Regions
Northwest Coastal.
There is the possibility that some parts of this region may see enhanced amounts of storm snow on Sunday. If this is the case, you may see local areas of high danger.
Confidence
Moderate - Forecast snowfall amounts are uncertain
Weather Forecast
Sunday: Around 10-15 cm new snow expected, with the freezing level around 1000 m. Winds are expected to be strong to extreme southwesterly, gusting 60-80 km/r at ridgetop. Monday: A short-lived break during the day, possibly with some flurries. Freezing level around 900 m, winds light to moderate southwesterly. Monday night and Tuesday: a storm is expected to start late on Monday, bringing in the region of 10-15 cm Monday night, and another 10 cm or so on Tuesday. Freezing levels around 1000 m, winds strong southwesterly.
Avalanche Summary
Avalanche activity has tapered off since the middle of last week, when warm temperatures and a tricky snowpack resulted in a lot of natural, human-triggered and explosive-triggered avalanches. My feeling is the expected storm snow this weekend will only have a moderate impact, and avalanches will be mostly confined to steep, wind-loaded features. But I don't totally trust the snowpack to be certain that avalanches couldn't happen on a lower weak layer.
Snowpack Summary
Temperatures have been gradually dropping and the freeze/crust line has been dropping as well. Breakable crusts have been reported as low as 1000m in the south of the region. In the Shames backcountry on Thursday, there was a supportive crust above 1300 metres that had developed overnight, on Wednesday there was no crust at 1300 metres ( the highest point the observer reached), and there was 50 cm of recent storm snow, with the top 25 cm being rain soaked. This moist and wet storm snow was sitting on the old crust from the previous warm storm at the end of January. Compression tests resulted in moderate shears within the storm snow and on the old crust. The January 9th surface hoar/facet layer is down 70-200 cm across the region and remains a concern for commercial operations. In the north of the region, we continue to get reports of natural and remotely triggered avalanches on the early January persistent weak layer, large avalanches have released where this layer steps down to weak basal facets. A very conservative approach to mountain travel is still required, due to another warm and wet storm system forecast over the next few days.
Problems
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.