Avalanche Forecast

Issued: Dec 18th, 2020 4:00PM

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

Avalanche Canada dsaly, Avalanche Canada

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We're in an active weather pattern with a complex upper snowpack, avalanche hazard will remain elevated through the weekend.

Summary

Confidence

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system.

Weather Forecast

FRIDAY NIGHT- Snow, 10-20 cm / southwest wind, 30-50 km/h / alpine high temperature near -7

SATURDAY - Flurries, up to 10 cm / southwest wind, 20-40 km/h / alpine high temperature near -4 / Freezing level rising to 1200 m

SUNDAY - Snow and wet flurries, 20-30 cm / southwest wind, 30-60 km/h / alpine high temperature near 0 / Freezing level 1700 m

MONDAY - Flurries, 10-15 cm / southwest wind, 10-20 km/h / alpine high temperature near -8

Avalanche Summary

Storm and wind slabs were underfoot/sled on Thursday (MIN report). Explosives triggered avalanches to size 2 along ridgelines and with good connectivity. Skiers reported reactive slabs 20-50 cm deep, most within the storm snow and in one case, steeping down to the early December crust.

A handful of natural avalanches were reported including one notable size 3 avalanche near Monashee Provincial Park - a 30-60 cm deep wind slab that failed on the early December crust, running for 1000 m.

On Wednesday, small storm and wind slabs showed reactivity on predictable, loaded features on the Dec 7 surface hoar/curst. Explosives and skiers triggered size 1-1.5 slab avalanches on leeward aspects above 2000 m. With the most reactivity, a small (size 1) avalanche was triggered remotely by a vehicle traveling along an exposed ridgeline. 

On Tuesday, small (size 1-1.5) storm slab avalanches were triggered by explosives and skier traffic on East, North, and West aspects above 2000 m; average depth 5-25 cm. 

Snowpack Summary

Ongoing flurries and snow has accumulated 30-50 cm fresh snow around the region. This new snow has buried a weak layer of surface hoar in many areas. This new snow combined with southwest winds has formed reactive slabs, especially at higher elevations. 

A weak layer that was buried in early December is now down 30-60 cm. This layer is variable and has been reported to exist as either a crust with facets or surface hoar. In any case, it is a persistent weak layer with a significant load above it now, and it may react easily to human triggers.

This MIN report from Thursday really captures the snowpack well, and documents reactivity both within the storm snow and on the early December crust.

Another crust that was buried in early November can be found near the base of the snowpack. There have been no recent avalanches reported on this layer, however it may be possible to trigger it from shallow, rocky terrain.

Terrain and Travel

  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Avoid freshly wind loaded terrain features.
  • Start with conservative lines and watch for clues of instability.
  • As the storm slab problem gets trickier, the easy solution is to choose more conservative terrain.

Problems

Storm Slabs

An icon showing Storm Slabs

Intermittent snow and flurries continue to stack up, 30-50 cm fresh snow overlies a layer of surface hoar. Reactive slabs have formed in the storm snow, expect to find increasingly reactive slabs as snow continues to accumulate, especially in wind-loaded areas.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

1.5 - 2.5

Persistent Slabs

An icon showing Persistent Slabs

A weak layer of surface hoar or crust layer from early December is now down 40-60 cm. Storm slab avalanches have and may easily step down to this layer, with surprising propagation.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

2 - 3

Valid until: Dec 19th, 2020 4:00PM