Avalanche Forecast

Issued: Apr 16th, 2013 10:05AM

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.

pgoddard, Avalanche Canada

Purcells.

This bulletin was produced using limited data. Significant variations in snowpack structure are likely to exist. If you've been traveling in the backcountry, we'd love to hear from you at forecaster@avalanche.ca.

Summary

Confidence

Fair - Timing, track, or intensity of incoming weather is uncertain on Thursday

Weather Forecast

Wednesday: Light snow. Light W winds. Freezing level near 1600 m.Thursday: Trace amounts of snow possible. Light W winds. Freezing level near 1800 m.Friday: Â Light snow. Light W winds. Freezing level 1900 m.

Avalanche Summary

Explosives triggered several size 2.5-3 slabs on northerly aspects above 2600 m on Monday. The largest failed a metre deep. On Sunday, a naturally triggered size 2.5 slab failed on a buried crust. Numerous small loose dry avalanches were also observed.Â In neighbouring Glacier National Park, skiers triggered a size 3 slab on a buried crust on an east aspect in the alpine on Monday. The slab above this interface may be reaching tipping point with incremental loading from snow and wind.

Snowpack Summary

Up to 40 cm recent storm snow has been shifted by variable winds into wind slabs at treeline and above.A weak interface down about 60-120 cm consists of a crust on solar aspects, and surface hoar on high-elevation northerly aspects. Cornices in the region are very large and potentially destructive. Periods of warming or solar radiation may increase the reactivity of cornices and storm slabs.

Problems

Wind Slabs

Wind slabs exist on a variety of aspects at treeline and above. Wind has also formed very large and potentially unstable cornices.

Be cautious as you transition into wind affected terrain.>Travel on ridgetops to avoid wind slabs on slopes below.>Avoid steep slopes below cornices.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 4

Persistent Slabs

A weak interface buried in the upper metre of the snowpack can create surprisingly large avalanches. It's been reactive in nearby regions, but there's limited information about its reactivity in the Purcells. Be cautious in alpine terrain.

Choose well supported terrain without convexities.>Be aware of thin areas that may propagate to deeper instabilites.>

Aspects: All aspects.

Elevations: Alpine.

Likelihood

Unlikely - Possible

Expected Size

1 - 6

Valid until: Apr 17th, 2013 2:00PM