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Avalanche Forecast

Archived

Dec 6th, 2021–Dec 7th, 2021

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

Regions

Northwest Coastal.

Choose wind sheltered terrain, and avoid features steeper than 25 degrees where the new snow is falling on a hard crust. 

Be wary of thin, rocky start zones. There is potential for a rider to trigger deep avalanches.

Confidence

Moderate - We are confident the likelihood of avalanche will increase with the arrival of the forecast weather.

Weather Forecast

Monday Night: Mostly cloudy. No new snow expected. Moderate southwest wind. Alpine temperature above -10, with a possible temperature inversion. 

Tuesday: Overcast. 20-30 cm snow expected. Strong to extreme southwest winds. Freezing level rising to 250 m, potentially up to 1000 m in the south of the region. Alpine temperature around -5. Temperature inversion breaking down.

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

Thursday: Partly cloudy. 0-5cm snow expected. Moderate southwest winds. Alpine temperature around -8.

Avalanche Summary

 On Sunday, near Ningunsaw, and in the Nass River valley, large avalanches were reported to have failed near the ground. This adds to the report late last week from Bear Pass of some very large, explosive triggered avalanches, keeping thoughts of a deep persistent weak layer in our minds.

Late last week, natural, explosive, and cornice triggered slab avalanches were reported up to size 3.

Expect natural and rider triggered avalanches to occur as we receive significant new snow and wind.

Snowpack Summary

There are recent reports of large to very large avalanches running on or near the ground in Bear Pass, Ningunsaw, the Nass River Valley, and around Smithers. We don't know how widespread this problem may be, or if all events 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.

There are reports from around the region of surface hoar forming at and below treeline. This could be a touchy new sliding layer when it gets buried on Tuesday, especially where it will be sitting on a crust.

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

  • Storm slab size and sensitivity to triggering will likely increase through the day.
  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.