Avalanche danger is expected to rise as new snow adds stress to a touchy and weak snowpack structure.
Confidence
Fair - Intensity of incoming weather systems is uncertain on Thursday
Weather Forecast
Snow is expected to begin on Thursday and become more intense overnight and into Friday (up to 30cm by Friday afternoon). Winds are forecast to become moderate to strong from the SW to NW. Freezing level at valley bottom. Snowfall becomes light on Saturday.
Avalanche Summary
Natural avalanches have been reported up to size 2 where the recent storm snow has been transported into a wind slab. Some of these avalanches have released on or stepped down to the mid-December persistent weak layer of surface hoar and crust. Human triggering continues to be likely to very likely due to the storm slab sitting on a hard sliding surface with a weak layer of surface hoar at the interface.
Snowpack Summary
Snow and wind forecast for Thursday and Friday will add stress to an already volatile snowpack. Storm slabs are expected to hide extensive recent wind slabs which formed at all elevations in response to strong northerly winds. Storm slabs or wind slabs may be easy to trigger, and may step down to the persistent weak layer of mid-December surface hoar. The touchy mid-December surface hoar layer is buried under a consolidated slab, up to a metre down. Below 2100 m this slab sits on a thick, solid crust/ surface hoar combination and has been acting as a perfect sliding layer. A hard rain crust with facets from early November is buried more than a metre down and is currently unreactive. However, triggering from shallow rocky and unsupported terrain remains a concern.
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.
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.