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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

Kootenay Boundary.

Avoid wind loaded areas where human triggering remains a concern. If winds pick up today, watch for wind transport creating fresh, reactive wind slabs. 

A buried persistent weak layer continues to warrant careful terrain selection 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: Mainly clear skies with alpine temperatures around -12 C. Light west winds at ridgetop.

Tuesday: Overnight temperatures dropping to around -20 C. Mainly clear with moderate west winds at ridgetop. Temperatures rising with an alpine high of -12 C. Cloud cover increasing into the evening.

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

Avalanche Summary

On Saturday, operators reported several natural and human triggered storm slab avalanches up to size 1 in the alpine and treeline.

Snowpack Summary

Saturday's storm brought 10-30 cm of new snow to the region. On Saturday afternoon, warm temperatures created a thin rain crust on the snow surface in the Nelson and Kootenay Pass area. 3-10 cm of snow overlies this thin crust. In wind exposed areas strong southwest wind has been the dominant feature, transported the new snow into wind slabs in lee features. 

Below this new snow, last week's storm snow overlies a layer of more consolidated snow over a substantial crust that formed in early December. This crust is 10cm thick on average and is present across all aspects to at least 2300m. A layer of weak facets (sugary snow) has been reported above this crust in Kootenay Pass and the Whitewater backcountry, and likely exists in other areas. 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. 

The lower snowpack is composed of several early season crusts. Snow depths at treeline average 150-200 cm. Below 1800m the snowpack remains relatively shallow with 80-120cm on average. The deepest snowpack can be found in the Kootenay Pass area.

Terrain and Travel

  • Be careful as you transition into wind affected terrain.
  • Pay attention to the wind, once it starts to blow fresh sensitive wind slabs are likely to form.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.

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.