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Avalanche Forecast

Archived

Jan 5th, 2022–Jan 6th, 2022

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

Purcells.

As winds pick up ahead of the next storm, fresh, reactive wind slabs will likely form at upper elevations. The region continues to deal with a tricky persistent slab problem and very large avalanches remain possible. Conservative terrain selection remains essential. 

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack. Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Wednesday night: Increasing cloud. Light NW wind switching SW. Treeline temperature around -16 °C. 

Thursday: Snowfall 4-8 cm. Moderate to strong SW wind. Treeline high around -12 °C. 

Friday: Snowfall 5-15 cm. Strong SW wind. Treeline high around -8 °C. 

Saturday: Light snowfall up to 5 cm. Moderate SW wind. Treeline high around -8 °C. 

Avalanche Summary

Reports from throughout the region Monday include natural, explosive and skier triggered wind slabs up to size 2. This MIN report from the south of the region shows an example of the instability in the storm snow and good decision making. Wind slabs remained reactive around Invermere into Tuesday, with continued natural and human triggered activity up to size 2. 

On Tuesday just east of the region in the Rockies near Invermere, a size 3.5 (very large) persistent slab avalanche was triggered remotely by a skier on an alpine ridgetop. The resulting crown was 2-3 m deep and the avalanche ran about 1 km to the valley bottom. More details in this MIN report. This and other notable persistent slab avalanches in neighboring regions feature in our latest blog, Photos of recent persistent slab avalanches in the southern interior.

Snowpack Summary

As winds pick up in advance of the next storm, 30-40 cm of recent snow will likely see redistribution into reactive slabs in lee terrain features. The recent snow sits over variable and potentially weak snow surfaces including widespread facets, wind affected snow, and/or surface hoar up to 3 mm in sheltered areas. Reactivity at this interface may persist for longer than usual.

The most notable layer of concern in the snowpack is a crust that was formed in early December and is now down 70-150 cm. Activity on this layer has been sporadic. It has generally been triggered on steep, rocky slopes, with a shallow or thin-to-thick snowpack. This layer has created a low likelihood, high consequence scenario which is best managed through conservative terrain choices and disciplined backcountry travel techniques.

Terrain and Travel

  • Fresh wind slabs will likely form throughout the day, diligently watch for changing conditions.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.
  • Seek out wind sheltered terrain below treeline where you can avoid wind slabs and find great riding.

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.