Avalanche Forecast

Issued: Dec 24th, 2017 5:01PM

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

cgarritty, Avalanche Canada

Purcells.

As storm snow settles, persistent slab problems emerge. Sheltered treeline elevations are a primary concern.

Summary

Confidence

High -

Weather Forecast

Monday: A mix of sun and cloud. Light variable winds. Alpine temperatures around -17.Tuesday: Mainly cloudy. Light southwest winds. Alpine temperatures around -16.Wednesday: Cloudy with isolated flurries bringing a trace of new snow, increasing overnight. Light southwest winds. Alpine temperatures around -15.

Avalanche Summary

A MIN report from the Dogtooth Range on Saturday showed a continuation of remote triggered Size 1 releases occurring in sheltered areas below treeline. Ski cutting (and explosives) also successfully produced several Size 1.5 releases over facets and surface hoar in the alpine. Another MIN report from Friday included observations of ski traffic easily producing shooting cracks between 1700-2000m in the Dogtooth. Persistent slab avalanche danger can be expected to persist in areas where last week's storm snow consolidates into a slab above the December 15 interface. In areas where new snow has not yet consolidated into a slab, expect continued potential for triggering loose dry avalanches in steep terrain.

Snowpack Summary

Last week's stormy weather brought 30-50cm of new snow to the region. Shifting winds over recent days have redistributed this storm snow into wind slabs on a variety of aspects at higher elevations while cold temperatures have worked to inhibit slab formation in wind-sheltered areas. The storm snow lies over a wide variety of old surfaces, including large surface hoar (weak, feather-like crystals), hard crusts formed by sun or wind, and sugary facets. The bond at this old snow interface is of critical importance where the overlying storm snow has consolidated into a slab. The most concerning areas are those that saw pronounced surface hoar development before the storm, particularly in sheltered areas at and below treeline. Recent snowpack tests show wide ranging reactivity at this interface but suggest that it may be most reactive on northerly aspects. A crust which was formed by rain in late November remains a major feature in the snowpack and is down approximately 60-100cm at tree line elevations. This interface shows good signs of bonding in the Purcells, but has indicated some reactivity in steep, variably loaded features in adjacent regions.

Problems

Persistent Slabs

Reactivity at the buried December 15 interface will likely be greater where the new snow has settled into a cohesive slab above buried surface hoar. Use extra caution around steeper, sheltered slopes at treeline and below.

Watch for signs of slab formation such as whumpfing, or cracking.Buried surface hoar is more likely to be preserved in sheltered openings below the alpine.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 2

Wind Slabs

Shifting winds have been redistributing storm snow into wind slabs on a variety of aspects. Northwest winds were the most recent. Exercise extra caution at lower elevations where winds may have formed a slab over a touchy surface hoar layer.

Be aware of variable wind loading patterns.Travel on ridges and/or ribs to avoid wind loaded slopes below

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

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 2

Valid until: Dec 25th, 2017 2:00PM