Avalanche Forecast

Issued: Dec 23rd, 2013 8:07AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. 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 Wednesday

Weather Forecast

A ridge of high pressure will set up over the Interior bringing a dry, cool northwest flow Tuesday-Wednesday. The next system should arrive Wednesday night bringing snow, wind and warming. Generally, the pattern is repeating itself. Tuesday: Cloudy with sunny breaks. Alpine temperatures -13. Ridgetop winds moderate gusting strong from the West.Wednesday: Snow amounts up to 10 cm. Alpine temperatures -5. Ridgetop winds moderate-strong from the West. Thursday: Cloudy. Alpine temperatures -5. Ridgetop winds light from the SW.

Avalanche Summary

On Sunday, one large size 2.5 natural slab avalanche occurred from a South aspect around 2650 m, this was said to have most likely run on a crust buried down 75 cm on a 40 degree slope. A couple of operators were out doing explosives avalanche control and triggered several large size 2-3 slab avalanches from NE-E aspects . Some of those ran on the deep persistent crust layer, and some were within the storm snow.

Snowpack Summary

Low density new snow covers stiff, dense wind slabs in exposed lee terrain at and above treeline. Average snowpack depths at treeline elevations are 100-130 cm. In the upper 80 cm of the snowpack a couple persistent weak layers 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.A bigger concern, especially in the Northern part of the region where the snowpack is thinner and more variable is weak faceted and depth hoar crystals combined with a crust from early October. This is now down around 80-120 cm. 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

New snow and wind will likely build touchy wind slabs over the old dense wind slabs. 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.

Stick to well supported, lower angle terrain.>Avoid lee and cross-loaded terrain near ridge crests.>Avoid steep lee and cross-loaded features>

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

Elevations: Alpine, Treeline.

Likelihood

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

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

2 - 5

Valid until: Dec 24th, 2013 2:00PM