Avalanche Forecast

Issued: Jan 20th, 2012 9:12AM

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

pgoddard, Avalanche Canada

Purcells.

Danger ratings are based on an expected 40cm of snowfall through Friday and Saturday, coupled with warming and strong winds. However, the weather forecast is uncertain. If less severe weather arrives, danger may be lower than forecast.

Summary

Confidence

Fair - Forecast snowfall amounts are uncertain on Saturday

Weather Forecast

Friday night: Moderate snow. Strong south-westerly winds. Alpine temperatures rising to -7C.Saturday: Moderate snow. Strong south-westerly winds. Alpine temperatures rising to -4C in the morning. Sunday: Cloud and flurries. Cooler temperatures. Moderate westerly winds. Monday: Flurries or light snow. Moderate to strong westerlies.

Avalanche Summary

Recent observations include a few cornice failures and isolated skier-triggered wind slabs in the size 1-2 range. Explosives testing released storm slabs, with little in the way of recent deep releases. Avalanche activity slowed down towards the end of the week, but is expected to increase again during the weekend.

Snowpack Summary

New storm snow is building up over wind slabs in exposed features and over loose dry snow in sheltered areas. Strong winds and warming temperatures forecast to accompany Friday night/Saturday's snowfall are likely to create a more widespread wind slab and storm slab problem. The mid-December surface hoar/facet persistent weakness, now down around 50cm on the eastern side of the range and as deep as 180cm on the western side, is still causing operators concern, especially in shallow snowpack areas. Recent snowpack tests show moderate to hard, sudden planar results on this layer. The consequences of a failure on this layer would be large. Facets and depth hoar exist at the base of the snowpack. Cornices are large and weak in some areas.

Problems

Storm Slabs

A classic recipe for storm slabs is evolving: new snow falling with rising temperatures onto loose dry snow. Storm slab avalanches are likely, especially in steep or convex terrain.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

1 - 5

Wind Slabs

Strong winds will shift available snow into wind slabs on downwind slopes. Older buried wind slabs also exist.

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

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 5

Persistent Slabs

Two layers to watch: 1. Buried surface hoar, most likely triggered on steep, unsupported slopes amongst the trees. 2. Basal facets, triggerable from thin snowpack areas or by a very heavy load (cornice fall, airborne sled). Avalanches could be large.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Unlikely - Possible

Expected Size

4 - 7

Valid until: Jan 21st, 2012 8:00AM