Avalanche Forecast

Issued: Jan 8th, 2018 5:16PM

The alpine rating is considerable, the treeline rating is high, and the below treeline rating is considerable. Known problems include Storm Slabs and Persistent Slabs.

Avalanche Canada mconlan, Avalanche Canada

New snowfall is loading numerous buried weak layers, which is reactive to natural and human triggers and producing slab avalanches. Storm slabs could step down to deeper layers and produce large avalanches. Maintain conservative terrain choices.

Summary

Confidence

Moderate - Intensity of incoming weather systems is uncertain

Weather Forecast

TUESDAY: Cloudy with snowfall, accumulation 10-15 cm, light to moderate westerly winds, alpine temperature near -7 C, freezing level near 1000 m.WEDNESDAY: Partly cloudy with intermittent flurries, light southwesterly winds, alpine temperature near -12 C, freezing level below valley bottom.THURSDAY: Cloudy with snowfall, accumulation 10-15 cm, moderate southwesterly winds, alpine temperature near -8 C, freezing level below valley bottom.

Avalanche Summary

On Sunday, numerous avalanches were triggered naturally and by skiers on the December 15 surface hoar layer at and below treeline on all aspects. The avalanches were small to large, being reported between size 1 and 2.5 with depths of about 40 to 50 cm. This is a consistent trend, with around 5-10 avalanches being reported on this layer each day for the past week.Numerous natural and skier-triggered storm slabs were also noted within the recent 20-40 cm of snow, releasing small to large (size 1 to 2) avalanches at all elevations and on all aspects. Reports of loading in northeasterly lee features were also noted, such as in this MIN post.Please share your recent observations through the Mountain Information Network (MIN).

Snowpack Summary

20-40 cm of new snow has accumulated on surface hoar in sheltered slopes and a sun crust on steep solar aspects. The snow is forming a storm slab that has produced numerous avalanches (see Avalanche Summary). The snow fell with strong southwesterly winds in the alpine, which produced wind slabs in lee features at treeline and alpine elevations. Below treeline, this new snow is mainly a concern in openings (cut blocks, gullies, cut banks).The additional snow has created a dangerous slab above numerous buried weak layers. 40-80 cm of snow overlies layers of weak and feathery surface hoar, which is most often found at and below treeline. This layer has the potential to create easily-triggerable destructive slab avalanches because the layers are buried at a depth prime to human triggering. Snowpack tests are generally showing that this layer could form large avalanches (sudden fracture characters, high propagation potential, rutschblock 1 to 3). See the Avalanche Summary for recent activity. Deeper in the snowpack (90 to 150 cm), a November crust is producing variable test results (sudden to no result). This layer is considered dormant but could be triggered where the snowpack is thin.

Problems

Storm Slabs

An icon showing Storm Slabs
Many avalanches have been human-triggered in the recent 20 to 40 cm of storm snow. The snow is sitting on variable surfaces and it is not bond well to them. The snow is likely deeper in northwesterly lee features due to strong winds.
If triggered, the storm slabs may step down to deeper layers resulting in large avalanches.Avoid exposure to terrain traps where the consequences of a small avalanche could be serious.Use caution when entering lee areas. Recent wind loading may have created wind slabs.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

1 - 3

Persistent Slabs

An icon showing Persistent Slabs
Skier traffic has triggered large avalanches on this layer, which is buried around 40 to 80 cm in the snowpack. Shallower storm slabs could step down to this layer.  If triggered, this layer could produce large, destructive avalanches that run far.
Avoid open slopes and convex rolls at and below treeline where buried surface hoar may be preserved.Use conservative route selection. Choose moderate-angled and well-supported terrain.Minimize overhead exposure during periods of heavy loading from new snow and wind-transported snow.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 3

Valid until: Jan 9th, 2018 2:00PM