Avalanche Forecast

Issued: Dec 17th, 2020 4:00PM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is low. Known problems include Wind Slabs and Deep Persistent Slabs.

Avalanche Canada dsaly, Avalanche Canada

Winds are building slabs along ridgelines and in lee terrain features, expect more rapid development as wind increases.

Summary

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Recent weather patterns have resulted in a high degree of snowpack variability within the region.

Weather Forecast

THURSDAY NIGHT- Mainly cloudy with isolated flurries. trace to 5 cm / west wind, 20-50 km/h / alpine high temperature near -6

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

SATURDAY - Mainly cloudy with scattered flurries, 10 cm / southwest wind, 25-55 km/h / alpine high temperature near -6

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

Avalanche Summary

Moderate southwest winds may form fresh windslabs that could be reactive to human triggers at upper elevations. Check out this MIN report from the KHMR backcountry - poor bonding between the new snow and old surface resulting in sluffing and loose, dry avalanches, and small windslabs in steep rocky terrain.

Avalanche control work on Tuesday produced a few small (size 1) storm slab and dry loose avalanches.

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

15-25 cm of snow overlies a rain crust found up to 2400 m in the south of the region and 1800 m in the north, the recent snow may sit over a weak layer of surface hoar or facets at elevations around treeline where this crust is not found. South-southwesterly winds are creating new, small cornices and wind slabs on lee slopes. The recent snow may not be bonding well to older surfaces. There have also been some reports of an isolated weak layer 40 cm below the surface around Golden.

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

  • 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.
  • Investigate the bond of the recent snow
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.

Problems

Wind Slabs

An icon showing Wind Slabs

Increasing winds impacting 10-30 cm of soft surface snow may blow into unstable slabs. Where the new snow overlies surface hoar (or a crust) a poor bond has been noted, and reactively will increase where soft snow gains cohesion and slab properties.

The problem is more likely found in northern parts of the region (e.g. Golden).

Aspects: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1.5

Deep Persistent Slabs

An icon showing Deep Persistent Slabs

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.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely

Expected Size

2 - 3

Valid until: Dec 18th, 2020 4:00PM

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