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Avalanche Forecast

Archived

Dec 18th, 2013–Dec 19th, 2013

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

Regions

Purcells.

Confidence

Fair

Weather Forecast

Thursday: Arctic air surges south giving colder and drier weather. Expect a mix of sun and cloud with increasing cloud later in the day. Alpine temperatures should be around -15 to -20. Winds are light to moderate from the northwest-west. Friday: Light snow should begin Thursday night with a few cms expected through Friday. The freezing level is at valley bottom and winds are moderate from the west-southwest. Saturday: Lingering flurries in the morning, then clearing. The freezing level is at valley bottom and winds are light from the northwest.

Avalanche Summary

Several small skier controlled avalanches (size 1) were reported on Tuesday. These appeared to be thin wind slabs in exposed north or east facing terrain.

Snowpack Summary

Small amounts of recent storm snow are beginning to add up on top of the sugary facets from early December. Snowpack depths at tree line currently vary from 100- 130 cm and reports speak of severe scouring on the windward side of ridges, moving snow onto lee sides and creating dense wind slabs. A persistent weakness of buried surface hoar and facets is down approximately 20-50cm. This layer has been producing variable results with snowpack tests. Recent reports indicate that the new storm/ wind slab is becoming reactive with warming temperatures and additional loads. At the base of the snowpack are weak facets and depth hoar combined with a crust from early October. This deep persistent weakness may be stubborn to trigger, especially in deeper snowpack areas, but the sensitivity to triggers likely increases in shallower locations, especially on steep, convex slopes. Snow depth is below threshold in many areas below treeline.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.