Regions
Kootenay Boundary.
Wind slabs are the main concern for Saturday. Watch for signs of reverse loading as the wind switches direction. West of Castlegar and north of Nelson, a spotty layer of surface hoar has recently been reactive and extra caution may be required.
Confidence
Moderate - Due to the number of field observations on Friday
Weather Forecast
Sunny conditions are expected on Saturday morning and cloudy conditions are expected in the afternoon. Alpine winds are forecast to be moderate to strong from the northwest and treeline temperatures are expected to be around -12C. Light snowfall is forecast to begin late Saturday afternoon or early evening. 3-6 cm of snowfall is expected between Saturday evening and Sunday afternoon. Alpine wind on Sunday is forecast to be moderate from the northeast and treeline temperatures are expected to be around -15C. A mix of sun and cloud is currently forecast for Monday with moderate outflow winds continuing and treeline temperatures falling to around -20C.
Avalanche Summary
On Thursday, ski cutting produced several size 1 wind slab avalanches on north aspects at around 2000 m elevation. These were 5-15 cm thick slabs. A skier also triggered a size 1.5 storm slab avalanche on a northeast aspect at 1850 m which was 15-25 cm thick. Three persistent slab avalanche were reported this past week in the mountains west of Trail and Castlegar, all of which released on the mid-December surface hoar. On Wednesday, a ski cut produced a size 1 persistent slab on a south aspect at 2100 m which was 40 cm thick. On Tuesday, a skier triggered a size 1.5 slab on a northeast aspect at 2100 m with a thickness of 40-60 cm. On Monday, a skier triggered a slab on this layer on a northeast aspect. Extra caution is still recommended in this area in places where surface hoar may still be preserved.On Saturday, wind slabs remain the main concern. Strong southwest winds and light snowfall during the week created wind slabs which may still be lingering and reactive. Winds have now switched to the northwest on Friday and by Saturday evening will be from the northeast. As a result, wind slabs should be expected on all aspects in exposed terrain.
Snowpack Summary
30-60 cm of snow has accumulated since Monday. The new snow sits over a variable interface which may consist of wind scoured surfaces or old wind slabs in exposed terrain, surface hoar up to 5 mm in sheltered areas and/or a freezing rain crust. Recent test results suggest the layer is gaining strength but may still be reactive in isolated areas. Recent strong winds have been from a variety of directions and have been loading leeward and cross loaded features in wind exposed terrain. The mid-December interface is now down 40-100 cm. This layer appears to be well bonded in many parts of the region but has recently been reactive in the mountains west of Trail and Castlegar where the interface is down around 40 cm. It appears that preserved surface hoar is creating an isolated problems for this area and three skier trigger avalanches have released on this layer between Monday and Wednesday. On Thursday, the interface was also reported to be reactive to snowpack tests in the Valhallas and the mountains north of Nelson. It will be important to continue to track how this layer evolves during the upcoming period of high pressure.
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.