Avalanche forecast

Purcells

Feb 23 – 24, 2021Avalanche Canada
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Avalanche danger

Thu
Fri
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Highlights

Human triggered avalanches are likely at upper elevations on Tuesday. Be especially wary of recently wind loaded features. Fresh slabs sit on a thick layer of weak, sugary facets and bonding at this interface may take some time.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely - Possible
    Size
    2–3

    There is a lingering possibility of triggering persistent slab avalanches in isolated steep terrain features. The primary layer of concern is layer of surface hoar, facets, and/or crusts buried 30-60 cm deep throughout the region. We are awaiting reports of avalanche activity during the storm, specifically whether they indicate step- downs to this layer, which may offer insight as to whether or it continues to be a problem.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    1–2

    10-30 cm of recent snow sits overtop of weak facets and/or extensively wind affected surfaces. Due to this weak interface, fresh slabs may be surprisingly sensitive to triggering. In the east of the region where less than 20 cm of snow fell, the problem is specific to wind loaded terrain features. 

    Cornices have likely experienced rapid growth with strong westerly winds. Unsupported tabs that break off can act as triggers on slopes below.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength. Recent weather patterns have resulted in a high degree of snowpack variability within the region.

Weather Forecast

MONDAY NIGHT: Up to 10 cm of snow, moderate to strong northwest wind, freezing level dropping from 1500 m to valley bottom.

TUESDAY: Scattered flurries with sunny periods, light northwest wind, freezing level 1000 m.

WEDNESDAY: Sunny, light northwest wind, freezing level 1000 m.

THURSDAY: Scattered flurries, moderate southwest wind easing to light west, freezing level 1000 m.

Avalanche Summary

Preliminary reports from Monday include several size 1-1.5 natural and explosive triggered wind slabs up to 40 cm deep in one of the lowest snowfall areas of the region.

Earlier in February, there were a few reports of large avalanches (size 2.5-3) on southeast aspects in the alpine, likely releasing on a layer of facets on a crust (see a photo here), and a large human-triggered avalanche (size 2.5) on surface hoar near Quartz Creek.

Snowpack Summary

10 to 40 cm of new snow sits overtop of the old, faceted and/or wind affected snow that sat on the surface during the extended cold, windy drought period. At upper elevations, the new snow has been blown around by strong to extreme westerly winds, forming reactive slabs in lee features.

A weak layer that formed in late January is now 30-70 cm deep. In the northern Purcells this layer has been reported as a surface hoar layer at treeline and below treeline elevations, but a combination of facets on crusts could exist at all elevations throughout the region. 

The northern Purcells also have an older surface hoar layer that can still be found 60-100 cm deep at treeline. Additional weak layers may exist near the bottom of the snowpack and steep rocky areas with a shallow or thin to thick snowpack should be considered suspect trigger points.

Terrain and Travel

  • Fresh snow rests on a problematic persistent slab, don't let good riding lure you into complacency.
  • Stay off recently wind loaded slopes until they have had a chance to stabilize.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

Regions covered