Purcells

Fair - Timing of incoming weather is uncertain on Friday

A low pressure system is moving inland from the coast on Friday. This system is forecast to bring snow and moderate southwest wind to the Columbia Mountains by early morning and to the Kootenay Boundary region by the afternoon. Snowfall amounts are forecast to be about 10 cm for the Purcells and 10-15 cm for the Kootenay Boundary on Friday. Another 10 cm for the Purcells and another 10-15 cm for the Kootenay Boundary are expected overnight and during the day on Saturday. Sunday is forecast to be a mixed bag of convective flurries and some sunny periods. The freezing level is expected to rise to about 700 metres on Friday, drop back to valley bottom overnight and then rise to near 1000 metres on Saturday.

The wind and new snow combined to overload the buried persistent weak layer (PWL) on Wednesday. Widespread avalanches were reported from may operators in the region. Some large avalanches up to size 3.5 were reported from the Bugaboo Range. Some avalanches released in very low angle terrain in cut blocks below treeline. A couple of slides released down to the basal facet layer near the ground. Natural avalanches are expected to be less frequent on Friday, but human triggering is expected to continue to be very likely from very light additional loads. Traveling near avalanche terrain may cause remotely triggered avalanches. Please look at the Special Avalanche Warning that was issued today.

Strong gusty wind has transported snow into a thick slab in the alpine and at treeline. The upper snowpack structure is very complex. There are buried layers of surface hoar, buried melt-freeze crusts, and some buried crusts with facets. These weak sliding layers are buried anywhere from 30 - 70 cm by several different storm layers. There are some shears in the storm layers on decomposed and fragmented snow crystals. As the snow above the surface hoar layers settles into a cohesive slab, we are seeing easier and more sudden shears that are a bit deeper. The surface hoar is more likely to produce wider propagations, and lower angle fractures in areas where it is associated with a crust. The crust has been reported to be 2-3 cm thick in some areas. The mid-pack is generally well settled. Basal facets have not been reactive, but operators continue to monitor this layer in tests. Triggering this deep persistent weak layer is unlikely, but shallow snowpack areas or shallow weak areas adjacent to deeper wind loaded slopes are the most suspect locations.