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Archived

Avalanche Forecast

Dec 18th, 2014–Dec 19th, 2014
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Purcells.

Conditions will change rapidly as we enter the weekend. Watch for sluffing from steep terrain at all elevations and for signs of early wind slab development in the alpine.

Confidence

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

Weather Forecast

The first wave of precipitation is expected to reach the interior late Thursday night or early Friday morning. 5-15cm of new snow is expected for Friday. Freezing levels are expected to be 1200-1500m and alpine winds moderate to strong from the SW. A weak ridge will keep Friday night and Saturday morning mainly dry before the next storm front arrives on Saturday afternoon or evening. Freezing levels on Saturday will rise to around 1500m or so and alpine winds will increase to strong from the SW. Saturday night and Sunday should yield another 15-30cm of snowfall up high. Freezing levels on Sunday may rise as high as 2000m and winds will remain strong.

Avalanche Summary

On Thursday, explosive control at a ski area produce several size 1 slabs on steep NE slopes in the alpine.  These slabs were around 20cm thick and were snow that reloaded slopes that had previously avalanched.  The bed surface was the early November rain crust/facets which remain an active concern in the region.

Snowpack Summary

The new snow sits on a layer of large surface hoar. Up to around 2200m elevation this surface hoar sits on a hard rain crust. Above 2200m the surface hoar sits on well settled and faceted snow. As wind increases with the progression of the storm, wind slabs are expected to form in leeward features. A thick rain crust with facets from early November is buried over 1 m down and may still be reactive in isolated areas. A weak layer from the end of November sits in the middle of the snowpack and may still be reactive in some areas.

Avalanche Problems

Loose Dry

The new snow will form a poor bond with the crust/surface hoar interface and fast sluffing is expected from steep terrain. Watch for early stages of wind slab development in the alpine.
Be cautious of sluffing in steep terrain.>Avoid exposure to terrain traps where the consequences of a small avalanche could be serious.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Possible - Likely

Expected Size: 1 - 2

Deep Persistent Slabs

Two weak layers remain a concern: an old rain crust near the bottom and a facet layer in the middle of the snowpack. Triggering these layers is unlikely but the consequences remain very high. Be wary of slopes that did not release during the storms.
Avoid common trigger zones including thin snowpack areas, near rocky outcrops, and steep alpine slopes.>Remote triggering is a concern, watch out for adjacent slopes.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Unlikely - Possible

Expected Size: 2 - 5