Avalanche Forecast

Issued: Dec 20th, 2014 8:04AM

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

pgoddard, Avalanche Canada

Purcells.

There's uncertainty about snowfall amounts. If you receive more than about 15cm, consider the alpine danger HIGH.

Summary

Confidence

Fair - Intensity of incoming weather systems is uncertain

Weather Forecast

A burst of snowfall is expected overnight Saturday/Sunday (5-25cm), accompanied by moderate to strong westerly winds. On Sunday and Monday, expect flurries or light snow with light to moderate NW winds. Freezing levels are expected to reach around 1500m on Sunday, gradually dropping to near valley floor by Tuesday. A second storm system is due to arrive some time on Tuesday and may bring moderate snow.

Avalanche Summary

On Saturday, explosives triggered size 2-2.5 slabs at the northern end of the Purcells. These failed on the lower layer of surface hoar, and in isolated spots stepped down to the early November crust. Smaller slabs were also failing on the most recently buried layer of surface hoar. I expect that reactivity will increase by Sunday as more snow builds above these touchy layers. A size 2.5 naturally-triggered slab was observed at 2600m on an east aspect on Friday.

Snowpack Summary

New snow is building up and being shifted by winds onto lee slopes. This process may continue over the next couple of days, building thick wind slabs and fragile cornices. Two layers of surface hoar are buried in the upper snowpack. The more recent of these sits on a crust below about 2100m, and on settled snow at higher elevations. A hard rain crust with facets from early November is buried over 1 m down and is still reactive to light loads in some locations.

Problems

Storm Slabs

Building storm snow is expected to be shifted by winds into slabs on lee slopes. In more sheltered areas, loose snow avalanches are possible. The new snow sits above a weak layer of buried crystals.

Avoid exposure to terrain traps where the consequences of a small avalanche could be serious.>Be cautious of sluffing in steep terrain.>Avoid freshly wind loaded features by sticking to ridges and ribs.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 3

Deep Persistent Slabs

There is still the potential to trigger a deeply buried weak layer, resulting in a surprisingly large avalanche.

Avoid common trigger zones including thin snowpack areas, near rocky outcrops, and steep alpine slopes.>Remote triggering is a concern, watch out for adjacent slopes.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

2 - 5

Valid until: Dec 21st, 2014 2:00PM