Avalanche Forecast

Issued: Dec 21st, 2018 5:25PM

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

shorton, Avalanche Canada

Purcells.

Persistent slabs continue to be a concern and require conservative terrain choices like avoiding steep slopes, convex rolls, and wind loaded terrain.

Summary

Confidence

High.

Weather Forecast

FRIDAY NIGHT: Mostly clear skies, moderate wind from the northwest, alpine temperatures drop to -16Â°c. SATURDAY: Clear in the morning then increasing cloud throughout the day, light wind from the south, alpine high temperatures around -10Â°c. SUNDAY: Isolated flurries with 2-4 cm of snow, moderate wind from the southwest, alpine high temperatures around -8Â°c. MONDAY: Mostly cloudy, light wind from the west, alpine high temperatures around -8Â°c.

Avalanche Summary

Evidence of a natural avalanche cycle was reported on Friday in the wake of Thursday's storm. The cycle involved large storm slab and persistent slab avalanches (size 2-3), primarily on north and east aspects. One very large natural avalanche (size 3.5) near Golden involved the full depth of the snowpack. Storm slabs were also very reactive to explosives. Extensive avalanche activity has occurred in the Purcells since December 12. On Wednesday, a large (size 2.5) persistent slab avalanche was triggered in the Golden area by a skier breaking a cornice onto a northeast-facing slope. Over the weekend, a small sluff triggered by a skier stepped down to a deeply buried weak layer (likely basal facets). Another very notable avalanche occurred in the Golden area on Saturday when a size 2.5 avalanche was remotely triggered from 50-100 m away on a northwest aspect at 2250 m. Persistent slab avalanches are likely to continue into the weekend given the weak structure of the snowpack.

Snowpack Summary

Thursday's storm delivered 10-20 cm of new snow and strong winds promoted widespread slab formation. Approximately 50-100 cm of recent snow sits on a weak layer of facets (sugary snow), surface hoar (feathery crystals), and a sun crust (on south facing slopes). Another layer of surface hoar and sun crust is now buried 80-150 cm. The potential may exist for smaller avalanches to step down to this deeper layer, resulting in very large avalanches. The most likely areas for this layer to be a problem is where the surface hoar sits on the sun crust, which is most likely on steep south facing terrain at treeline.At the base of the snowpack is a crust that formed in late October. This layer is likely only a problem on large, steep alpine features with a shallow snowpack. It would likely take a large trigger, such as a cornice fall, to produce an avalanche on this layer.

Problems

Persistent Slabs

Persistent weak layers have become very sensitive under the weight of the new snow and have produced numerous large avalanches. It's critical to give these layers more time to heal.

Pay attention to overhead hazards like cornices which easily trigger persistent slabs.Use conservative route selection, choose moderate angled and supported terrain with low consequence.Avoid convexities or areas with a thin or variable snowpack.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

2 - 3

Wind Slabs

20-40 cm of snow from the most recent storm will be reactive, especially on steep and wind loaded terrain features.

The new snow will require several days to settle and stabilize.Use ridges or ribs to avoid pockets of wind loaded snow.If triggered the wind slabs may step down to deeper layers resulting in large avalanches.

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

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 2

Valid until: Dec 22nd, 2018 2:00PM