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Avalanche Forecast

Archived

Dec 17th, 2020–Dec 18th, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Purcells.

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

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

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.