Regions
Northwest Inland.
Stormy conditions have elevated the avalanche danger. Incremental loading above buried persistent weak layers is the recipe for an "avalanche surprise". Conservative terrain selection is required where slabs have formed over buried surface hoar.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
3-5 cm of snow above 700 metres with moderate westerly winds overnight. Flurries or light snow on Saturday with moderate westerly winds and freezing level dropping back to valley bottoms. 5-10 cm of snow with strong southwest winds on Sunday and freezing level at or just above valley bottoms. On Monday, expect strong winds, moderate to heavy precipitation and freezing levels rising rapidly to at least 1500 metres.
Avalanche Summary
No new avalanches reported. On Wednesday there was a report of an accidentally triggered size 1.0 avalanche on a short steep roll below treeline at 1500 metres in the Ashman area. The crown was 30-50 cm deep and 25 metres wide; compression tests on this layer produced easy-moderate sudden shears. Some loose dry avalanches up to size 1.0 were reported from the Hankin area on Tuesday in the alpine on steep east aspects.
Snowpack Summary
Thin new storm slabs are developing on Friday, From the Babines on Thursday, we have a report of a very shallow and weak snowpack that is about 80 cm on average. The persistent weak layers were found down about 15 cm and 30 cm, the deeper layer is suspected to be surface hoar above the December melt-freeze crust. Below this depth there is about 50 cm of weak facetted snow that "barely carries the weight of a sled." There is about 20 cm of recent snow above a new surface hoar layer in the Hankin area that may consollidate into a cohesive slab with forecast warming. Below 1200-1400 m the new snow probably sits on a crust. Fresh soft wind slabs are likely in exposed lee terrain. The early or mid January surface hoar layer is reported throughout the region and is generally 30-50 cm deep. Observers have found this persistent weakness on all aspects and at all elevations. It consistently produces moderate "pops" results in snowpack tests. Below this, the Boxing Day surface hoar problem may also be lurking. The mid and lower snowpack is generally quite weak and faceted, especially in lower snowpack areas.
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.