Avalanche forecast

Purcells

Jan 13 – 14, 2022Avalanche Canada
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Archived · expired 5 years ago

Avalanche danger

Sat
Sun
Alpine
4 · High
Treeline
4 · High
Below
3 · Considerable
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Highlights

A rising freezing level up to 2500 m Wednesday night in the east of the region will result in very dangerous avalanche conditions with warmth, some sunshine and a persistent weak layer in the snowpack.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–2.5

    Up to 20 cm of snow fell with moderate southwest wind which formed a reactive wind slab. Be especially mindful around steep and convex openings in the trees and ridge features in the alpine.

    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.

  • Cornices

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2–3

    Cornices are reactive to human traffic with the warm temperatures and natural cornice falls become more likely. Large cornice falls are dangerous on their own. 

    A failed cornice can trigger buried persistent weak layers and result in very large and destructive avalanches. 

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely - Possible
    Size
    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 will affect the snowpack and if we will see more activity on this layer during the warm period.

    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. Uncertainty is due to difficult to forecast freezing levels.

Weather Forecast

Wednesday night: Mostly cloudy, up to 3 cm new snow, moderate to strong southwest wind, treeline low around -2 °C, freezing level rising to around 2500 m in the east of the region and staying around 1100 m in the west of the region.

Thursday: Mix of sun and cloud, trace of new snow, moderate southwest wind, treeline high around 0 °C, freezing level around 2200 in the east of the region and around 1400 m in the west of the region. 

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

Saturday: Mix of sun and cloud, trace of new snow, strong southwest wind, treeline high around -3 °C, freezing level around 1200 m.

Avalanche Summary

On Tuesday, a large (size 2) natural storm slab avalanche was observed. Skiers and explosives triggered many small avalanches up to size 1.5.

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.  

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

Up to 20 cm new snow fell with southwest wind and warming temperatures. The new snow sits on wind affected surfaces in exposed and open areas. In sheltered areas the new snow might sit on a weak layer of feathery surface hoar. Below that 40-90 cm of previous storm snow sits on 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

  • Minimize overhead exposure; avalanches triggered by warming or cornice fall may be large and destructive.
  • 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.

Regions covered