Lawinenlagebericht

Purcells

Jan 12 – 13, 2022Avalanche Canada
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Lawinengefahr

Fri
Sat
Hochgebirge
3 · Erheblich
Waldgrenze
3 · Erheblich
Unterhalb
3 · Erheblich
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Kernpunkte

Strong wind combined with new snow, warming temperatures and some sun require continuing assessment of rapidly changing snow conditions. The south of the region might see freezing levels rising to 2000 m on Wednesday afternoon.

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1.5–2.5

    Moderate to strong southwest wind will redistribute 5-15 cm new snow and continue to form fresh wind slabs. Be especially mindful around steep and convex openings in the trees and ridge features in the alpine. Wind slabs might have formed lower down slope than expected with the strong wind.

    The new and recent snow may overlie a weak interface which could result in deeper and touchier than expected slabs. Small avalanches may step down to deeper weak layers and result in large and destructive avalanches.

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Unlikely - Possible
    Größe
    2–3

    A persistent slab problem formed by a crust down 90-160 cm has created a low likelihood, high consequence scenario that is difficult to forecast. It is uncertain how the forecast warm temperatures in the south of the region will affect the snowpack and if we will see more activity on this layer during the warm period in that area.

    Avoid likely trigger spots such as steep, rocky slopes with a shallow or thin-to-thick snowpack. Large additional loads like small avalanches or failed cornices can trigger deeper weak layers. The same feature or path can slide repeatedly when the persistent weak layer gets reloaded with new snow. 

Confidence

Moderate - Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather.

Weather Forecast

Tuesday night: Mostly cloudy, 5-10 cm new snow, strong southwest wind, treeline low around -4 °C, freezing level around 800 m.

Wednesday: Mix of sun and cloud, up to 3 cm new snow, moderate to strong southwest wind, treeline high around 0 °C, freezing level around 1200 m, in the south of the region the freezing level will rise to 2000 m.

Thursday: Mix of sun and cloud, up to 3 cm new snow, moderate to strong southwest wind, treeline high around 0 °C, freezing level rising to 1400 m, in the south of the region freezing levels might stay around 2000 m. 

Friday: Mix of sun and cloud, trace of new snow, light southwest wind, treeline high around -3 °C, freezing level around 1000 m.

Avalanche Summary

On Monday, two cornices failed naturally and triggered a storm slab resulting in size 2 and 2.5 avalanches. The cornices did not step down to the early December persistent weak layer. Explosives triggered a small wind slab avalanche of size 1.5. 

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 is being reported since 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 Monday, a skier triggered a size 2 persistent slab avalanche below treeline on an E aspect. The avalanche ran on the early December layer.
  • 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.

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