Avalanche Forecast

Issued: Dec 4th, 2013 8:32AM

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.

rbuhler, Avalanche Canada

Purcells.

Highly variable early-season snowpack conditions exist across the region. Investigate conditions in your local area before heading out.

Summary

Confidence

Fair - Due to the number of field observations

Weather Forecast

A strong Arctic air mass will continue to dominate conditions in the southern interior. Thursday: Mostly sunny, light NE alpine winds, alpine temperatures around -20CFriday/Saturday: Mostly sunny, light NE alpine winds possibly increasing on Saturday, alpine temperatures as low as -30C

Avalanche Summary

We are still receiving limited reports. Explosive control on Tuesday produced numerous avalanches to size 2, primarily on north aspects. Recent snowpack tests suggest the continued potential for human triggered avalanches. If you have seen any recent avalanche activity, please send your reports to forecaster@avalanche.ca

Snowpack Summary

Snow depths varies greatly across the region and average 70-110cm at treeline. The southern portion of the region received substantially more new snow (up to 70cm ) compared to the northern portion (as little as 10cm). The recent storm snow is sitting on top of the late-November interface which may consist of surface hoar on sheltered/shaded slopes, a melt-freeze crust on south facing slopes, or a combination of both in isolated locations. Strong NE winds after the storm have caused reverse loading and wind slab formation on SE through W aspects in wind exposed areas. In the southern part of the region there is 40-70cm of new snow over the late-November interface which has created a persistent slab problem. The current cold temperatures will contribute to the persistence of the late-November weak layer. In the northern part of the region the lower/mid snowpack is fundamentally weak, composed of an early season crust (more prevalent on north aspects) and weak faceted crystals. While many areas did not receive enough new snow to create a new slab problem on top of the late-November interface, there are likely isolated areas where a persistent slab does exist.

Problems

Wind Slabs

Strong NE winds have redistributed loose surface snow into reactive wind slabs on southwest facing terrain. This is an atypical loading pattern which has the potential to catch skiers/riders by surprise.

Avoid traveling in areas that have been reverse or cross-loaded by winds.>Avoid freshly wind loaded features.>

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

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

2 - 4

Persistent Slabs

Primarily a concern in the southern part of the region. This persistent slab will be tricky to manage as it overlies a variety of weak surfaces. A conservative and investigative approach is required for safe mountain travel.

Dig down to find and test weak layers before committing to a line.>Whumpfing, shooting cracks and recent avalanches are all strong indicators of unstable snowpack.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 4

Deep Persistent Slabs

A weak crust/facet layer near the base of the snowpack has the potential to create large, destructive avalanches. This is primarily a concern in shallow snowpack areas but may also be a problem on unsupported or convex slopes.

Be aware of the potential for large, deep avalanches due to the presence of a crust/facet combo.>Avoid convexities or areas with a thin or variable snowpack.>

Aspects: North, North East, East.

Elevations: Alpine, Treeline.

Likelihood

Unlikely

Expected Size

3 - 5

Valid until: Dec 5th, 2013 2:00PM