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Avalanche Forecast

Archived

Jan 12th, 2021–Jan 13th, 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 avalanches possible, human triggered probable.
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.

Regions

Kootenay Boundary.

 Widespread avalanche activity is expected avoid avalanche terrain including overhead hazards. The new load and stress on the snowpack may be enough to trigger deep weak layers. 

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

The storm will start to diminish overnight as a ridge of high pressure builds on Wednesday bringing dryer and cooler conditions.

Tuesday Night: Snow 20-30 cm with freezing levels rising to 1500 m and possibly spiking to 2000 m early Wednesday morning. Ridgetop wind strong from the southwest.

Wednesday: Cloudy with possible sunny periods. Treeline temperatures near -2 and freezing level 1400 m.

Thursday/ Friday: Mix of sun and cloud. Treeline temperatures -4 and freezing level 900 m rising to 1100 m on Friday.

Avalanche Summary

On Tuesday, explosive control triggered numerous size 2-2.5 slab avalanches and human triggered avalanches up to size 1.5 were also reported. 

Natural and human triggered slab avalanches will continue to exist on Wednesday.

Of note, a naturally-triggered size 3 deep persistent slab avalanche released on a southeast aspect at 2200 m in the Rossland range last Tuesday or Wednesday. The avalanche likely failed on the weak layers described in the Snowpack Summary and scrubbed to ground. Persistent slab avalanche activity has quieted in the past few days since this report. Although the likelihood of triggering these layers has reduced, the consequence of doing so remains high. 

Snowpack Summary

Up to 30 cm of new snow fell on Tuesday with another 20-30 cm forecast for overnight into Wednesday. The new snow accompanied by strong winds are building reactive storm and wind slabs. These will likely be extra sensitive where they overlie surface hoar in sheltered terrain. 

Two weak layers are buried in close proximity to one another. They are 70 to 100 cm deep around Nelson and 50 to 80 cm deep around Rossland and other western parts of the region. The layers are composed of surface hoar and faceted grains and they overlie a hard melt-freeze crust. Many MIN reports describe these layers and their snowpack test results. The results suggest that it remains possible for riders to trigger these layers.

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

  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.
  • Cornice failure may trigger large avalanches.

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.