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Avalanche Forecast

Archived

Dec 21st, 2018–Dec 22nd, 2018

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Purcells.

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

Confidence

High.

Weather Forecast

FRIDAY NIGHT: Mostly clear skies, moderate wind from the northwest, alpine temperatures drop to -16°c. SATURDAY: Clear in the morning then increasing cloud throughout the day, light wind from the south, alpine high temperatures around -10°c. SUNDAY: Isolated flurries with 2-4 cm of snow, moderate wind from the southwest, alpine high temperatures around -8°c. MONDAY: Mostly cloudy, light wind from the west, alpine high temperatures around -8°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 Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.