Regions
Northwest Coastal.
Storm snow and wind are building reactive slabs particulary on leeward slopes at upper elevations, while buried weak layers have produced recent avalanches at lower elevations.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
TONIGHT: Light flurries. Accumulation trace. Ridge wind light, southeast. Temperature -8. Freezing level valley bottom.THURSDAY: Mostly cloudy, light flurries. Accumulation trace. Ridge wind light, southeast. Temperature -5. Freezing level 300 m. FRIDAY: Mostly cloudy, flurries. Accumulation 2-6 cm. Ridge wind light to moderate, south. Temperature -6. Freezing level valley bottom.SATURDAY: Mix of sun and cloud. Ridge wind light, east. Temperature -9. Freezing level valley bottom.
Avalanche Summary
On Tuesday there were reports of natural storm slab avalanches up to size 2 failing on mid-January interface and ski cut results producing storm slab avalanches to size 1.5 near ridge crest. In the Bear Pass area numerous natural wind slab failures from Size 1-2.5 were observed on wind-loaded northerly features as well as Size 1.5-2.5 naturals that ran on the mid-December crust layer between 600-1200 m.Monday, areas north of Stewart reported numerous natural loose storm snow avalanches on most aspects, and one skier remote Size 1.5 storm slab avalanche on a southerly aspect at 1400 m and is suspected to have failed on the mid-December interface following a large whumpf.Reports from Saturday included one observation of a remotely (from a distance) triggered Size 1.5 storm slab releasing on a steep north-facing roll at 1100 m. This is suspected to have run on surface hoar buried beneath the recent snow accumulations.
Snowpack Summary
Approximately 40-80 cm of recent storm snow covers a widespread crust as well as surface hoar in isolated areas at mid elevations that were buried mid-January. This surface hoar is more likely to exist in sheltered areas at treeline and below. Professionals have also been monitoring a few mid-pack layers within the snowpack including a crust and 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 produced recent large, natural avalanches particularly in northern areas of the region.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.