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Avalanche Forecast

Archived

Feb 27th, 2023–Feb 28th, 2023

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

Regions

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

The winds have decreased in most of the region, but the extent of the windslab development at upper elevations is widespread. Reports from today and over the weekend show this problem continues to be reactive to human triggering.

Confidence

Moderate

Avalanche Summary

With improved visibility today, ski hills reported evidence of a widespread windslab avalanche cycle to size 2 over last weekend. Today, skiers on Wawa near Sunshine triggered a size 1.5 avalanche near the slide triggered on Saturday. Lake Louise conducted some heli bombing in previously uncontrolled terrain in West Bowl and Maintenance cliffs. They got results to size 2 with crowns 50-60 cm deep failing on an old snow surface, possibly one of the January interfaces.

Snowpack Summary

15-20 cm of recent snow overlays a facet layer from last week's cold snap which capped the 30-60 cm of storm snow received early last week. Last weekend moderate to strong W winds formed widespread windslabs at alpine and treeline elevations. These windslabs overlie the various Jan PWL interfaces of sun crusts, facets and surface hoar down 80-120 cm. The weaker Nov 16 basal facet layer is down 100-200 cm and still produces mod-hard sudden collapse test results. Deeper snowpack areas west of the divide have a stronger and more supportive lower snowpack than areas to the east.

Weather Summary

Tuesday, expect light winds, very light morning flurries and alpine temperatures to rise to near -10C.

For Wednesday expect partly cloudy skies with isolated flurries in the afternoon. Winds will increase through the day with values reaching 50-70 km/hr by Thursday.

Terrain and Travel Advice

  • Make conservative terrain choices and avoid overhead hazard.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Cornice failure may trigger large 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.

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.

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.