Avalanche Forecast
Issued: Dec 8th, 2020 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Deep Persistent Slabs.
dsaly,
Avalanche Canada
Reactive slabs may lurk on windloaded slopes and steep and convex features in the alpine.
Summary
Confidence
Weather Forecast
TUESDAY NIGHT: Isolated flurries, trace to 5 cm / Moderate to light west ridgetop wind / alpine low temperature -4 / Freezing level 1100 m
WEDNESDAY: Mostly cloudy, isolated flurries near the Coquihalla / Light to moderate southwest ridgetop wind / alpine high temperature -5 / Freezing level 1000 m
THURSDAY: Cloudy with sunny breaks / Light southwest ridgetop wind / alpine high temperature -2 / Freezing level 1200 m
FRIDAY: Sun and cloud / Light southwest ridgetop wind / alpine high temperature -6 / Freezing level 800 m
Avalanche Summary
Loose wet and wet slab avalanches to size 2 were observed around the Coquihalla Tuesday, these were triggered by heavy rain and high freezing level.
Around Whistler (which received more snow than the Duffy), storm slab avalanches to size 2 were triggered by explosives, and evidence of a natural avalanche cycle to size 2 was also reported.
On Monday afternoon around the Duffy, small size 1 loose-snow and low density slab avalanches failed naturally and with skier traffic as snow began to accumulate and load ridgetops. Small loose wet avalanches were observed below 1600 m.
A notable observation in the Duffy Area: On Tuesday Dec 1, a large size 3 natural avalanche was observed on a NE aspect at 2400 m. It was reported as a cornice failure triggering the slope below and appeared to have slid on the early November crust near the base of the snowpack.
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Snowpack Summary
Overnight Monday-Tuesday, rain and high freezing levels overnight produced a moist snowpack above 2000 m before freezing levels began falling.
By Tuesday morning, 10-20 cm new snow accumulated above 1800m and the snow/rain line with moderate ridgetop winds. At upper (drier) elevations, the new snow covers a melt-freeze crust and old wind slabs. Dry snow accumulated around Cayoosh and the Duffy area, but sustained warm temps and rain saturated slopes around the Coquihalla.
A saturated snowpack is found at lower elevations, this will cool and freeze with falling temperatures.
The lower snowpack is characterized by a few crusts that formed during warm periods in November.
- In the north, treeline snowpack depth sits around 80 to 150 cm, and decreases rapidly below 1600 m. The crust of note here is sitting near the base of the snowpack, likely surrounded by a weak layer of sugary facets. We have a lot of uncertainty on the reactivity of this layer - we're keeping it on our radar because we don't have enough information. Recent snowpack tests in the Cayoosh zone found this layer present and down 60 cm, and unreactive to snowpack tests.
- In the south of the region, treeline snowpack depth sits around 70-120 cm. The deeper crusts in this area are generally down around 60-80 cm in the alpine and well bonded to surrounding snow.
Terrain and Travel
- Be careful as you transition into wind affected terrain.
- Be alert to conditions that change with elevation and wind exposure.
- Continue to make conservative terrain choices while the storm snow settles and stabilizes.
Problems
Wind Slabs
10-20 cm of new snow with strong winds from the southwest formed reactive slabs at upper elevations. These will be more sensitive to triggering and larger in size in areas where strong winds have transported the snow into deeper slabs.
This problem is confined to areas that accumulated dry snow - its is likely not a concern where sustained warm temps and rain saturated slopes (such as around the Coquihalla). The saturated snowpack should freeze and dry with falling temperature, however a loose wet avalanche problem should be considered anywhere the snowpack remains moist or wet.
Aspects: North, North East, East, South East, West, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Deep Persistent Slabs
In the north of the region (e.g., Duffey Lake), a weak layer of sugary faceted grains around a hard melt-freeze crust sits near the base of the snowpack. There is a lot of uncertainty on the reactivity of this layer - we're keeping it on our radar because we don't have enough information. Be wary of overhead hazards, a large impact (like a cornice failure - see avalanche summary) could trigger the slope below. Keep in mind if this layer is triggered, it will likely produce large avalanches. This problem does not exist in the south of the region.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 9th, 2020 4:00PM
