Regions
Northwest Coastal.
A touchy weak layer has the potential for large avalanches, and is primed for triggering. Conservative terrain selection is critical.
Confidence
Moderate - Intensity of incoming weather systems is uncertain on Friday
Weather Forecast
Friday evening: 10-15cm of new snow / Extreme westerly winds / Alpine temperatures of -3Saturday: Lingering flurries in the morning with clearing in the afternoon / Extreme northerly winds / Alpine temperatures of -10Sunday: Clear skies / Extreme northerly winds / Alpine temperatures of -10Monday: Clear skies / Moderate northerly winds / Alpine temperatures of -3
Avalanche Summary
Persistent slab avalanches to size 2 were reported to be triggered remotely or under light loads in the Terrace backcountry on a layer of surface hoar buried 30-60 cm below the surface. One report, accessible on the MCR website mountainconditions.com, mentions wide propagations and avalanches running in low angled terrain. Check out the great Mountain Information Network posts as well for other detailed reports.
Snowpack Summary
Recent storms have deposited 50-80 cm new snow combined with strong, mainly southwest winds. The storm snow has buried a layer of feathery surface hoar (up to 15 mm in sheltered areas), making wide propagations possible. Recent reports from Shames indicate the slab properties overlying this layer of surface hoar are supporting easily triggered avalanches, including avalanches running in low angled terrain. An earlier (and therefore deeper) weak interface that formed during the early December cold snap can be found in isolated areas buried 100-150 cm deep. The layer consists of preserved surface hoar or weak faceted (sugary) snow. The lower snowpack is well consolidated in deep snowpack areas. In shallow snowpack areas, especially north of Ningunsaw, an old rain crust near the bottom of the snowpack has developed weak facets and might be triggerable from a thin or rocky area on a convex slope.
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.