Regions
Northwest Coastal.
Watch for areas where wind and storm snow have formed cohesive slabs above the recently buried crust/ surface hoar layer. These slabs have been reactive to both human and natural triggers.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
TONIGHT: Light flurries. Accumulation 1-5 cm. Ridge wind moderate, southeast. Temperature -7. Freezing level 200 m.FRIDAY: Cloudy, light flurries. Accumulation 2-7 cm. Ridge wind moderate, south. Temperature -7. Freezing level 200 m.SATURDAY: Mix of sun and cloud. Ridge wind moderate, east. Temperature -11. Freezing level valley bottom.SUNDAY: Snow. Accumulation 15-25cm. Ridge wind strong, southeast. Temperature -10. Freezing level 200 m. Alpine Inversion.
Avalanche Summary
Explosive control work on easterly aspects between 1100-1330 m, near Terrace on Thursday produced several storm slab avalanches up to Size 1.5 which failed on the most recently buried, mid-January crust layer.Wednesday, a naturally triggered cornice fall released a Size 2 storm slab avalanche on a steep, north aspect, and skier traffic in steep terrain produced only small, loose sloughs in the recent storm snow. On Tuesday there were reports of natural storm slab avalanches up to Size 2 failing on the mid-January interface and ski cut results produced 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 between 600-1200 m that ran on the mid-December crust layer. 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.
Snowpack Summary
Approximately 50-80 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 and has produced easy, sudden results in recent snowpack tests on northerly aspects between 1300-1450 m in areas north of Terrace.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 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.