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Avalanche Forecast

Archived

Dec 9th, 2021–Dec 10th, 2021

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
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.

Safe riding today will mean dense trees with no exposure to overhead hazard. Concern for buried weak layers means that avalanches have the potential to pull back into low angled terrain, that we might otherwise consider 'safe' in high danger. 

Confidence

Moderate - Uncertainty is due to whether buried persistent weak layers become active, triggering avalanches, with the arrival of the forecast weather. Uncertainty is due to limitations in the field data. We are confident the likelihood of avalanche will increase with the arrival of the forecast weather.

Weather Forecast

A strong pacific front impacts the northwest ranges on Friday. Expect heavy snowfall and extreme southwest winds.

Thursday Overnight: The incoming system will bring moderate precipitation overnight, with 10-20cm of accumulation. Moderate to strong west winds will increase into the morning, shifting southwest. Freezing levels rising to ~300m, with alpine temperatures around -10 C.

Friday: A stormy day. Heavy snow and strong to extreme (80-120km/h) southwest winds will persist throughout the day, tapering overnight. Another 30-80 cm can be expected, heaviest amounts in coastal areas. Freezing levels reach ~500m, expect snow level to stay at valley bottom. Alpine temperatures around -8 C. 

Saturday: A mainly cloudy day with light to moderate precipitation, 5-15 cm accumulation. Winds easing into the moderate to strong range from the southwest. 

Sunday: Mainly cloudy with isolated flurries. Freezing levels at valley bottom with alpine temperatures around -10 C. Moderate to strong southwest winds at ridgetop.  

Avalanche Summary

We expect natural and rider triggered avalanches to occur today. If you do head out in the backcountry, please share your pictures/observations to the Mountain Information Network!

On Wednesday, several natural loose dry avalanches up to size 1.5 where observed by highways in the north of the region and northwest of Terrace. One natural snow slab size 2.5 was observed from the highway in Bulkley. 

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.

Snowpack Summary

In some areas, over 80 cm of new snow has fallen in the past 72 hours, with another 20-40 cm to come throughout the day today. This new snow will add significant load to the snowpack, and is almost certainly creating instability in the snowpack in most areas. In wind exposed areas, we can expect strong southwesterly winds to be creating deep deposits of wind slab in lee features. In sheltered areas, this new snow will acts as a storm slab or loose dry problem. 

There is still some concern for buried weak layers, will the possibility for buried surface hoar and faceting above the Nov 29th crust, now buried up to a meter deep. In the southern part of the region the December 1st crust has been reported up to 2000m, tapering to 500m in the northern reaches. The Terrace area has reported the crust to around 1200m.

Snowpack depths range from 150 cm- 300 cm at treeline and 200 cm- 400 cm in the alpine.

Terrain and Travel

  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Any steep opening in the trees should be treated as suspect right now.
  • Storm slab size and sensitivity to triggering will likely increase through the day.
  • Persistent slabs have potential to pull back to lower angle terrain.

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.