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Avalanche Forecast

Archived

Dec 5th, 2021–Dec 6th, 2021

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Northwest Coastal.

Choose terrain sheltered from the wind, and avoid thin, rocky start zones. Right now, wind slabs are the common problem, but triggering a high consequence, deep slab avalanche is possible. 

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

Sunday Night: Mostly cloudy. Possible trace of snow. Light west wind. Alpine temperature around -13 C.

Monday: Mostly clear. No new snow expected. Light northeast wind, becoming moderate southwest by the evening. Alpine temperature above -10 C with a possible temperature inversion. 

Tuesday: Overcast. 10-30 cm snow expected. Extreme southwest winds. Freezing level rising to 250 m. Alpine temperature around -5 with a possible temperature inversion.

Wednesday: Mostly cloudy. 5-20 cm snow expected. Strong west winds. Freezing level dropping back to valley bottom with alpine temperatures around -10 C.

Avalanche Summary

With some improved visibility, observations of avalanches that occured during the last storm are starting to trickle in. 

Natural, explosive, and cornice triggered slab avalanches have been reported up to size 3.

Some very large explosive triggered avalanches were reported in Bear Pass, having us thinking about the potential for a deep persistent weak layer.

Snowpack Summary

There are recent reports of large to very large avalanches running on or near the ground in the Bear Pass area and around Smithers. We don't know how widespread this problem may be, or if both events even reflect the same avalanche problem, but we are definitely keeping an eye on it, and would be avoiding large features and areas with thin to thick snowpack transitions for now. 

Due to recent, variable winds, you may find wind slabs on many aspects, and on both sides of ridges. Some exposed areas may be stripped back to the Nov 30/Dec 1 Crust. This crust exists up to 2000 m in the southwest of the region, and may only be found up to 1200 m in areas east of Terrace.

At higher elevations where this crust is not present, the storm snow overlies old wind slab or wind stripped ridge tops.

Treeline snowpack depths are estimated to be around 2-3 m, and a prominent crust can be found near the base of the snowpack.

Terrain and Travel

  • Avoid lee and cross-loaded slopes at and above treeline.
  • Keep in mind the crust offers an excellent bed surface for avalanches.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.