Avalanche Forecast
Issued: Dec 9th, 2021 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Persistent Slabs.
zryan,
Avalanche Canada
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.
Summary
Confidence
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
50-100 cm of new snow has fallen in the past week, with another 30-40 cm to come throughout the day today. This amount of new snow will put significant new load on the snowpack, and is likely to create avalanches in many areas.
In wind exposed areas, strong southwest winds will create deep deposits of wind slab. In sheltered areas, the new snow will acts as a storm slab or loose dry problem.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Persistent Slabs
There is still some concern for buried weak layers, with the possibility for buried surface hoar and faceting above the December 1st crust. These layers where buried during the last storm cycle and will now be down 40-80 cm in sheltered areas.
The December 1st crust is present to around 1200m in the Terrace area, varying from 800m-1600m throughout the region. At treeline and below 5-10mm surface growth was observed prior to the last storm cycle. With limited visibility and observations in the past 3 days we are unsure if this surface hoar is preserved in any locations.
These weak layers will add heightened danger to an already significant storm. The presence of these weak layers mean that avalanches have the potential to pull back into low angled terrain, that we might otherwise consider 'safe' in high danger.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 10th, 2021 4:00PM
