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Avalanche Forecast

Archived

Dec 7th, 2021–Dec 8th, 2021

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

Regions

Northwest Coastal.

Fresh wind slabs will overlie a weak layer of facets or surface hoar, making human triggering likely. Stick to low angled, sheltered terrain as this complex scenario plays out. 

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Forecast precipitation (either snow or rain) amounts are uncertain.

Weather Forecast

Tuesday Overnight: Snow and wind will continue with another 10-20cm expected overnight with strong westerly winds. Freezing levels at valley bottom with alpine temperatures around -10 C. 

Wednesday: Unsettled conditions continue. A cloudy day with another 5-15cm of new snow and continued strong westerly winds. Freezing levels at valley bottom with alpine temperatures around -15 C. 

Thursday: A brief reprieve with a weak high pressure system forming inland. A partially cloudy day with flurries, accumulation up to 7cm throughout the day. Moderate to strong northwest winds and alpine temperatures remaining at -15 C. 

Friday: A strong pacific system develops over White Pass and NW ranges. Strong to extreme SW winds with freezing levels rising to 1000m. 20-30cm of new snow can be expected, heaviest in the afternoon and increasing into the evening. 

Avalanche Summary

Expect natural and rider triggered avalanches to occur as we receive significant new snow and wind. If you do head out in the backcountry, please share your observations to the Mountain Information Network!

On Monday, a size 2 skier-accidental was reported on a south aspect at treeline in the Blue Boy area in the Shames backcountry. The avalanche character is unknown, either shallow wind slab or loose dry. See full report here.

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.

 

Snowpack Summary

10-30cm of new snow has been redistributed by moderate to strong westerly winds, creating new wind slabs in the alpine and treeline. Below this new snow, a weak layer of surface hoar or facets can be found from previous cold temperatures and clear skies. This weak layer will add to the reactivity of new wind slabs, especially where they are sitting on a crust.

Snowpack depths range from 150 cm- 300 cm at treeline and 200 cm- 400 cm in the alpine. The Dec 1st crust extends up to 2000m in the southwest of the region, but may only be found to around 1200m in the Terrace area. This crust is down 70-100cm in sheltered areas. 

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. 

Terrain and Travel

  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Sheltered low elevation terrain will likely offer the best and safest riding today.

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.