Northwest Coastal

Friday's warming and rain will promote slab formation in our low density storm snow and test the weak layer buried below it. Mid elevations are the greatest concern for holding the best preserved surface hoar.

Moderate - Freezing levels are uncertain

Thursday night: Cloudy with scattered flurries bringing a trace to 5 cm of new snow. Light southwest winds. Freezing level rising from 500 metres to about 1000 metres overnight.Friday: Cloudy with wet flurries bringing 5-10 cm of new snow to high elevations. Rain below about 1500 metres, possibly higher. Precipitation continuing overnight. Moderate to strong southwest winds, increasing over the day. Alpine high temperatures around 0 to +1 with freezing levels between 1900-2500 metres. Saturday: Cloudy with continuing wet flurries bringing up to 20 cm of new snow to high elevations. Rain below about 1300 metres with new snow elevations dropping over the day as temperatures cool. Strong to extreme southwest winds. Alpine temperatures dropping from around -1 to -2 as freezing levels decline from 1700 to 1300 metres over the day.Sunday: Sunny. Light northeast winds. Alpine high temperatures around -4 with freezing levels to 1000 metres.

A large (size 2.5) wind slab was triggered remotely (from 20 metres away) just south of the Seven Sisters Provincial Park area on Wednesday. This occurred on a southeast aspect at 1700 metres and is believed to have failed on the roughly 50 cm-deep persistent weak layer described in our snowpack summary. This event highlights increasing slab properties of our storm snow in wind-affected areas as well as the potential for our persistent weak layer to exist in protected areas above 1500 metres.Looking forward, warm temperatures will be another factor promoting slab formation in the new snow while rain adds load to this snow and tests the weak layer buried beneath it. Friday will be a day to back off into simple terrain, stay clear of overhead hazards, and away from avalanche runout zones.

Around 50 cm recent low density snow covers a variable surface of wind-affected and faceted snow, a weak layer of surface hoar in more protected alpine areas, but more predominantly a rain crust/surface hoar combination below 1500 m. Thin breakable crusts have been observed both at the new snow surface as well as within the recent snow. Reports from the Shames area suggest a strengthening bond between the recent snow and the supportive underlying crust now down about 50 cm. Below the interface described above, old wind-affected storm snow is expected to have formed a good bond with the underlying strong mid snowpack, although exceptions may exist in the far north of the region. Here, the old storm snow may overlie a weak layer of surface hoar (feathery crystals). This layer was identified as the failure plane in several large avalanches in the north of the region last week. It has not shown in reports elsewhere.For most of the region, a weak layer of facets (sugary snow) buried in early December exists down 180-250 cm. The likelihood of triggering this layer is generally low, but it could be triggered by humans in shallow snowpack areas, particularly in the north of the region.