Avalanche Forecast

Issued: Apr 19th, 2013 10:29AM

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

swerner, Avalanche Canada

Purcells.

Summary

Confidence

Fair - Due to limited field observations

Weather Forecast

An upper trough will move through the Interior tomorrow, while the NW flow will bring cooler, dryer air and decent pulses of precipitation.Saturday: Overcast. Light-moderate precipitation amounts. Ridgetop winds blowing moderate from the NW. Freezing levels near 1600 m.Sunday/Monday: A ridge of high pressure will bring a mix of sun and cloud, with cooling and dryer conditions. Freezing levels will be near 1000 m during the afternoon and falling to valley bottom overnight.

Avalanche Summary

No new avalanche activity has been reported.

Snowpack Summary

Up to 10 cm of new snow overlies a variety of old snow surfaces. These consist of melt-freeze crusts, buried surface hoar and surface facets. Touchy wind slabs exist on lee slopes and behind terrain features. Cornices on ridgelines have grown large, and pose a threat to slopes below.Deeper in the snowpack a weak interface buried in early April is down about 60-120 cm and consists of a crust on solar aspects, and surface hoar on high-elevation northerly aspects. Earlier this week, very large avalanches were reactive on this interface in the neighboring Glacier National Park. The bond may be getting stronger, but I would still use caution and be suspicious of large, steep upper elevation slopes.

Problems

Wind Slabs

Wind slabs exist on a variety of aspects at treeline and above. Cornices have grown very large, so be aware of overhead hazards and stay back on ridgelines.

Avoid lee and cross-loaded terrain near ridge crests.>Watch for whumpfing, hollow sounds, shooting cracks or recent avalanches.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

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, and remains a concern with professionals. Be cautious in steep, alpine terrain.

Be aware of thin areas that may propagate to deeper instabilites.>Dig down to find and test weak layers before committing to a line.>

Aspects: All aspects.

Elevations: Alpine.

Likelihood

Unlikely - Possible

Expected Size

1 - 6

Valid until: Apr 20th, 2013 2:00PM