Regions
Kootenay Boundary.
The snowpack is complex with buried weak layers being overloaded by recent storm snow. Travel in avalanche terrain is not recommended.
Confidence
Moderate - Forecast snowfall amounts are uncertain
Weather Forecast
Thursday: New snow 10-25 cm by early afternoon. Ridgetop winds strong from the southwest. Alpine temperatures near -5 with freezing levels 1600 m. Friday: Mix of sun and cloud with alpine temperatures near -10 and freezing levels at valley bottom. Ridgetop winds light from the north.Saturday: Mostly clear with no forecast snow. Alpine temperatures near -10 and ridgetop winds light from the northeast.
Avalanche Summary
On Tuesday, a couple of natural wind slab avalanches up to size 2.5 from east aspects above 2000 m were reported. Avalanche control, using explosives also triggered numerous persistent slab avalanches up to size 2.5. These were mostly from N-SW aspects above 1900 m. One of these large avalanches stepped down to ground in some places on a early season November crust. With forecast snow and strong winds expect large natural avalanche activity to continue through the forecast period.
Snowpack Summary
Up to 20 cm of new storm snow accompanied by strong winds adds stress to the upper snowpack. This now brings 90-140 cm of settled snow over two surface hoar/ crust layers that were formed early to mid-January. Digging deeper 160-200 cm sits the mid-December surface hoar layer. All of these persistent weak layers lurk within the snowpack and continue to be reactive, producing large and destructive avalanches. Near the base of the snowpack exists a crust/ facet interface which could be triggered by a step down avalanche, cornice failure, thin to thick spot triggering by people or reaching its threshold with new load from above. The snowpack is extremely complex and requires respect and diligence at this time.
Problems
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.
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.