Avalanche danger is expected to rise as new snow overloads a weak snowpack.
Confidence
Fair - Forecast snowfall amounts are uncertain on Friday
Weather Forecast
Snowfall is expected to become more intense overnight and into Friday (up to 40cm 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 before a second pulse of snow arrives late on Sunday.
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.
Snowpack Summary
Snow and wind forecast for Friday will add stress to a snowpack that varies across the region, but is volatile in some places. New snow is expected to hide wind slabs which formed recently in response to strong northerly winds. Storm slabs or wind slabs may overload the persistent weak layer of mid-December surface hoar. This layer is buried under a consolidated slab, anywhere from 15-80 cm deep. Below 2100 m, the buried surface hoar sits on a thick, solid crust and has been acting as a perfect sliding layer. In the worst affected areas, this layer sports well preserved 10-50 mm crystals, especially within a few hundred metres of treeline. 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.