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Avalanche Forecast

Archived

Jan 7th, 2021–Jan 8th, 2021

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

Regions

Kootenay Boundary.

Skies are clearing after a stormy week. It may be tempting to get into bigger terrain, however persistent slab avalanches continue to be reported. The best and safest riding will be found in mellow wind sheltered terrain. 

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Thursday Night: Mainly cloudy with clear periods, light to moderate south wind, alpine high -6.

Friday: Mainly cloudy with sunny breaks and isolated flurries, moderate south wind, alpine high -4, freezing level 700 m.

Saturday: Isolated flurries, moderate southwest wind, alpine high -5, freezing level 600 m.

Sunday: Flurries, light southwest wind, alpine high -5.

Avalanche Summary

Reports on Wednesday and Thursday indicate several explosives triggered size 2 storm slab avalanches on northerly and southerly aspects at treeline and in the alpine. There was also a report of a size 2 storm slab avalanche that was triggered by another smaller avalanche running nearby. This was at 1950 m on a northeast aspect. As well on Thursday there was a report of 3 explosives triggered size 2.5-3 persistent slab avalanches on north through southwest aspects at 2100 m.

On Tuesday there was a report of a naturally triggered size 2.5-3 wind slab avalanche which stepped down mid-path to the early December persistent weak layer. This was on an east aspect near ridgetop and occurred in the central part of the region. As well there were reports of several explosives controlled size 2 storm snow avalanches in Kootenay Pass on southerly aspects between 1800 and 2100 m.

A remotely triggered (from a distance) size 2 persistent slab avalanche was reported near Rossland on Sunday. The details can be seen in MIN posts here and here.

Snowpack Summary

40-70 cm of recent storm snow accumulating over the past week has seen wind affect at upper elevations. In sheltered areas, the recent snow may sit over surface hoar.

Two weak layers are buried in close proximity to one another. They are 70-100 cm deep around Nelson and 50-80 cm deep around Rossland and other western parts of the region. The layers are composed of feathery surface hoar and sugary faceted grains and they overlie a hard melt-freeze crust. 

Terrain and Travel

  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Be especially cautious as you transition into wind affected terrain.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger 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.