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Avalanche Forecast

Archived

Dec 20th, 2018–Dec 21st, 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 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.

Regions

South Rockies.

The storm and associated natural avalanches have tapered off, but the snowpack remains generally weak. Any avalanche triggered within the storm snow is likely to step down to the base of the snowpack, resulting in full depth avalanches.

Confidence

Moderate - Due to the number of field observations.

Weather Forecast

THURSDAY NIGHT: Mainly cloudy with scattered flurries, up to 5 cm, southwest winds, 60-80 km/h, alpine low temperature near -5°c, freezing level 2000m. FRIDAY: A mix of sun and cloud, west winds, 50-60 km/h, alpine high temperature near -8°c, freezing level 1200m and dropping to valley bottom by the evening. SATURDAY: Sunny with cloudy periods, southwest winds, 20-40 km/h, alpine high temperature near -12°c, low temperature near -14°c. SUNDAY: Cloudy with sunny periods and isolated flurries, southwest winds, 50-70 km/h, alpine high temperature near -8°c, low temperature near -10°c.

Avalanche Summary

One size 1.5 explosives triggered avalanche was reported on Thursday. No new natural avalanches were reported in the region. Several explosives triggered size 2 avalanches on northeast aspects at 2200-2300 m were reported in the region on Wednesday. Widespread natural avalanche activity was observed around Elkford and Sparwood on Tuesday during the height of the storm. 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.

Snowpack Summary

30-70 cm of recent snow is sitting on a weak layer of facets (sugary snow) that formed during the dry weather in early December. Wind and warm temperatures have promoted widespread storm slab formation over this layer. The lower snowpack has a weak structure composed primarily of facets. It is likely that an avalanche triggered in the storm snow, would entrain the facets at the base of the snowpack, resulting in a full depth avalanche.

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.