Avalanche Forecast

Issued: Feb 9th, 2013 8:55AM

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

swerner, Avalanche Canada

Purcells.

Intensity from the sun can make the snowpack weaken and the avalanche danger rise rapidly. Be aware of this and keep travel options open to adjust with this change.

Summary

Confidence

Fair - Timing or intensity of solar radiation is uncertain on Sunday

Weather Forecast

Sunday: As the ridge shifts to the East, the region will remain under a NW flow which will keep alpine temperatures cooler but under a high solar influence. However, some areas may still see high cloud cover through the day. Ridgetop winds will blow Light out of the North. Treeline temperatures near-1 and freezing levels rising to 1400 m.Monday: The flattening ridge will start to retreat to the South allowing a more zonal flow to set up off the coast. Ridgetop winds will switch and blow moderate from the West. Treeline temperatures will be near -2 and freezing levels near 1300 m.Tuesday: A cold front embedded in the zonal flow will move through the region bringing light precipitation amounts accompanied by strong winds from the SW. Treeline temperatures will fall to -6 and freezing levels will be near 1200 m.

Avalanche Summary

Over the past couple of days a few natural slab avalanches occurred on steep solar aspects up to size 2.5. On Friday a skier triggered size 1.5 slab avalanche occurred from a NE aspect at 2300 m and natural loose dry sluffing continues from steep unsupported terrain.

Snowpack Summary

Wind slabs

exist in the alpine and at treeline on lee slopes and behind ridgelines. The recent storm slab continues to settle over old interfaces comprising of surface hoar, crusts and facets down 30-50 cm. However, areas that have buried surface hoar seem to be most touchy. Continued freezing temperatures overnight have helped to strengthen the snowpack at lower elevations. The mid-pack is generally well settled and the average snowpack depth at treeline elevations is near 130. Watch the duration and intensity of the sun in your local riding area; it may weaken the upper snowpack. Cornices have grown and may threaten slopes below.

Problems

Wind Slabs

New wind slabs continue to be reactive to light additional loads like skiers and sledders. Areas of concern are in the immediate lees of slopes and behind terrain features like ridgelines and ribs. Cornices have grown and may threaten slopes below.

Use ridges or ribs to avoid pockets of wind loaded snow.>Highmark or enter your line well below ridge crests to avoid wind loaded pillows.>

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

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 4

Persistent Slabs

The recent storm snow continues to settle into a cohesive slab above weak layers and variable old surfaces that comprise of buried wind slabs, surface hoar, and crusts. These weak layers exist 30-50 cm below the surface. They can be tricky to manage.

Use extra caution on open slopes and convex rolls at and below treeline where buried surface hoar may be preserved.>Dig down to find and test weak layers before committing to a line.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

2 - 5

Valid until: Feb 10th, 2013 2:00PM