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Avalanche Forecast

Dec 28th, 2011–Dec 29th, 2011
Alpine
4: High
The avalanche danger rating in the alpine will be high
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Purcells.

Confidence

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

Weather Forecast

Overnight Wednesday: Snow amounts 10cms. Moderate ridgetop winds from the SW. Thursday: A ridge of high pressure will likely build. This will bring dryer conditions, with possible sunny breaks in the Purcells. Ridgetop winds from the SW 30-70km/hr. Freezing levels around 1000m. Friday: Expect another frontal system spreading light-moderate amounts of snow. Winds will pick up from the South. Saturday: Mainly dry conditions.

Avalanche Summary

Numerous small size 1 slab and loose avalanches occurred on Tuesday. 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 30cms of snow has buried a new surface hoar layer (December 24th). 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 50cms 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.

Avalanche 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

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