There will be gradual accumulation of low density new snow in the coming days.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Thursday
Weather Forecast
THURSDAY: Scattered flurries, accumulation up to 5cm / Wind light west. Alpine temperature -10 FRIDAY: Scattered flurries, accumulation a trace. Wind light southeast. Alpine temperature -10 SATURDAY: Scattered flurries / Wind light northeast. Alpine temperature -9
Avalanche Summary
Recent reports are limited to both natural and human triggered loose dry avalanches in steep terrain to size 1 in areas that have not been affected by wind. Expect continued potential for triggering loose dry avalanches in steep terrain and consider the potential to trigger persistent slabs where last week's storm snow has settled into a cohesive slab.
Snowpack Summary
Last week's storm brought 30-50cm of new snow to the region. Predominantly northwest winds have since redistributed this storm snow into wind slabs in lee terrain at higher elevations while cold temperatures have inhibited slab formation elsewhere. Cold has also worked to decrease the cohesion and reactivity of older wind slabs.The storm snow lies over a variety of old surfaces, including large surface hoar (weak, feather-like crystals), sun crust and sugary facets. The bond at this old snow interface is of critical importance where the overlying storm snow has consolidated into a slab. The most concerning areas are those that saw pronounced surface hoar development before the storm, such as sheltered areas at and below treeline. Recent snowpack tests show wide ranging reactivity at this interface but suggest that it may be most reactive on northerly aspects. A crust which was formed by rain in late November remains a major feature in the snowpack and is down approximately 60-100cm at tree line elevations. This interface shows good signs of bonding in the Purcells, but has indicated some reactivity in steep, variably loaded alpine features in adjacent regions.
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.