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Avalanche Forecast

Archived

Dec 15th, 2018–Dec 16th, 2018

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

South Rockies.

Watch for unstable snow on steep and wind loaded terrain features.

Confidence

Moderate - Wind effect is extremely variable

Weather Forecast

SATURDAY NIGHT: Cloudy with clear periods, moderate wind from the south, alpine temperatures drop to -10 C.SUNDAY: Increasing cloud throughout the day, moderate wind from the southwest with strong gusts, alpine high temperatures around -2 C.MONDAY: Light flurries, strong wind from the southwest, freezing level rising to 1800 m.TUESDAY: 10-15 cm of snow, strong wind from the southwest, freezing level around 1800 m.

Avalanche Summary

Widespread natural avalanche activity was reported on Thursday and Friday, primarily size 1-2 wind slab avalanches on east-facing lee terrain in the alpine. Some appear to have stepped down to the deep persistent weak layer near the bottom of the snowpack. Heavy triggers, including, explosives, also produced some large and very large deep persistent slab avalanches (size 2-3) that failed on the weak snow at the bottom of the snowpack. These avalanches were primarily on north and east aspects and ranged in depth from 50-120 cm. On Thursday, the storm snow was also very reactive to skier and snowmobilie traffic, producing widespread cracking, whumpfing, and some small avalanches.See some photos of avalanche activity north of Crowsnest Pass from out field team's MIN report here.

Snowpack Summary

About 20-30 cm of new snow has buried previous snow surfaces that ranged from soft power to hard wind slab and sun crust. Strong winds have likely been aggressively forming wind slabs with the new snow at higher elevations.Beneath the new snow and old surface, the snowpack has a thin, weak structure, with the bottom half of the snowpack composed of weak facets and crusts. This basal layer has been reactive to heavy triggers, and terrain features like smooth alpine bowls with variable snowpack depths are suspect for human triggering.Currently only 30-100 cm of snow can be found in alpine areas and much less at lower elevations.

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.