Avalanche Forecast

Issued: Feb 1st, 2013 9:15AM

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

swerner, Avalanche Canada

Northwest Inland.

Summary

Confidence

Poor - Due to variable snowpack conditions

Weather Forecast

Saturday: No significant precipitation expected. Alpine temperatures near zero degrees with freezing levels rising to 1800 m. Ridgetop winds will blow moderate from the South.Sunday: The brunt of the frontal system will take effect, bringing light precipitation accompanied by strong SW ridgetop winds. Alpine temperatures near -4.0 and freezing levels falling to 700 m. Monday: Cloudy skies with light precipitation. Ridgetop winds will blow light from the SW. Alpine temperatures dropping to -9.0 with freezing levels lowering to valley bottoms.

Avalanche Summary

Natural sluffing from steep terrain features. On Thursday, a remote triggered slab avalanche size 2.0 initiated 50 m away from the skier at treeline elevation. Another report of a size 2.0 slab avalanche (skier triggered) also occurred. Both incidents were at 1200-1300 m on North aspects with crowns 50-60 cm deep, 30-40 m wide, and running up to 150 m in length.

Snowpack Summary

Touchy wind slabs have built on lee slopes (N-SW) and behind terrain features likes ribs and ridges. Up to 50 cm of snow sits on a variety of old surfaces including old wind slabs, scoured thin slopes, crusts and surface hoar. This interface has shown reactivity to rider triggers, especially around treeline and below treeline elevations. Warmer temperatures have promoted some settlement and consolidated the recent snow into slab-like characteristics. With little observations from the field it's hard to get a good handle on how widespread this layer is. I stress the importance to dig down and test weak layers. Become familiar with the snowpack in your neck of the woods. The average snowpack depth at treeline is near 100 cm but remains quite inconsistent across the region. A strong mid-pack currently overlies a weak base layer of facets/depth hoar. Triggering of this basal weakness may still be possible from thin spots, rocky outcrops or under the weight of larger triggers such as cornice fall and larger amounts of new storm snow.

Problems

Wind Slabs

New snow and strong shifting winds will build touchy wind slabs that are likely found behind terrain breaks such as ridges and ribs. Hollow sounds and cracking snow is a good indicator of unstable snow.

Avoid freshly wind loaded features.>Assess start zones carefully and use safe travel techniques.>Use extra caution on lee and cross-loaded terrain near ridge crests.>

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

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 5

Persistent Slabs

Buried beneath the new snow sits a surface hoar/crust/facet weakness. This may be sensitive to rider triggers in steeper sheltered terrain or over convex rolls, especially at treeline and below treeline elevations.

Dig down to find and test weak layers before committing to a line.>Whumpfing is direct evidence of a buried instability.>Avoid slopes that are showing signs of instability due to warming.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

2 - 5

Deep Persistent Slabs

A deeply buried weak layer near the base of the snowpack could be triggered by large loads such as a cornice collapse or from a thin-spot trigger point.

Be aware of thin areas that may propagate to deeper instabilites.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

2 - 6

Valid until: Feb 2nd, 2013 2:00PM