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Avalanche Forecast

Archived

Dec 17th, 2019–Dec 18th, 2019

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Sea To Sky.

Continuing snowfall will build up storm slabs while bringing increasing stress to an active and deeply buried weak layer at higher elevations. Expect stability to deteriorate over the day.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

TUESDAY NIGHT: Cloudy with continuing flurries bringing 10-15 cm of new snow. Strong southwest winds.

WEDNESDAY: Cloudy with continuing flurries bringing another 10-15 cm of new snow, continuing overnight. New snow totals of 20-30 cm by end of day. Moderate to strong south wind. Alpine high temperatures near -4 with freezing level to 1000 metres.

THURSDAY: Cloudy with continuing flurries bringing approximately 10-20 cm of new snow and 3-day snow totals of up to 50cm. Snowfall increasing overnight. Light to moderate south winds. Alpine high temperatures around -4 with freezing levels near 1000 metres.

FRIDAY: Cloudy with easing flurries and 20-30 cm of snow from the overnight period. Light southwest winds. Alpine high temperatures around -5.

Avalanche Summary

We haven't yet received reports of avalanches from Monday and Tuesday's new snow, but there were 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.

On Friday, there were several reports of natural, human and explosives triggered avalanches up to size 2. The majority of these avalanches released on the persistent weak layer that was buried in mid November.

On Thursday, there were several reports of human and explosives triggered avalanches up to size 2. Some of these avalanches stepped down to the mid November weak layer.

Looking forward, activity of this nature can be expected to continue and perhaps intensify as snowfall increases stress on buried weak layers.

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. 

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

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.