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Avalanche Forecast

Archived

Jan 10th, 2022–Jan 11th, 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 avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
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 possible, human triggered probable.

Regions

Purcells.

Fresh wind slabs will form in lee terrain features and below ridge crests. Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches. 

Confidence

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

Weather Forecast

Monday night: Increasing cloud coverage, trace of new snow, strong southwest wind, treeline low around -8 °C, freezing level below valley bottom. 

Tuesday: Mix of sun and cloud, up to 5 cm new snow, strong southwest wind, treeline high around -3 °C, freezing level rising to 1000 m, possible inversion.

Wednesday: Mix of sun and cloud, 5-10 cm new snow, strong southwest wind, treeline high around 0 °C, freezing level rising to 1500 m, possible inversion. 

Thursday: Sunny with cloudy periods, up to 5 cm new snow, strong southwest wind, treeline high around +1 °C, freezing level around 2000 m, inversion. 

Avalanche Summary

On Sunday, a natural size 2 wind slab avalanche that released in extremely steep terrain was observed. A few small (up to size 1.5) wind slab and dry loose avalanches were triggered by skiers.  

On Saturday, several wind slab avalanches up to size 2 released naturally and were triggered by explosives. A small size 1.5 wind slab avalanche was triggered remotely by a skier. 

On Friday, explosives triggered storm stab avalanches to size 2, on average 20 cm deep. A rider triggered a size 2 wind slab avalanche 30 cm deep. Skiers triggered numerous small (size 1) avalanches. A natural avalanche cycle to size 1.5 occurred Thursday-Friday night with rapid loading of new snow.

An increasing amount of large, persistent slab avalanches was reported on Sunday. Almost all the reported avalanches over the past week ran on the early December weak layer. Deeply buried persistent problems like these don't go away overnight, and it remains a serious concern. 

  • On Sunday, two size 3 and 3.5 deep persistent avalanches were triggered with explosives up to 1 m deep. They released at an elevation around 1900 m on SE and SW aspects. And a persistent slab avalanche 2 m deep and 200 m wide was reported in this MIN report. 
  • Last 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 ridge top. 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. 

Notable persistent slab avalanches in neighbouring regions feature in our latest blog, Photos of recent persistent slab avalanches in the southern interior.

Snowpack Summary

40-60 cm of recent storm snow fell with southwesterly wind and warming temperatures. In open terrain and upper elevations, wind developed more reactive deposits around lee and convex features. An accumulated total of 40-90 cm of new and recent snow sits now over variable and potentially weak snow surfaces including widespread facets, wind affected snow, and/or surface hoar up to 3 mm in sheltered areas.

The most notable layer of concern in the snowpack is a crust that was formed in early December and is now down 90-160 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

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

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.