Avalanche Forecast
Purcells
Cold temperatures mean slow change in the snowpack. Signs of wind effect and slab formation will be the best indicators of avalanche danger - especially at lower elevations.
Avalanche Problem 1: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- Surface hoar distribution is highly variable. Avoid generalizing your observations.
- Watch for signs of slab formation such as whumpfing, or cracking.
- Buried surface hoar is more likely to be preserved in sheltered openings below the alpine.
Avalanche Problem 2: Wind Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Shifting winds have been redistributing storm snow into wind slabs on a variety of aspects. Northwest winds have been the most consistent. Exercise extra caution at lower elevations where winds may have formed a slab over a touchy surface hoar layer.
Travel and Terrain Advice
- Be aware of variable wind loading patterns.
- Travel on ridges and/or ribs to avoid wind loaded slopes below
Avalanche Summary
Sunday's reports include observations of numerous explosives-controlled persistent slab releases east of Invermere. Sizes were typically smaller, with one Size 1.5 recorded. These all occurred on north to northwest aspects in the alpine. A MIN report from the Dogtooth Range on Saturday showed remote triggered Size 1 releases occurring in sheltered areas below treeline. Another MIN report from Friday included observations of ski traffic easily producing shooting cracks in mellower terrain between 1700-2000m in the Dogtooth. These releases and cracks likely failed at the December 15th interface.
Persistent slab avalanche danger can be expected to persist in areas where last week's storm snow consolidates into a slab above the December 15 interface. In areas where new snow has not yet consolidated into a slab, expect continued potential for triggering loose dry avalanches in steep terrain.
Snowpack Summary
Last week's storm brought 30-50cm of new snow to the region. Predominantly northwest winds have since redistributed this storm snow into wind slabs in lee terrain at higher elevations while cold temperatures have inhibited slab formation elsewhere. Cold has also worked to decrease the cohesion and reactivity of older wind slabs.
The storm snow lies over a variety of old surfaces, including large surface hoar (weak, feather-like crystals), crusts formed by sun or wind, and sugary facets. The bond at this old snow interface is of critical importance where the overlying storm snow has consolidated into a slab. The most concerning areas are those that saw pronounced surface hoar development before the storm, such as sheltered areas at and below treeline. Recent snowpack tests show wide ranging reactivity at this interface but suggest that it may be most reactive on northerly aspects.
A crust which was formed by rain in late November remains a major feature in the snowpack and is down approximately 60-100cm at tree line elevations. This interface shows good signs of bonding in the Purcells, but has indicated some reactivity in steep, variably loaded alpine features in adjacent regions.
Weather Forecast
Wednesday: Mainly cloudy with isolated flurries and a trace of new snow. Light southwest winds. Alpine high temperatures of -16.
Thursday: Increasing cloud with scattered flurries bringing 2-4 cm of new snow, increasing overnight. Light west winds. Alpine high temperatures of -12
Friday: Continuing flurries bringing 3-6 cm of new snow, increasing overnight. Light southeast winds. Alpine high temperatures of -10.
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A weak layer of surface hoar buried 30-50 cm deep has potential to produce large avalanches where the overlying snow has settled into a cohesive slab. The greatest caution is needed around steeper, sheltered slopes at treeline and below.