Regions
Kootenay Boundary.
Confidence
Fair - Due to variable snopack conditions
Weather Forecast
The warm front associated with the weekend storm has delivered little precipitation to the Kootenay Boundary as a ridge of high pressure has been "protecting" the region. The ridge slides to the south Sunday night allowing a cold front to impact the area Monday. I expect 5 - 10 cm out of this wave. Winds will switch from the W to NW late in the day Monday with enough oomph to create fresh windslabs. Expect 1500 m temps to be near 0 C. A ridge of high pressure moves in Tuesday which should break up the cloud cover and provide some good visibility.
Avalanche Summary
A skier-triggered avalanche near Nelson was reported from Thursday, running on wind slab over a crust. The avalanche was a size 2 and occurred on a north aspect, 40deg slope at 2125 m. The crown was 20-30 cm deep and the slide ran for 300-350 m. Recent activity on the mid-December surface hoar includes a snowcat remotely triggering a 70cm deep Size 2 slab avalanche from 10m away on a log-haul landing in a cutblock. Recent storm snow overlying a crust is also highly reactive to human triggers with slope-cuts producing numerous loose snow and 10-30cm thick slab avalanches up to Size 1.5. Continued whoomphing reported.
Snowpack Summary
Recent new snow and gusty winds are keeping wind slabs and cornices fresh and weak, and in some places buried old wind slabs are a concern. A lightly buried thin crust can be found as high as 1900m. Compression tests have been producing easy to moderate sudden results on the mid-December surface hoar, down 70-105cm, and propagation tests, whoomphing, and remote triggering suggest that avalanches associated with this persistent slab have a high propensity to propagate over large areas. Other weaknesses within the slab create the potential for step-down avalanches. Basal facets and depth hoar remain a concern in shallow rocky areas.
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.