Weak surface hoar is preserved under our new snow in many areas, so expect storm slabs to take significantly longer to heal than usual. With a bout of north wind and sunny skies in the forecast, south aspects are a special concern for Saturday.
A persistent weak layer 50-100 cm deep continues to show reactivity in snowpack tests, despite not producing avalanches in the past week. This surface hoar/crust interface may remain human triggerable in steep convex features at treeline and potentially steep alpine features where it presents as a crust.
20-40 cm of new snow and strong westerly winds have formed storm slabs that are expected to remain reactive to human triggers on Saturday. Wind loaded and sun-exposed south-facing slopes will be especially suspect on Saturday.
FRIDAY NIGHT: Clearing. Light to moderate northwest winds.
SATURDAY: Mainly sunny. Light to moderate northwest winds, shifting west and increasing over the day and overnight. Alpine high temperatures around -10.
SUNDAY: A mix of sun and cloud. Moderate to strong winds, southwest at lower elevations, possibly from northwest at ridgetop. Alpine high temperatures around -5 with freezing levels to 1300 metres.
MONDAY: Cloudy with flurries continuing from the overnight period bringing 10-15 cm total of new snow. Strong southwest winds, possibly extreme at ridgetop. Alpine high temperatures around -4 with freezing levels to 1500 metres.
Reports from Thursday showed ski cutting and explosives control in the Nelson area yielding 20-40 cm-deep size 1.5 (small) storm slabs on north through west slopes above 1900 metres. Impressive sensitivity to triggering and slab propagation were noted in these reports.
We don't yet have observations from Friday, but another 10-25 cm of overnight snow, coupled with elevated and shifting winds likely maintained active avalanche conditions through the day.
Looking forward, conditions aren't expected to improve greatly before Saturday, when clear skies and solar exposure add another destabilizing ingredient to our avalanche problem mix.
Another 10-30 cm of new snow over Thursday night (with Kootenay Pass as the hotspot) and elevated winds (shifting from south to north) have formed fresh storm slabs that will likely remain reactive to human triggers on Saturday.
Collectively 20-40 cm of new and recent snow now overlies a layer of weak surface hoar at treeline and below. Reports from Kootenay Pass show an especially problematic structure with this surface hoar sitting on a crust on solar aspects. Storm slabs will be more sensitive to human triggering in areas where this weak layer remains preserved - especially where a combo of surface hoar and crust exists.
The main persistent layer of concern below our storm snow interface is a combination of surface hoar, facet, and crust layers that formed in January that is now buried 60-150 cm deep. Steep, open slopes at treeline elevations are the most likely spots to trigger a large avalanche on this layer.
