Avalanche Forecast

Issued: Dec 30th, 2011 2:53AM

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

ghelgeson, Avalanche Canada

Purcells.

Summary

Confidence

Fair - -1

Weather Forecast

One last pulse out of the recent active weather system will affect the region Friday evening before a strong ridge of high pressure anchors itself over Eastern BC for the weekend. Skies clear and cooler temperatures pervade under the ridge which will bring freezing levels down to valley bottom. On Saturday you can expect moderate winds out of the W and a daytime high of -9 with overnight lows dropping slightly to -12. A weakening system slides into the region Monday bringing light precipitation and cloudy skies.

Avalanche Summary

An operation in the North end of the region saw a natural avalanche cycle up to size 3.5 on Wednesday. Additionally naturals up to size 1.5 were reported in the Southern sections. There was also a skier remote avalanche that was triggered from 25m away. This failed on the December 24th surface hoar layer. Where this layer exists is quite touchy and has wide propagation propensity.

Snowpack Summary

The region has received up to 40cms of new snow since Christmas. New snow and consistent winds have deposited soft slabs onto leeward exposed slopes, and terrain features. This new snow has buried the December 24th surface hoar layer. I'm not sure how widespread this exists in the Purcells, but it also formed in adjacent regions which makes me suspicious. With additional load this layer may fail stepping down to the mid-December layer. There is about 60-80cms of snow sitting over the Mid-December interface of surface hoar and facets. Some observations suggest this layer is gaining strength, but still remains within the threshold of human triggering where there are sufficient load/slab properties. This will be the layer to watch as more snow and wind accumulates this week. Below this weak interface the mid pack is well-consolidated and strong. Near the base of the snowpack there are a few layers that have the potential to trigger with very heavy loads or from shallow spots. These include a surface hoar/crust/facet layer from early November, and the basal facets/depth hoar that are currently unreactive but still a concern in areas that have not previously avalanched.

Problems

Wind Slabs

Predominantly on North through South East aspects, but some localized northerlies have formed wind slabs into unusual places. Stiff snow surfaces, cracking and hollow drum like sounds are signs of unstable snow.

Aspects: North, North East, East, South East.

Elevations: Alpine, Treeline.

Likelihood

Likely - Very Likely

Expected Size

1 - 4

Storm Slabs

Storm slabs can be destructive, run far and fast. The slabs are sitting on two weak layers in the upper 100cms of the snowpack. The weight of the slab may trigger the initial weakness, then step down to the deeper layer creating large avalanches.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

1 - 4

Persistent Slabs

As the load increases the mid-December buried surface hoar layer may become reactive. The failure of this layer could initiate large, destructive avalanches.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

2 - 6

Valid until: Dec 31st, 2011 8:00AM