Regions
Northwest Coastal.
Extreme winds and heavy snowfall on Tuesday night will push the avalanche danger to HIGH.
Confidence
Moderate - Intensity of incoming weather systems is uncertain on Wednesday
Weather Forecast
Tuesday night and Wednesday: Up to 45cm of new snow in the overnight period with an additional 7-15cm during the day / Extreme southwest winds / Freezing level at 650mThursday: 5-20cm of new snow / Light and variable winds / Freezing level at 300mFriday: Light flurries / Light and variable winds / Freezing level at surface
Avalanche Summary
Observations were highly limited on Monday, but I suspect there was an impressive natural avalanche cycle on Sunday night in response to heavy storm loading. More snow on Tuesday night will promote ongoing storm slab activity. There is some uncertainty regarding the reactivity of the buried weak layers below Sunday's storm snow. Surprisingly large avalanches at this interface remain possible and may remain sensitive to human triggering or a surface avalanche in motion.
Snowpack Summary
On Sunday night the region got pummeled by heavy snowfall and extreme westerly winds. By Monday morning, 24 hour totals were around 40cm in Bear Pass, and between 40 and 85 cm along the Hwy 16 corridor west of Terrace. With heavy snowfall and extreme winds forecast for Tuesday night, the developing storm slab will increase in depth and destructive potential. The new snow sits over a highly variable interface resulting from last week's cold snap. This mid-December interface consists of hard wind slabs in wind exposed terrain, widespread faceting of the upper snowpack, and 5-10 mm surface hoar in sheltered areas. The new snow may bond poorly to this interface. A layer of surface hoar which was buried mid-November was down 50-80 cm in many parts of the region prior to the storm. This layer has been dormant recently but may increase in reactivity with storm loading. In many areas in the region, the snowpack is reported to be very thin and faceting has been reported through the entire snowpack. In these areas, old rain crusts from November are developing weak facets. Concerns in the north of the region are basal weaknesses that may exist in the shallower 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.