Avalanche Forecast

Issued: Dec 18th, 2012 9:43AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is considerable. Known problems include Storm Slabs, Persistent Slabs and Deep Persistent Slabs.

ccampbell, Avalanche Canada

Northwest Coastal.

Summary

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.

Problems

Storm Slabs

New snow will likely be sensitive to human triggers, particularly in wind-exposed terrain. Natural avalanches are possible, particularly on northwest through east aspects.

Assess start zones carefully and use safe travel techniques.>Be cautious as you transition into wind affected terrain.>Avoid freshly wind loaded features.>Choose well supported terrain without convexities.>Give cornices a wide berth when travelling on or below ridges.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

1 - 4

Persistent Slabs

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.

Avoid open slopes and convex rolls at and below treeline where buried surface hoar may be preserved.>Make observations and assess conditions continually as you travel.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 5

Deep Persistent Slabs

Deeply buried weaknesses exist. Large triggers, such as cornice falls, could release deeper avalanches on steep, smooth terrain, especially in thinner snowpack areas.

Carefully evaluate and use caution around thin snowpack areas.>Avoid thin, rocky or sparsely-treed slopes.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely

Expected Size

3 - 7

Valid until: Dec 19th, 2012 2:00PM