Avalanche Forecast

Issued: Dec 21st, 2013 9:05AM

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.

swerner, Avalanche Canada

Purcells.

The snowpack structure is complex and varies throughout the region. A conservative approach is essential.

Summary

Confidence

Fair - Timing, track, or intensity of incoming weather system is uncertain on Monday

Weather Forecast

The Interior will see generally dry, cool conditions under the current NW flow. A Pacific frontal system will arrive Sunday, bringing light- moderate amounts of precipitation and warmer temperatures.Sunday: Cloudy with some sunny breaks. Alpine temperatures near -11.0. Light NW winds.Monday: Snow amounts 10-25 cm. Alpine temperatures near -7. Strong westerly ridgetop winds. Freezing levels 1500 m.Tuesday: Cloudy. Alpine temperatures near -7. Ridgetop winds light from the West.

Avalanche Summary

On Friday, a skier triggered a size 2 slab avalanche outside of the ski area boundary in the Dogtooth Range. The person involved was fully buried and not wearing an avalanche transceiver. This slowed rescue efforts and unfortunately the person did not survive. The avalanche failed on the deep persistent October crust/ facet layer near the base of the snowpack. The depth of the crown was 40-80 cm and the width was near 70 m. Further south a size 2.5 natural slab avalanche was reported from a steep SE aspect.On Wednesday, a size 3 accidentally triggered slab avalanche was reported near Golden on Wednesday. This slide was triggered by a skier on a north-northeast aspect in the alpine and likely released on a persistent weakness near the bottom of the snowpack. The skier was uninjured. This type of avalanche activity may be a good indication of the potential for large avalanches during periods of loading.

Snowpack Summary

Average snowpack depths at treeline elevations are 100-130 cm. Low density new snow covers stiff, dense wind slabs in exposed lee terrain at and above treeline. In the upper 80 cm of the snowpack a couple persistent weak interfaces exist comprising of surface hoar and a facet/ crust combo. This interface has produced variable results with snowpack tests, and operators are keeping a close eye on them as the load above increases and/or a slab develops. 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, north-facing slopes.

Problems

Wind Slabs

Stiff, dense wind slabs can be found on exposed lee terrain and cross-loaded features. Rider triggering is likely and a small wind slab release may be enough to trigger a deep weak layer, producing a large-very large avalanche.

Use ridges or ribs to avoid pockets of wind loaded snow.>Be cautious as you transition into wind affected terrain.>Avoid steep lee and cross-loaded features>

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

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 4

Deep Persistent Slabs

May be stubborn to trigger, especially in deeper snowpack areas. The sensitivity to triggers will likely increase in shallow locations, especially on steep, convex, north-facing slopes. If triggered, consequences can be devastating.

Be aware of the potential for large, deep avalanches.>Be aware of thin areas that may propogate to deeper instabilites.>Avoid convexities or areas with a variable snowpack.>

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

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

2 - 6

Valid until: Dec 22nd, 2013 2:00PM