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Avalanche Forecast

Archived

Dec 28th, 2011–Dec 29th, 2011

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

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.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.