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Avalanche Forecast

Archived

Dec 29th, 2011–Dec 30th, 2011

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
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 - Intensity of incoming weather is uncertain on Friday

Weather Forecast

Friday: Snow amounts 5-10cms. Ridgetop winds 40km/hr from the West. Freezing levels could rise to 1200m. Saturday/Sunday: A ridge of high pressure is expected to move into the region for the weekend. Mainly sunny skies, and seasonal temperatures with a light-moderate NW flow.

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

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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.