Avalanche Forecast

Issued: Mar 4th, 2013 9:11AM

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

swerner, Avalanche Canada

Purcells.

Summary

Confidence

Good

Weather Forecast

Tonight and Tuesday: A weak ridge will remain over interior ranges with generally dry conditions. Ridgetop winds will blow moderate from the South. Light snow amounts. Alpine temperatures near -10 and freezing levels 1100 m.Tuesday night and Wednesday: A low pressure system will move inland. Most of this system will remain South of the border, but some cloud and light precipitation will spread into the Southern Interior ranges. Ridgetop winds light from the West. Alpine temperatures near -8 and freezing levels at 1400 m, falling to valley bottom overnight.Thursday: Generally clear and dry. Ridgetop winds light from the West. Alpine temperatures near -7 and freezing levels 1600 m the dropping overnight.

Avalanche Summary

Over the weekend the region saw a widespread large natural avalanche cycle, with avalanches up to size 3.5. On Sunday, numerous natural slab avalanches were reported up to size 2.5 on a a variety of aspects. Explosive avalanche control in region revealed several large slab avalanches up size 3.5.On Monday, explosive avalanche control produced a size 2.5 on a West aspect around 2900 m. No natural avalanche activity has been reported.

Snowpack Summary

Over the past week 60-80 cm snow fell. This new snow added to an ongoing storm snow instability, particularly on the west side of the region where accumulations have been highest. Buried surface hoar and sun crust layers exist up to 80cm below the surface and seem most prevalent on the west side of the region. Large avalanches initiated on these weak layers during the weekend storm. Large looming cornices exist on ridgelines and may become weak, especially when the sun comes out. Cornice fall could trigger slab avalanches on slopes below.A surface hoar layer buried on January 23 is still on the radar of some operators. This layer has been less likely to trigger by skiers, but may still be sensitive to large loads like avalanches in motion or cornice fall.Below treeline exists a melt-freeze crust which may be bridging instabilities deeper in the snowpack. If temperatures warm, you'll notice the crust breaking down and the snowpack becoming weaker.

Problems

Storm Slabs

Recent new 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 likely. Watch sluffing from steeper terrain.

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

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

1 - 5

Persistent Slabs

Loading from new snow and wind may add reactivity to persistent buried surface hoar layers. In deeper snowfall areas, resulting avalanches may be large and destructive.

Be aware of the potential for large, deep avalanches due to the presence of buried surface hoar.>Avoid paths that have not avalanched recently.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 6

Cornices

Large looming cornices exist on ridgelines. These cornices may be weak and fail, especially if the sun shines through. The weight of cornice fall could trigger a slab avalanche on the slope below.

Avoid areas with overhead hazard.>

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

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 5

Valid until: Mar 5th, 2013 2:00PM