Avalanche Forecast

Issued: Mar 5th, 2013 9:35AM

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

swerner, Avalanche Canada

Purcells.

Natural avalanche activity has tapered off, but skier triggered avalanches are possible. Solar aspects seem to be most reactive with large avalanches failing on an old crust. Evaluate terrain carefully and conservative decision making is recommended.

Summary

Confidence

Good

Weather Forecast

Tonight and Wednesday: A low pressure system moves inland to the South of the border. Light-moderate precipitation will begin this evening over the Southern ranges. Ridgetop winds light from the South. Alpine temperatures near -6 and freezing levels 1300 m. Thursday: Mainly cool and dry with some clouds. Ridgetop winds light from the West. Alpine temperatures near -7 and freezing levels at 1200 m. Friday: The ridge will remain over the region with dry, sunny conditions. Ridgetop winds will blow light from the West. Alpine temperatures near -9 and freezing levels at 1400 m.

Avalanche Summary

On Monday, one size 2 natural slab avalanche occurred from an East aspect. Many operators in the region did control work using explosives and triggered slab avalanches size 1-3.5 on all aspects generally above 2200 m. The large avalanches that initiated from Southerly aspects ran on a buried crust interface down 80-100 cm.

Snowpack Summary

Up to 80 cm of snow fell over the past week. The new snow added to ongoing storm snow instabilities, particularly on the west side of the region where accumulations have been highest. Buried beneath (60-100 cm) exists a surface hoar and a sun crust layer, which continue to be lingering concerns. Large looming cornices exist on ridgelines. Cornice fall could trigger a large slab avalanche on the slope below.Below treeline exists a melt-freeze crust with moist snow beneath it. The crust may be bridging instabilities deeper in the snowpack. If temperatures warm at lower elevations, you'll notice the crust breaking down and the snowpack becoming weaker.

Problems

Storm Slabs

Recent snow and wind have created new slabs. These may be particularly touchy on slopes lee of the wind and it may take several days for the storm snow to settle out. Skier and rider triggering is possible.

Avoid freshly wind loaded features.>Be alert to conditions that change with elevation.>Assess start zones carefully and use safe travel techniques.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

1 - 5

Persistent Slabs

Persistent weak layers exist buried down 60-100 cm, comprising of surface hoar and a sun crust. Avalanches failing on these layers will likely be large and destructive.

Be aware of the potential for large, deep avalanches due to the presence of buried surface hoar and a sun crust.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 6

Valid until: Mar 6th, 2013 2:00PM