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Avalanche Forecast

Archived

Dec 19th, 2021–Dec 20th, 2021

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

Regions

Purcells.

Conditions are more complicated than meets the eye. Wind-drifted snow and buried weak layers warrant a cautious approach and diligent decision making. 

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Sunday Overnight: High pressure will continue to strengthen into the evening. Light northerly winds and clear skies. Temperatures dropping to -15 C in the alpine. 

Monday: Clear skies with alpine temperatures around -15 C. Light southwest winds at ridgetop.

Tuesday: Mainly clear with moderate west winds at ridgetop. Temperatures rising with an alpine high of -8 C. 

Wednesday: Mainly cloudy with flurries, 3-15 cm accumulation. Moderate to strong southwest winds at ridgetop. Alpine temperatures around -8 C.

Avalanche Summary

On Sunday, operators reported ski cutting and explosive storm slab results to size 1 in the alpine and treeline.

On Saturday, operators reported reactive wind slabs developing throughout the day.

On Friday, operators in the eastern Purcells triggered several size 2-2.5 deep persistent avalanche with explosives on N and NW aspects in the alpine.

Snowpack Summary

Saturday's storm brought 20-50cm of new snow to the region. This new snow was accompanied by moderate to strong southwest winds which have formed wind slabs in the alpine and treeline.

Below this new snow, 40-60cm of consolidated snow sits over a substantial crust that formed in early December. This crust is 10-30 cm thick and can be found down 60-120 cm. It is present across aspects below 2300m. A thin layer of weak facets (sugary snow) can found above this crust that has demonstrated reactivity in snowpack tests. This problem is particularly hard to predict and tricky to manage. For this reason wide, conservative terrain margins and disciplined backcountry travel techniques will be very important. Get more details and photos in our forecaster blog. 

A late October facet/crust layer of concern sits at the bottom of the snowpack above 1900m. Although this layer has been trending less reactive, it remains on our radar.

The snowpack depth at treeline is around 115-200 cm. The deepest snowpack can be found near the Bugaboos. 

Terrain and Travel

  • Be careful as you transition into wind affected terrain.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Avoid shallow snowpack areas, rock outcroppings and steep convex terrain where triggering is most likely.

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.