalanching is not restricted to a particular aspect or elevation. The only requirement seems to be adequate coverage. Another concern is whether the problem has been eradicated in start zones that have avalanched. In one instance, the bed surface was the weak temperature crust, and the 10 cm. layer of facets was still well preserved below. In rocky terrain, the facets are still very much in evidence around the crown lines. How well the new snow will bond in these areas is something that the usual storm variables will determine.

The only sure thing is that we will probably be faced with this weakness in the snowpack for a while yet. Travel with extreme caution in the backcountry. Remember that with each new snowfall, more terrain will receive the coverage needed to promote a failure on the weak layer.

As of Nov. 28, the backcountry avalanche danger is trending to CONSIDERABLE from HIGH. The forecast storms will likely put it to HIGH again.

Conditions may vary and can change rapidly. For more information, call the CAA Public Bulletin @ 1-800-667-1105 or contact your local ski patrol.

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