The new storm in the forecast is expected to drive avalanche danger up to HIGH over the next few days.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Mostly cool (alpine temperatures around -14) with flurries overnight and light snow (3-5 cm) on Sunday. On Sunday night a moist Southwest flow is expected to collide with the cold arctic air that has descended from the North, making it difficult to forecast snowfall amounts. Expect 10-20 cm of light dry snow combined with light to moderate Southwest winds by Monday morning in most areas, with enhanced snowfall amounts expected for the Western upslope areas of the Monashees. Snow (10-15 cm) is expected to continue on Monday as the Southwest winds increase to strong and the alpine temperatures rise to about -6. Another 15-20 cm of snow combined with strong Westerly winds is forecast by Tuesday morning. Snow is expected to continue during the day on Tuesday and the alpine temperature should rise up to about -3.
Avalanche Summary
Persistent slabs continue to be triggered by skiers, vehicles and remote triggers (from a distance away). These have been up to size 2.5 and failing 50-70 cm down on the mid-December crust/surface hoar layer. Many of these have been around treeline elevation, although some smaller sized avalanches were triggered as low as 1750 m, and one natural avalanche size 2.0 was reported from the Selkirks at 2500 metres. Several size 1-1.5 wind slabs also failed naturally or with skier triggers following strong northerly winds.
Snowpack Summary
10-20 cm of new snow has fallen in the past few days combined with moderate winds. Winds were sufficiently strong to set up new wind slabs in exposed areas mostly on east-facing aspects, but previous wind slabs on south-facing aspects remain a concern and may now be buried by a small amount of new snow, making them harder to spot. Storm slabs or wind slabs may be easy to trigger, and could step down to a persistent weak layer from mid-December that comprises of a crust with well-preserved surface hoar crystals sitting above it. It is most prevalent between 1700 m and 2200 m and has been the weak layer responsible for a large number of natural and human-triggered avalanches over the last several days. A hard rain crust with facets from early November is buried more than a metre down and is currently unreactive.
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.