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Avalanche Forecast

Archived

Dec 22nd, 2024–Dec 23rd, 2024

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

Regions

Banff Yoho Kootenay, Little Yoho, Banff, East Side 93N, Kootenay, Lake Louise, LLSA, Sunshine, West Side 93N, Field.

The recent snow has helped the ski quality, but strong alpine winds have created new wind slabs in lee alpine areas. While the wind slabs may seem manageable, the primary concern is the threat of superficial slabs stepping down to the deep persistent weakness resulting in full-depth avalanches.

Confidence

Moderate

Avalanche Summary

The ski hills continued to trigger deep persistent slabs with explosives on Saturday and Sunday, up to size 2.5, failing on the basal facets/crust.

On flights and field trips we have observed failures at both the new wind slab interface and the deep persistent problem since the latest storm on Dec 18, but natural avalanche activity has slowed down in the past couple days.

Snowpack Summary

4-8 cm of snow on Sunday over ~20 cm of storm snow from last week. This snow, combined with strong W/SW winds, created wind slabs in alpine lee areas and down into treeline.

The mid and lower snowpack is faceted and weak, with facet/crust interfaces near the ground. This is more pronounced east of the divide, while western regions display a deeper snowpack with fewer facets.

Snowpack depths at tree-line are about 60 cm in eastern areas and closer to 100 cm west of the divide.

Weather Summary

Becoming cloudy overnight on Sunday with moderate to strong SW winds at ridgetops that continue into Monday. A mix of sun and cloud on Monday with more cloud to the west, and treeline temperatures steady around -6°C.

Terrain and Travel Advice

  • Avoid freshly wind-loaded features, especially near ridge crests, rollovers, and in steep terrain.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Avoid thin areas like rocky outcrops where you're most likely to trigger avalanches on deep 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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.