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Avalanche Forecast

Archived

Jan 16th, 2021–Jan 17th, 2021

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

Regions

Kootenay Boundary.

Find the best riding in wind sheltered terrain above 2000 m but keep your guard up around steep pockets where recent storm snow may be settling over a fragile layer of surface hoar or a slippery crust. 

Confidence

High -

Weather Forecast

Saturday night: Flurries, a few cm, moderate southwest ridgetop wind, freezing level valley bottom.

Sunday: Flurries, a few cm, moderate northwest ridgetop wind, freezing level 800 m.

Monday: Sunny, light northerly ridgetop wind, freezing level 800 m.

Tuesday: Mix of sun and cloud, ridgetop wind shifting southwest, building to strong, freezing level 800 m.

Avalanche Summary

A couple of natural avalanches since the storm are suspected to have run on the surface hoar 20-40 cm down, including a size 2.5 storm slab out of a northwest facing gulley feature around 2000 m near Nelson.

On Wednesday, a widespread natural avalanche cycle up to size 2.5 was reported. A recent MIN report observed from Rossland shows a size 3 natural persistent slab avalanche that occurred during or post-storm.

Last week, a naturally-triggered size 3 deep persistent slab avalanche released on a southeast aspect at 2200 m in the Rossland range. The avalanche likely failed on the weak layers described in the snowpack summary and scrubbed to the ground. 

Snowpack Summary

A surface crust exists up to around 2000 m. The top 20-40 cm of the snowpack is composed of some elevation-dependent permutation of a crust sandwich, formed by fluctuating freezing levels during the last storm. 

A layer of surface hoar (and/or another crust) may be found beneath the most recent storm snow in sheltered terrain. A couple of recent avalanches are suspected to have run on this layer and where it has been found in snow profiles, it has been producing moderate to hard planar test results.

An older weak layer is buried 70 to 130 cm deep. It is composed of faceted grains over a crust. This recent MIN report observed from Rossland on Wednesday the 13th shows good evidence of a large natural slab avalanche failing on this layer during or post-storm. This suggests that it remains possible for riders to trigger this layer.

Another layer of faceted grains above a melt-freeze crust that formed in early November is upwards of 200 cm below the surface. This has been the suspected failure layer of a few very large avalanches that released last week in the Rossland range.

Terrain and Travel

  • Be careful as you transition into wind affected terrain.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Conditions may have improved, but be mindful that deep instabilities are still present.

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.

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.

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.