Touchy new slabs are expected on Monday as the storm progresses and the region continues to deal with a persistent slab problem.
Southern areas of the region may see larger snowfall amounts. If more than 25 cm of new snow is observed, the hazard should be treated as HIGH.
A persistent slab problem formed by a crust down 50-120 cm has created a low likelihood, high consequence scenario that is difficult to forecast. Avoid likely trigger spots such as steep, rocky slopes with a shallow or thin to thick snowpack. Smaller storm slabs or wind slabs have the potential to step down to a deeper weak layer.
New slab formation is expected on Monday with the incoming storm system. These new slabs may overlie a weak surface and could become touchy very quickly. In the south of the region, the slabs will be more widespread like a storm slab problem. In the north and east, the slabs will be in more specific wind loaded terrain like a wind slab problem.
A storm system is forecast to bring substantial snowfall to the south of the region on Sunday night and Monday with strong winds.
Sunday night: Snowfall 10-15 cm in the south, 5-10 cm in the north, moderate to strong SW wind, treeline temperature around -12 °C.
Monday: Snowfall 15-20 cm in the south, 5-10 cm in the north, moderate to strong SW wind, treeline high around -10 °C.
Tuesday: A mix of sun and cloud with a chance of flurries, light to moderate SW wind, treeline high around -10 °C.
Wednesday: A mix of sun and cloud, light to moderate NW wind, treeline high around -12 °C.
On Sunday, ski cutting was triggering size 1 wind slabs. No new avalanches were reported on Friday or Saturday.
There has been consistent reports of persistent slab avalanches on reloaded bed surface crusts from the Invermere area over the past month. On Thursday, a size 1 was triggered remotely by bootpacking on a thin northeast aspect in the alpine. Explosive control work on December 26th produced large avalanches (size 2) on the crust found 40-70 cm deep.
On Monday, new wind slabs are expected to form throughout the region with the incoming storm system and strong winds. In the south of the region, these slabs have the potential to be more widespread. These new slabs will sit on a weak snow surface and may be very reactive. The persistent slab problem remains a serious concern and the weight of the new snow may increase the reactivity of these deeper layers. Smaller storm slab avalanches also have the potential to step down to deeper layers resulting in very large avalanches.
The new storm snow buries a variable snow surface which is likely very weak and includes widespread facets, wind affected surfaces, and/or surface hoar up to 3 mm in sheltered areas. The new snowfall is expected to be accompanied by strong winds and reactive new slabs are expected to be widespread in the south of the region where the most new snowfall is expected. In the north, new slabs may be more isolated to wind loaded terrain.
The most notable layer of concern in the snowpack is a crust that was formed in early December and is now down 60-120 cm. Activity on this layer has been sporadic. It has generally been triggered on steep, rocky slopes, with a shallow or thin-to-thick snowpack. This layer has created a low likelihood, high consequence scenario which is best managed through conservative terrain choices and disciplined backcountry travel techniques.
A late October facet/crust layer sits at the bottom of the snowpack above 1900 m.
