Avalanche Forecast
Issued: Dec 18th, 2019 3:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Persistent Slabs.
cgarritty,
Avalanche Canada
New snow accumulations are steadily building up storm slabs while adding incremental strain to a deeply buried weak layer. Higher elevations hold the trickiest combo of wind loading and more widespread snowpack weakness.
Summary
Confidence
Moderate - Forecast snowfall amounts are uncertain.
Weather Forecast
WEDNESDAY NIGHT: Cloudy with continuing flurries bringing another 10-15 cm of new snow. Moderate south winds.
THURSDAY: Cloudy with continuing flurries bringing approximately 5-10 cm of new snow and 3-day snow totals to 30-50 cm. Snowfall increasing overnight. Moderate to strong south winds. Alpine high temperatures around -4 with freezing levels near 1100 metres.
FRIDAY: Cloudy with easing flurries bringing about 5 cm of new snow in addition to 20-30 cm of snow from the overnight period. Light southwest winds. Alpine high temperatures around -5.
SATURDAY: Mainly cloudy. Light southwest winds. Alpine high temperatures around -6.
Avalanche Summary
Explosives avalanche control yielded 2 large (size 2) storm slab releases as well as one smaller (size 1.5) persistent slab in the Whistler area on Wednesday morning.
Reports from Tuesday showed ski cuts producing small (size 1-1.5) storm slabs with crown depths of 10-50 cm, showing good evidence of wind redistribution. These and the above mentioned storm slabs were noted to have failed on the rime crust and surface hoar noted in our snowpack description.
A few persistent slab avalanches reported on Saturday, both natural and human triggered, up to size 2.5. Click here to check out a MIN report of a persistent slab avalanche in the Whistler backcountry on Saturday. These avalanches released on the persistent weak layer that was buried in mid November.
Looking forward, human triggering of unstable new snow accumulations as well as more dangerous persistent slab avalanches will be an ongoing concern.
Snowpack Summary
New snow from a series of storms has begun to bury a new weak layer of surface hoar at lower elevations as well as a glaze of rime crust above 1900m. The new snow adds to 15-25 cm of recent snow sitting on another layer of surface hoar in sheltered areas, as well as sugary, faceted snow elsewhere.
Perhaps most importantly, continuing snowfall will increasingly stress a persistent weak layer from mid November that consists of a crust/facet combination, recently found approximately 35-70 cm deep. This layer has been responsible for several recent avalanches. Snowpack tests consistently indicate that slabs overlying it can be triggered by humans and propagate widely, resulting in large avalanches.Â
The snowpack is unusually shallow and weak for the Sea to Sky region. Persistent weak layers like the above mentioned mid-November crust/facet combination can occur in any season, but they are not the norm here. They demand especially thoughtful and conservative terrain selection.
Total snowpack depths range between 80-200 cm and taper quickly at lower elevations.
Problems
Storm Slabs
New snow is accumulating above a new array of weak surfaces. Expect new accumulations to trigger increasingly easily as the depth and cohesion of new snow increases. For this reason wind loaded areas should be especially concerning. Lower elevations may be affected in areas where new snow falls on smooth surfaces
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Persistent Slabs
A persistent weak layer has been responsible for numerous recent avalanches. This scenario is atypical for the Sea to Sky region, and needs to be treated with serious caution. The likelihood of triggering a large persistent slab avalanche will be increasing as new snow accumulates.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 19th, 2019 5:00PM
