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Avalanche Forecast

Archived

Dec 19th, 2018–Dec 20th, 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 possible, human triggered probable.

Regions

Purcells.

Avalanche hazard will rise with incoming snow and increased winds. In areas that accumulate more than 30 cm snow, bump the avalanche danger to HIGH.

Confidence

Moderate - Intensity of incoming weather systems is uncertain.

Weather Forecast

WEDNESDAY NIGHT: Cloudy with isolated flurries, accumulation up to 5cm. Light to moderate south-southwest wind. Freezing level below 1000m. THURSDAY: Snow, 10-25cm new snow, increasing through the day. Moderate southwest wind with strong to extreme gusts. Freezing level rising to 1600m. FRIDAY: Mix of sun and cloud, isolated flurries. Light west wind with strong gusts. Freezing level valley bottom. SATURDAY: Mix of sun and cloud. Light west wind. Freezing level valley bottom.

Avalanche Summary

Extensive recent avalanche activity has occurred in the Purcells since December 12. Explosives and skier traffic are triggering large (size 2) avalanches, and although natural avalanche activity has started to subside, human-triggered avalanches remain a concern. Over the weekend, a small sluff triggered by a skier stepped down to a deeply buried weak layer (likely basal facets) causing a size 2 avalanche on Sunday. Another very notable avalanche occurred on Saturday when a size 2.5 avalanche was remotely triggered from 50-100m away on a northwest aspect at 2250m.

Snowpack Summary

Approximately 60-90 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). Strong winds have promoted widespread slab formation, particularly with the most recent new snow available for transport. 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

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.

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.