Regions
Northwest Coastal.
Christmas surface hoar layer is a present that keeps on giving into the New Year! Conservative terrain selection remains important.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain
Weather Forecast
Monday: strong northerly winds continue / warming trend starting / mix of sun & cloud / dryTuesday: moderate to strong northerly winds / slow warming trend continues / some sun & dryWednesday: light winds / warm temperatures with near zero possible close to treeline elevations / clear & dry.
Avalanche Summary
In recent days, several persistent slab avalanches to size 2 were triggered remotely or under light loads in the Terrace backcountry on a layer of surface hoar buried 30-60 cm below the surface. Many of these avalanches failed in low angle terrain. Check out the latest Mountain Information Network posts as well as the ACMG Mountain Conditions Report (mountainconditions.com) for some great reports of related avalanche activity. Information from Dec 31 shows this touchy problem remains and is likely to continue. Strong Northerly winds and rapid reverse loading led to a natural avalanche cycle on Dec 30 near Ningunsaw. I suspect this may illustrate a region wide wind slab problem.
Snowpack Summary
Recent storms deposited 50-80 cm of snow combined with strong winds, most recently from the north. The storm snow overlies a layer of feathery surface hoar (up to 15 mm in sheltered areas) buried on December 25, making wide propagations possible. Ongoing reports indicate this combination of layers is susceptible to 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.