Avalanche Forecast

Issued: Dec 17th, 2012 9:32AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Storm Slabs, Persistent Slabs and Deep Persistent Slabs.

jfloyer, Avalanche Canada

Northwest Coastal.

Summary

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 snow instabilities are possible on steeper slopes where new snow accumulations are greater than 30 cm. New wind slabs are likely behind ridges, ribs, or similar terrain breaks.

Assess start zones carefully and use safe travel techniques.>Be cautious as you transition into wind affected terrain.>Highmark or enter your line well below ridge crests to avoid wind loaded pillows.>Watch for whumpfing, hollow sounds, shooting cracks or recent avalanches.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

1 - 5

Persistent Slabs

A persistent weak layer of surface hoar, buried 60-80 cm below the surface, has been reported from around Shames. The chance of avalanche activity on this layer will increase as the slab above it gets stiffer.

Whumpfing is direct evidence of a buried instability.>Choose well supported terrain without convexities.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Unlikely - Possible

Expected Size

2 - 5

Deep Persistent Slabs

Deeply buried weaknesses exist. Large triggers, such as cornice falls, could release deeper avalanches on steep, smooth terrain, especially in thinner snowpack areas.

Avoid shallow snowpack areas where triggering is more likely.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely

Expected Size

3 - 7

Valid until: Dec 18th, 2012 2:00PM