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Avalanche Forecast

Archived

Dec 17th, 2012–Dec 18th, 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 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.

Regions

Northwest Coastal.

Confidence

Fair - Due to limited field observations

Weather Forecast

Tuesday: Expect flurries with some clearer spells, treeline temperatures around -8C and light SW winds that may pick up slightly in the afternoon.Wednesday: A Pacific frontal system makes landfall overnight Tuesday/Wednesday bringing 15-20 cm new snow. At the onset of the storm, winds will blow up to 80 km/h from the SW, then reduce in strength to around 40 km/h from the south through most of Wednesday. Treeline temperatures should be around -4C.Thursday: The tail end of the storm will bring further light snowfall (around 5 cm), moderate SE'ly winds and treeline temperatures rising to around -1C.

Avalanche Summary

Natural avalanche activity was observed in several parts of the region up to size 2. The avalanche activity affected all aspects but appeared to be isolated to the recent storm snow, producing avalanches 30 to 75 cm deep depending on the local depth of the new snow.

Snowpack Summary

Storm snow accumulations in the north and near the coast (for example the mountains near Stewart) are as high as 65 cm; away from the coast there was much less. For example near Shames it was closer to 20 cm of storm snow but the total snowpack depth only added 5 cm due to settling. The upper snowpack should be mostly settling powder depending on elevation. I expect wind slabs or wind affected snow in higher elevation exposed areas. A surface hoar layer, reported to be 5 to 10 mm thick, has been reported from the Shames backcountry. It may exist in other areas too. Snowpack tests showed sudden "pop" results on this layer, indicating the possibility for avalanche releases on this layer (although none have been definitely attributed to it as of 17-Dec). Deeper in the snowpack professionals continue to monitor a crust down 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.

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.