Avalanche Forecast

Issued: Dec 27th, 2011 9:53AM

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

swerner, Avalanche Canada

Purcells.

Summary

Confidence

Fair - Timing, track, or intensity of incoming weather is uncertain

Weather Forecast

Overnight Tuesday: Strong SW ridgetop winds. Snow amounts up to 15cms. Freezing levels could rise to 1000m. Wednesday: Ridgetop winds 60-70km/hr from the SW. Snow amounts 5-15cms. Freezing levels could rise to 1200m. Thursday: Ridgetop winds 55-90km/hr from the W. Snow amounts 5cms. Freezing levels dropping back down to valley bottom. Friday: Snow amounts 10-20cms. Freezing levels at valley bottom starting to rise in the evening.Check out the new Forecasters Blog Post to learn more about weather patterns, and timing in regards to how it affects our avalanche bulletins.

Avalanche Summary

On Monday up to four different skier triggered avalanches occurred. They sized 1-1.5 released in facets down 30cms, and a weak storm snow interface also down 30cms. In the Dogtooth Range natural cornice occurred that released a size 2. As more snow and wind enters the region its likely that we'll start to see more avalanche activity on these weak layers.

Snowpack Summary

Consistent winds have deposited soft slabs onto leeward exposed slopes, and terrain features. Up to 20cms of snow has buried a new surface hoar layer. I'm not sure how widespread this exists in the Purcells. With additional load this layer may fail stepping down to the mid-December layer. There is about 40cms 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 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, South, South West.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 4

Persistent Slabs

Storm snow will continue and slab avalanches will be a concern on the persistent mid-December surface hoar/facet interface.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

2 - 6

Valid until: Dec 28th, 2011 8:00AM