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Avalanche Forecast

Archived

Dec 21st, 2024–Dec 22nd, 2024

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 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.

A bit of new snow overnight Saturday, with a forecast for strong winds into the start of the week will extend the possibility of natural avalanches.

While wind slabs may seem manageable, the primary concern is the threat of superficial slabs stepping down to the deep persistent weakness to result in full-depth avalanches.

Confidence

Moderate

Avalanche Summary

The ski hills continued to trigger wind slabs with ski cuts and explosives today, ranging up to size 2, failing 10 to 40 cm deep.

Sunshine Snow Safety reported using a 1lb charge to trigger a sz 2.5-3 slab that failed up to 2m deep and 30m wide that ran 450m in W-NW alpine ridge-crest terrain. The October crust was the bed surface.

On flights and field trips we have observed failures at both the wind slab interface and the deep persistent problem since the latest storm on Dec 18.

Snowpack Summary

Up to 20 cm of storm snow fell last week. This snow, combined with strong winds, formed wind slabs throughout the alpine and into treeline.

The mid and lower snowpack is faceted and weak with the two deep persistent slab interfaces sitting near the ground. This is more pronounced east of the divide, while western regions display a deeper snowpack that is more settled.

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

Weather Summary

Overnight Saturday expect up to 5 cm of snow with higher amounts closer to the divide as temperatures at treeline remain around -4C. Moderate west winds should shift northwest and diminish into Sunday. West winds are forecast to rebuild into Monday and may increase further into Tuesday.

Terrain and Travel Advice

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

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.