Regions
Kootenay Boundary.
Progressive storm loading and a buried weak layer have created tricky conditions. Watch for fresh wind slabs on Thursday and use extra caution on south facing slopes when the sun is out. Conservative terrain selection remains critical.
Weather Forecast
A mix of sun and cloud is expected for most of the day on Thursday. Alpine wind is expected to be light to moderate from the southwest and treeline temperatures are forecast to reach around -5C. A storm system is expected to reach the region Thursday evening. 20-30 cm of snow is forecast between Thursday evening and Friday afternoon. Alpine wind is forecast to be strong from the southwest and freezing levels may climb as high as 1800 m on Friday. Saturday is currently forecast to be dry and sunny before the next storm system arrives Saturday night.
Avalanche Summary
On Tuesday, a natural size 1.5 storm slab was observed on a north aspect at 1950 m elevation which was 30 cm thick. Ski cutting triggered a size 1 storm slab on a north aspect at 2050 m which was 15 cm thick. A skier remotely triggered a size 2.5 slab avalanche from 5 m away on a north aspect at 2125 m which failed on the late-February surface hoar down 60 cm. On Monday, skiers and explosives triggered numerous storm slab avalanches up to size 2. Over the weekend, a natural size 3.5 deep persistent slab avalanche occurred in an alpine bowl in the Bonnington range on west and northwest aspects at 2300 m elevation. The crown line was 200+ cm thick and it was described as a climax event.On Thursday, recently formed wind slabs are expected to remain touchy. Large persistent slab avalanches remain a concern with the snow from the past week releasing on the late-February weak layer. It may still be possible for a person to directly trigger a persistent slab or a smaller avalanche could step down.
Snowpack Summary
Another 10-15 cm on Tuesday night brings the typical storm accumulation in the past week to 50-80 cm. This snow has settled into a slab that sits over the late-February interface which includes sun crust, surface hoar, and weak faceted snow. Moderate to strong southwest wind Tuesday night redistributed the most recent storm snow forming new wind slabs in leeward terrain at higher elevations. The thick mid-February crust layer is now down 80-120 cm and exists on solar aspects and at lower elevations. This layer appears to have gone dormant but is still a concern which could wake up in the future with heavy loading or a major warming event. In most deeper snowpack areas, the snowpack is generally well settled and stable below this crust layer. However, some areas have a lingering basal weakness at the bottom of the snowpack. This is especially prevalent in shallow snowpack areas (less than ~150 cm).
Problems
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.
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.