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Avalanche Forecast

Archived

Jan 8th, 2021–Jan 9th, 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 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.

Regions

Purcells.

Keep buried weak layers and potential for persistent slab avalanches in your mind. The likelihood of triggering them may be diminishing but if you're unlucky enough to do so, consequences are high. A complex snowpack like this is best managed with conservative terrain choices.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast. Uncertainty is due to field data and reports showing a wide variation in conditions throughout the region.

Weather Forecast

Friday night: Clear, calm to light west wind, freezing level valley bottom.

Saturday: Mainly sunny, light southwest wind, alpine high -7, freezing level valley bottom.

Sunday: Mix of sun and cloud, light west wind, alpine high temperature near -7, freezing level valley bottom.

Monday: Mix of sun and cloud, moderate southwest wind, alpine high -7, freezing level 900 m.

Avalanche Summary

A few natural, human and explosive triggered wind slabs size 1-2 were reported across the region in the latter part of the week.

Two human triggered avalanche incidents occurred on Tuesday; a size 1.5 on surface hoar in a northeast facing burn near Glacier National Park (view MIN report here), and a size 2.5 avalanche on an east aspect at treeline in the Lower Holt area, near Golden (view report here). Thankfully there were no injuries.

A notable avalanche from the natural storm cycle that occurred last weekend include a natural size 2.5 deep persistent slab avalanche reported on a north aspect at 2500 m near Invermere.

Snowpack Summary

Recent snow with strong southerly wind has formed wind slabs in leeward terrain features at upper elevations.

There are currently several layers of concern in the regional snowpack. 40-60 cm of snow sits on a weak layer of surface hoar that was reported in the Golden area. 

The mid-December crust and/or surface hoar is now down 90-140 cm. Although there have been no new reports of avalanches on this layer in the last few days, it remains possible to trigger where it is well preserved.

The lower snowpack is characterized by more crusts, the most notable is a rain crust from early November that is near the base of the snowpack, surrounded by a weak layer of sugary facets. It is most likely to be reactive to human triggers in steep, shallow, rocky areas with a thin to thick snowpack.

Terrain and Travel

  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
  • Uncertainty is best managed through conservative terrain choices at this time.

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.