Regions
Kootenay Boundary.
If winds are strong and new snowfall amounts exceed 20cm in your area, Saturday`s Avalanche Danger may be HIGH in alpine.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Friday
Weather Forecast
A brief pulse will bring 15-25cm of new snow to the region on Friday night. Expect gradual clearing throughout the day on Saturday and mainly clear skies on Sunday. On Sunday night the region may see up to 5cm of new snow, but more clearing expected on Monday. Ridgetop winds will be generally moderate from the southwest with Friday and Sunday nights` systems; otherwise, they will remain mainly light. Freezing levels should hover between 1300 and 1500m for the forecast period.
Avalanche Summary
On Thursday, numerous naturally triggered storm slab avalanches were observed in the size 1.5-2 range. They failed in response to heavy storm loading on Wednesday night and Thursday. Throughout the week, a few skier triggered size 1.5-2 avalanches were also reported. A few of these avalanches failed on the February crust interface mentioned in the Snowpack Discussion section. Of note, a size 2 slab avalanche northwest of Kaslo (in the South Columbia region) partially buried 2 people and completely buried another person. The avalanche, which failed on the early February crust interface, occurred on an east aspect at 1700m. Nobody was injured in the avalanche, but it points to the touchy conditions which also extend south into some parts of the Kootenay Boundary region. New snow and wind on Friday night will deliver its own mix of wind slab activity, but storm loading will also add to the likelihood and consequences of avalanches failing on the buried early February crust.
Snowpack Summary
On Friday night, 15-20cm of new snow is expected to fall and overlie settling storm snow from the past few days. Moderate southwest winds are forecast to shift these accumulations into deeper and more reactive wind slabs in exposed lee terrain. 60-100 cm below the surface you'll find a hard melt-freeze crust which formed during the 2nd week of February. This crust is quite widespread and may co-exist with weak surface hoar or facets. This variably reactive interface is the critical layer to watch in the region as it has caught a few people by surprise in recent days. A few buried surface hoar layers exist in the top 100 cm in drier areas on the west side of the region near Rossland. In other areas the notable early January surface hoar layer is 100-170 cm deep and poses a low probability/ high consequence threat. It should remain on your radar, especially in thin snowpack areas and during periods of significant warming or rapid loading.
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.