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Avalanche Forecast

Archived

Feb 26th, 2023–Feb 27th, 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 begun to decrease, but the extent of the windslab development at upper elevations is widespread. Reports over the weekend have shown that this problem is reactive to human triggering.

Confidence

Moderate

Avalanche Summary

The field team Monday observed active loading at upper elevations, with sloughing and audible avalanches out of extreme terrain. Local ski hills reported reverse loading with small reactive windslabs at ridge crest.

Saturday, several human-triggered avalanches to size 2.5 were reported in the forecast region.

Snowpack Summary

10-15cm of new snow overlays a faceting upper snowpack. Moderate to Strong W winds continue to form windslabs at alpine and treeline elevations. These windslabs overlie the various Jan PWL interfaces of sun crusts, facets and surface hoar down 70-110 cm. The weaker Nov 16 basal facet layer is down 100-180 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

Partly cloudy conditions with light precip are expected Monday. Winds will gradually decrease through the day with peak values of moderate from the West in the morning. Daytime high temperatures will be near 0 degrees near valley bottom. By late Monday into Tuesday, increased flurries are expected with a few centimeters of accumulation.

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.