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Avalanche Forecast

Archived

Jan 10th, 2021–Jan 11th, 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 possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Lizard-Flathead.

  

We continue to receive sporadic reports of large persistent slab avalanches failing on a deeply buried crust. Human triggering of this layer is most likely on steep, rocky terrain with a shallow or thin to thick snowpack.

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

  

SUNDAY NIGHT: Cloudy / Light, west ridgetop wind / alpine low temperature -11 / Freezing level valley bottom.

MONDAY: Cloudy with isolated flurries; 0-3 cm. / Moderate, southwest ridgetop wind / alpine high temperature -6 / Freezing level 1200 m.

TUESDAY: Flurries; 3-5 cm, with another 15-25 cm. overnight / Extreme, southwest ridgetop wind / alpine high temperature -1 / Freezing level rising to 1800 m.

WEDNESDAY: Snow, 5-15 cm. / Moderate, west ridgetop wind / alpine high temperature -4 / Freezing level 1400 m.

Avalanche Summary

On Saturday, explosive testing near Fernie produced several large persistent slab avalanches up to size 2.5 failing on weak facets above a crust buried in early December.

A couple large (size three) naturally triggered persistent slab avalanches were reported on large alpine features on Thursday. These avalanches were triggered by either smaller wind slabs in motion or cornice falls. These are continued reminders of the "low probability; high consequence" scenario that persistent slab problems often create. See MIN reports HERE.

Snowpack Summary

Lingering wind slabs formed by recent snow and wind remain possible to human trigger in isolated areas. Roughly 30-50 cm of recent snow is settling above a crust that extends up to 1900 m. In isolated areas below treeline, this recent snow may be sitting on a weak layer of surface hoar.

 The main feature we are monitoring in the snowpack is a layer of weak facets(sugar snow) over a hard melt-freeze crust found around 150 cm down. Recent sporadic, large naturally triggered avalanches have occured on this layer on large alpine slopes and were triggered by either smaller wind slabs in motion or cornice falls.

Terrain and Travel

  • Be carefull around freshly wind loaded features.
  • 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.
  • Avoid exposure to slopes that have cornices overhead.

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.