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Avalanche Forecast

Archived

Dec 19th, 2018–Dec 20th, 2018

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

Regions

South Rockies.

Natural avalanche activity has tapered off to some degree, but human triggered avalanches remain likely. Conservative terrain selection is recommended.

Confidence

Moderate.

Weather Forecast

WEDNESDAY NIGHT: Mainly cloudy with isolated flurries, up to 5 cm, moderate west to southwest winds, alpine low temperature near -9°c, freezing level 1000m. THURSDAY: Cloudy with flurries, 5-10 cm beginning to accumulate mid to late afternoon, southwest winds, 20-40 km/h, gusting to 80 km/h, alpine high temperature near -2, freezing level 1700m. FRIDAY: A mix of sun and cloud, west winds, 20-40 km/h, gusting to 90 km/h, alpine high temperature near -7°c, low temperature near -13°c, freezing level 1000m. SATURDAY: Cloudy with sunny periods and isolated flurries, west winds, 20-35 km/h, alpine high temperature near -10°c, low temperature near -15°c.

Avalanche Summary

Several explosives triggered size 2 avalanches on northeast aspects at 2200-2300m were reported in the region on Wednesday. Widespread natural activity was observed around Elkford and Sparwood on Tuesday. Many avalanches failed on deep persistent weak layers in the alpine. Snowmobiles remotely triggered several small avalanches below treeline on cutbanks north of Sparwood on Tuesday. Although these avalanches were small, they involved the entire depth of the snowpack and failed on weak layers at the base of the snowpack. See this MIN report. Deep persistent slabs were also reactive over the past few days. On Monday, a skier triggered a size 2 avalanche on an east aspect around 1800m. The avalanche failed on a persistent weak layer 40cm below the surface. On Friday and Saturday, explosive control produced numerous large (size 2-3) wind slab and deep persistent slab avalanches on north and east aspects above 2000m.

Snowpack Summary

25-60 cm of recent storm snow has formed  storm slabs at upper elevations. This new snow substantially increases snowpack depths throughout the region, which prior to the storm ranged from 30-120 cm in the alpine and much less at lower elevations. The lower snowpack has a weak structure composed of facets and crusts, and the additional load of the new snow has pushed these weak layers past the tipping point. Any avalanche could step down to the bottom of the snowpack.

Problems

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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.