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Avalanche Forecast

Archived

Jan 8th, 2021–Jan 9th, 2021

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

Regions

Kootenay Boundary.

 Heightened avalanche conditions exist on wind loaded slopes and steep rocky terrain. Although the likelihood of triggering deeply buried weak weak layers is reducing, the consequences of doing so are high. 

Confidence

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

Weather Forecast

Friday Night: Mainly cloudy, light west wind, alpine high -7 C, freezing level at valley bottom.

Saturday: Isolated flurries, moderate southwest wind, alpine high -5, freezing level 500 m.

Sunday: Isolated flurries, moderate southwest wind, alpine high -5, freezing level 500 m.

Monday: Flurries, moderate southwest wind, alpine high -2, freezing level 1000 m.

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. 

A naturally triggered size 3 deep persistent slab avalanche running on a southeast aspect at 2200 m was reported in the Rossland range. It was suspected to have run on Tuesday or Wednesday.

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. 

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

Recent new snow from earlier in the week has been redistributed by southwest wind at upper elevations. In sheltered areas, recent new 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

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.