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Avalanche Forecast

Archived

Apr 20th, 2023–Apr 21st, 2023

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

Regions

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

The only way to reduce risk when dealing with the basal weak layer is avoidance, as we can't know which slopes will slide and when. Factor this into your terrain choices.

Confidence

Moderate

Avalanche Summary

A large deep release was reported out of the north side of Surprise Pass on Tuesday, and Sunshine ski Hill reported a natural size 3 deep persistent avalanche in the backcountry on the Monarch.

Snowpack Summary

15-40 cm of snow in past week (most around Lake Louise) overlies previous storm snow at upper elevations and rain-crusts below 1800 m. Recent strong winds have created wind-slabs in the alpine. Multiple buried crusts are present in the mid-pack on solar aspects. The basal snowpack remains weak with facets and depth hoar. A field team today on Mt. Whymper found large (10mm) depth hoar and a generally weak snowpack in the alpine.

Weather Summary

Friday: Light upslope winds and 2-4cm or more possible within convective cells. 1700-1900m freezing levels in the afternoon.

Saturday: A trough will begin to produce very light precipitation. Freezing levels 2000-2200m in the PM.

Terrain and Travel Advice

  • Avalanche hazard may have improved, but be mindful that deep instabilities are still present.

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.