Avalanche Forecast

Archived

Dec 14th, 2020–Dec 15th, 2020

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

Purcells.

Wind slabs are likely to be encountered at upper elevations and may be reactive to human triggering, especially in lee features. Watch for signs of instability such as whumphing, cracking and recent avalanches.

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

MONDAY NIGHT - Cloudy with clear periods and isolated flurries / light southwest wind / alpine low temperature near -10

TUESDAY - Cloudy sunny periods and isolated flurries / southwest wind, 20-50 km/h / alpine high temperature near -6

WEDNESDAY - Cloudy sunny periods and isolated flurries / southwest wind, 20-40 km/h / alpine high temperature near -6

THURSDAY - Mainly cloudy with scattered flurries / light southwest wind / alpine high temperature near -6

Avalanche Summary

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

A MIN report from the Clamshell near Golden reported a natural size 2.5 avalanche on Wednesday. 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

10-30 cm of soft snow overlies a rain crust found up to 2400 m in the south of the region and 1800 m in the north. 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. The recent snow may not be bonding well to these old 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

Investigate the bond of the recent snow
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.

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.