The consequence of triggering a large persistent slab avalanche warrants a cautious approach to steep open terrain.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
SATURDAY: Cloudy with sunny periods, light winds, alpine temperatures around -5 C.SUNDAY: Cloudy, light to moderate southeast winds, alpine temperatures around -7 C.MONDAY: Cloudy with isolated flurries, light to moderate east winds, alpine temperatures around -10 C.
Avalanche Summary
No new avalanches were reported on Friday. On Thursday, avalanche reports were limited to a size 1 skier-triggered wind slab on a steep convex below treeline feature.Wind slabs may remain possible to trigger in isolated locations over the weekend. The main concern continues to be the possibility of triggering the weak faceted layers deeper in the snowpack, although uncertainty revolves around what type of loading or weak spots are required for human triggering.
Snowpack Summary
Recent flurries and winds have formed hard wind slabs on lee features in exposed terrain, while warm temperatures have moistened the upper snowpack below treeline. The snowpack is quite variable throughout the region. In deeper snowpack areas, the snowpack appears to be well settled with isolated concerns about the mid-December facet layer buried 50 to 100 cm deep. In shallow snowpack areas and lower elevations, the snowpack is heavily faceted. Triggering a persistent slab in thinner snowpack area may result in a full depth avalanche.
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.