Regions
Northwest Coastal.
Snowfall accumulations combined with moderate to strong winds will raise avalanche danger in the coming days. There is uncertainty with forecast snowfall amounts. Avalanche danger may be higher than indicated if amounts exceed estimates on Monday.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Monday
Weather Forecast
MONDAY: Periods of snow, accumulations 15-25cm overnight Sunday through Monday / Moderate to strong southeast wind / Alpine temperature -4 TUESDAY: Periods of snow, accumulation 15-20cm / Moderate southwest wind / Alpine temperature -3 WEDNESDAY: Flurries / Light southwest wind / Alpine temperature -2
Avalanche Summary
On Saturday a size 2 remotely (from a distance) triggered persistent slab avalanche was reported to have run on a northeast aspect in the alpine on buried layer of surface hoar.Reports from the Shames area on Thursday detailed "touchy" conditions with natural, remote and skier triggered avalanches up to Size 2 (including an involvement of a canine member of one group), between 1000-1400 m, on south to west aspects where the mid-January crust is quite prevalent. While explosive control work on easterly aspects between 1100-1330 m, produced several storm slab avalanches up to Size 1.5 which also failed on the recently buried, mid-January crust layer.
Snowpack Summary
Approximately 50cm of recent storm snow covers a widespread crust as well as surface hoar in isolated areas at mid elevations that were buried mid-January. Surface hoar is more likely to exist in sheltered areas at treeline and below with crust elsewhere. This interface has produced easy, sudden results in recent snowpack tests in the Shames area and also recent avalanche activity at upper treeline and alpine elevations.Professionals have also been monitoring a few mid-pack layers within the snowpack including a crust/ surface hoar layer that was buried early-January and now lies 60-100 cm below the surface, and a similar layer buried mid-December that now lies 80-120 cm below the surface. Both of these layers have been reactive in recent snowpack tests producing moderate to hard, sudden results and have also produced recent large, natural avalanches in northern parts of the region near Bear Pass.The lower snowpack is generally strong, with the exception of areas around Stewart and further north where a basal crust and facets exist.
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.