Avalanche Forecast

Issued: Dec 24th, 2015 8:38AM

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

rbuhler, Avalanche Canada

Purcells.

In some areas, wind slabs are still reactive to the weight of skiers and riders. Old weak layers may still be reactive in isolated areas. Continually assess conditions as you travel.

Summary

Confidence

Moderate

Weather Forecast

A ridge of high pressure should bring mostly sunny conditions for Christmas day. Temperatures at treeline should be around -15C and alpine winds are expected to increase to moderate from the NW in the afternoon. Mostly sunny conditions are expected for Saturday with moderate NW winds and treeline temperatures around -10C. A weak storm system is expected for Sunday but models are currently showing only a couple centimeters of new snow.

Avalanche Summary

Evidence of a few natural wind slab avalanches to size 2 was observed in lee alpine terrain on Monday and Tuesday. The avalanches occurred in response to new snow and wind on the weekend. On Wednesday, two skier accidental avalanches were reported in the Golden backcountry. The first occurred in the morning and was a size 2 wind slab avalanche which occurred on a northwest aspect at about 2400m. It was up to 50cm in depth and ran approximately 600m. The second occurred in the afternoon and was a size 2.5 wind slab at ridgetop. This also occurred on a NW aspect at 2400m elevation. The avalanche stepped down to a deeper layer lower down in the start zone, likely the early December interface.Â On Thursday, a large settlement was felt in a flat meadow in the Invermere backcountry.Â This occurred at 2200m in a below treeline terrain feature and the failure layer is unknown.

Snowpack Summary

Throughout last weekend there was up to 20cm of new snow. Strong southwesterly winds have redistributed much of this snow into wind slabs in exposed lee features in the alpine and at treeline. With the current cold weather, I'd suspect continued faceting in the snowpack. Areas most affected would include windward or cross-loaded rocky areas at higher elevations. Between 40 and 80cm below the surface you'll likely find a layer of weak buried surface hoar which is most prominent between 1400m and 1800m although it may extend to higher elevations in some parts of the region. Snow pit tests suggest that human triggering of this interface is becoming unlikely, although a release at this interface could be destructive in nature.

Problems

Wind Slabs

A couple skier-triggered wind slabs were triggered on Wednesday. This is a good reminder that recently formed wind slabs should remain on our radar and may still be sensitive to human triggering.

Avoid lee and cross-loaded terrain near ridge crests.>Be cautious as you transition into wind affected terrain.>Stay off recent wind loaded areas until the slope has had a chance to stabilize.>

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

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 3

Persistent Slabs

A layer of buried surface hoar can be found between 1400m and 1800m. A weak crust/facet/surface hoar layer is expected at higher elevations. Smaller avalanches may step down to these layers.

Avoid steep open slopes and convex rolls below treeline where buried surface hoar may be preserved.>Conditions are greatly improved, but be mindful that persistent weak layers are still present.>Avoid steep convexities or areas with a thin or variable snowpack.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Unlikely - Possible

Expected Size

2 - 4

Valid until: Dec 25th, 2015 2:00PM