Avalanche Forecast

Issued: Dec 22nd, 2013 8:10AM

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 Monday

Weather Forecast

The Pacific frontal system will push through the interior regions Sunday night through Monday. This will bring light-moderate snow amounts and strong SW-W winds.Monday: Snow amounts 10-20 cm. Alpine temperatures near -5. Ridgetop winds moderate from the West. Freezing levels 800 m.Tuesday: Mainly cloudy with some flurries. Alpine temperatures near -14. Ridgetop winds moderate from the West gusting strong. Freezing levels back to valley bottom.Wednesday: Cloudy, and dry. Alpine temperatures near -8. Ridgetop winds light from the West.

Avalanche Summary

On Sunday, explosive controlled size 2-3 slab avalanches were reported. These all failed on the early October crust. Currently widespread loose dry sluffing from steeper terrain slopes and features has been reported. With new snow and strong ridgetop winds, you can expect the avalanche danger to rise Monday/ Tuesday.

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.

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

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.

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

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

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

2 - 5

Valid until: Dec 23rd, 2013 2:00PM