A storm will bring high avalanche danger to the South Columbia.
Weather Forecast
Thursday: 15-20 cm new snow expected with the heaviest amounts likely to be in the south of the region. Strong southwest winds up to 80 km/h expected at ridgetop. Freezing levels expected to rise to 1800 m near noon. Friday: mostly dry with some lingering flurries. Some clearing expected. Winds becoming light southeasterly. Freezing level around 1700 m. Saturday: 5-10 cm new snow, with moderate southwesterly winds and freezing level remaining around 1700 m.
Avalanche Summary
On Tuesday, several slab avalanches were noted on solar aspects in recent storm snow, including one very large size 3 that started on a hanging convex feature at 2400 m and ran all the way to valley bottom. The previous two days before that (Sunday and Monday) had seen an avalanche cycle mostly on northwest to northeast aspects above 1800 m with sizes averaging size 2 and crown depths averaging 50 cm. Most of these avalanches appeared to be initiating in a layer of surface hoar buried on Feb-27. On Sunday, there was a fatal avalanche near Sicamous that occurred on a north aspect at 2025 m that most likely ran on the Feb-27 surface hoar layer.
Snowpack Summary
A surface crust that formed on all aspects up to at least 1700 m is expected to get buried by new snow. In the Monashees, you may find an additional thin crust (from Mar-5) in the top 40 cm of snow that was reactive up to size 2 on Sunday. Approximately 70 cm below the surface lies a variable interface that comprises surface hoar, facets, a crust, or some combination of all three. This layer was buried on or around Feb-27 and has been very active since the weekend. I anticipate this layer will continue to be reactive to sled or skier traffic and/or additional loading from new snow. A previous surface hoar/crust layer buried Feb-10 is now down over a metre. This layer has become less likely to trigger, but continues to be discussed as a possible failure plane by commercial operations. It's presence keeps open the possibility of very large avalanches, as a small surface slide could step down into this deeper layer.
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.