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Avalanche Forecast

Archived

Dec 20th, 2012–Dec 21st, 2012

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

Regions

Northwest Coastal.

Confidence

Fair - Due to variable snowpack conditions

Weather Forecast

Overnight Thursday and Friday: Light snowfall / Light to moderate southeast winds / Temperature inversion with ridgetop temperatures of -12.0Saturday: Mostly clear / Light southeast winds / Temperature inversion with ridgetop temperatures of -12.0Sunday: Trace amounts of snowfall / Light east winds / Ridgetop temperatures of -16.0

Avalanche Summary

Size 1 slabs have been reported from the south of the region, while slabs to size 2.5 were reported from steep alpine chutes in the Bear Pass area. All activity is suspected to have occurred within recent storm layers.

Snowpack Summary

Very light amounts of new snow have been redistributed by winds at higher elevations. This overlies the past week's storm snow which is settling rapidly, although weaknesses may still exist within storm snow interfaces. A buried surface hoar layer, reported to be 5 to 10mm thick, recently produced sudden snowpack test results down 60-80cm in the Shames backcountry, and may exist in other areas too. Avalanche professionals continue to monitor a faceted crust near the ground. It seems this problem is confined to alpine areas and likely needs a large trigger and/or a shallow snow pack feature to trigger. Cornices are also really big and forecast strong winds and cooling temperatures could make them especially heavy and brittle.

Problems

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.

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.