Regions
Kootenay Boundary.
Large amounts of recent storm snow may push the danger to higher than forecast. This is a good time to be conservative with route selection.
Confidence
Moderate - Due to the number and quality of field observations
Weather Forecast
A ridge of high pressure is building behind the front that passed through yesterday. Fairly dry conditions are expected for tonight. Another Pacific front will slowly make its way to the South Coast by mid-day Sunday. We may see precipitation in the Kootenay Boundary by late Sunday night into Monday morning. Another Pacific front is scheduled to arrive on the coast on Monday morning, but is not expected to produce much precipitation in the southern interior as a ridge of high pressure builds over the whole province. The interior should remain dry through mid week.
Avalanche Summary
Numerous skier controlled, skier accidental, and naturally triggered storm slab avalanches have been reported in the size1.5,-2 range. These failed in response to heavy storm loading on Wednesday night and Thursday. 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 from Friday night has delivered another round of wind slab activity. The new storm snow loading will add to the likelihood and consequences of avalanches failing on the buried early February crust.
Snowpack Summary
On Friday, up to 35 cm of new snow fell in the region and now overlies settling storm snow from the past few days. Moderate to strong southwest winds are transporting the new snow into deeper and more reactive wind slabs in exposed lee terrain. 90-120 cm below the surface there is 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 and it has caught a few people by surprise in recent days. A few buried surface hoar layers exist in the top 120 cm in drier areas on the west side of the region near Rossland. In other areas the notable early January surface hoar layer is 100-190 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.