Avalanche forecast

Purcells

Dec 19 – 20, 2020Avalanche Canada
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Archived · expired 6 years ago

Avalanche danger

Mon
Tue
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
1 · Low
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Highlights

Wind has blown loose snow into slabs along ridgelines and lee terrain features, use caution moving into wind affected terrain.

Avalanche problems

  • Wind Slabs

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

    Winds have formed reactive slabs in lee features at upper elevations. Where the new snow overlies surface hoar (or a crust) a poor bond has been noted, and reactively will increase as winds encourage cohesion and slab properties. Dig down to investigate developing slabs in the upper snowpack and a poor bond to older surfaces. The problem is more likely found in northern parts of the region, in areas that have received the higher amounts of incremental snowfall (e.g. Golden).

  • Deep Persistent Slabs

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

    A facet/crust layer from early November sits near the base of the snowpack. This layer has shown signs of recent reactivity with explosive control in shallow rocky locations producing large avalanches. It is most likely to be triggered by humans in shallow, rocky areas or in a step-down from a smaller avalanche.

Confidence

Moderate - Recent weather patterns have resulted in a high degree of snowpack variability within the region.

Weather Forecast

FRIDAY NIGHT - Mainly cloudy with scattered flurries, 5-10 cm / west-southwest wind, 20-40 km/h / alpine high temperature near -8

SATURDAY - Mainly cloudy with scattered flurries, trace to 5 cm / southwest wind, 25-55 km/h / alpine high temperature near -4 / Freezing level rising to 1300 m

SUNDAY - Snow, 10-20 cm / southwest wind, 20 gusting to 80 km/h / alpine high temperature near -1 / Freezing level rising to 1700 m

MONDAY - Flurries, 10 cm / southwest wind, 20-30 km/h / alpine high temperature near -9 / Freezing level returning to valley bottom

Avalanche Summary

On Thursday in the Kicking Horse backcountry, small (size 1-1.5) wind slabs 15-25 cm deep were reactive to skiers, and were found in predictable wind loaded areas.

This MIN report from the KHMR backcountry documents poor bonding between the new snow and old surface resulting in sluffing and loose, dry avalanches, and small windslabs in steep rocky terrain. 

A MIN report from the Clamshell near Golden reported a natural size 2.5 avalanche last Wednesday (Dec 9). This avalanche appears to have run on a weak layer roughly 40 cm below the surface, which seems to be an isolated problem in this area.

Snowpack Summary

Incremental loading has stacked 15-30 cm loose snow around the region (with the higher amount to the north around Golden). Southwest winds have built slabs in lee features at upper elevations, reactive cornices have grown too. 

This recent snow overlies a crust up to 2400 m in the south of the region and 1800 m in the north. Around treeline, where there crust isn't found, the recent snow may sit over a weak layer of surface hoar or facets. Reports on this weak layer come from the Golden area (MIN report), finding weak surface hoar in isolated locations, 30-40 cm below the surface. Keep this layer in mind a forecasted snow (on Monday) accumulates, a touchy weak layer could emerge.

The lower snowpack is characterized by a couple of crusts, the most notable is from a rain event in early November that is 50 to 100 cm deep. This crust is sitting near the base of the snowpack, surrounded by a weak layer of sugary facets. It is most likely to be reactive to human triggers or in a step-down from a smaller avalanche in shallow, rocky areas.

Terrain and Travel

  • Investigate the bond of the recent snow before committing to your line.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Be careful with wind slabs, especially in steep, unsupported and/or convex terrain features.
  • In areas where deep persistent slabs may exist, avoid shallow or variable depth snowpacks and unsupported terrain features.

Regions covered