Avalanche Forecast

Issued: Dec 18th, 2013 7:46AM

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

Peter, Avalanche Canada

Purcells.

Summary

Confidence

Fair

Weather Forecast

Thursday: Arctic air surges south giving colder and drier weather. Expect a mix of sun and cloud with increasing cloud later in the day. Alpine temperatures should be around -15 to -20. Winds are light to moderate from the northwest-west. Friday: Light snow should begin Thursday night with a few cms expected through Friday. The freezing level is at valley bottom and winds are moderate from the west-southwest. Saturday: Lingering flurries in the morning, then clearing. The freezing level is at valley bottom and winds are light from the northwest.

Avalanche Summary

Several small skier controlled avalanches (size 1) were reported on Tuesday. These appeared to be thin wind slabs in exposed north or east facing terrain.

Snowpack Summary

Small amounts of recent storm snow are beginning to add up on top of the sugary facets from early December. Snowpack depths at tree line currently vary from 100- 130 cm and reports speak of severe scouring on the windward side of ridges, moving snow onto lee sides and creating dense wind slabs. A persistent weakness of buried surface hoar and facets is down approximately 20-50cm. This layer has been producing variable results with snowpack tests. Recent reports indicate that the new storm/ wind slab is becoming reactive with warming temperatures and additional loads. At the base of the snowpack are weak facets and depth hoar combined with a crust from early October. This deep persistent weakness may be stubborn to trigger, especially in deeper snowpack areas, but the sensitivity to triggers likely increases in shallower locations, especially on steep, convex slopes. Snow depth is below threshold in many areas below treeline.

Problems

Wind Slabs

New snow and moderate to strong westerly winds have created dense wind slabs in exposed lee terrain and cross-loaded features. Rider triggering is possible, particularly in steep unsupported terrain.

Avoid cross loaded slopes at or above treeline.>Stay off recent wind loaded areas until the slope has had a chance to stabilize.>

Aspects: North, North East, East, South East, South.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 4

Deep Persistent Slabs

Triggering the persistent weakness of facets and/or a crust at the base of the snowpack is unlikely, but if it is triggered the resulting avalanche would likely be very large and destructive.

Avoid convexities or areas with a thin or variable snowpack.>Dig down to find and test weak layers before committing to a line.>Whumpfing is direct evidence of a buried instability.>

Aspects: North, North East, East, South East, South.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

2 - 6

Valid until: Dec 19th, 2013 2:00PM