Avalanche Forecast
Issued: Dec 19th, 2020 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
mconlan,
Avalanche Canada
Snow and strong wind tonight will form new slabs and may critically load buried weak layers.
Summary
Confidence
Weather Forecast
SATURDAY NIGHT: Snow, accumulation 10 to 20 cm with local amounts up to 30 cm possible, 60 to 80 km/h southwest wind, alpine temperature -2 C, freezing level 1500 m.
SUNDAY: Cloudy with isolated flurries, 40 to 60 km/h west wind, alpine temperature -7 C, freezing level 1000 m.
MONDAY: Snow, accumulation 20 to 30 cm, 30 to 50 km/h south wind, alpine temperature -7 C.
TUESDAY: Clear skies, 10 to 20 km/h north wind, alpine temperature -9 C.
Avalanche Summary
On Saturday, a naturally-triggered wind slab avalanche was observed on a southeast aspect in the north of the region. Other observations were limited due to poor visibility.
On Friday, a few storm slab avalanches were observed out of steep terrain and small wind slabs were triggered by riders on northerly aspects.
Slabs could be triggered by riders on Sunday from within the most recent storm snow, older storm snow, or on the weak layers described in the snowpack summary.
Snowpack Summary
Saturday night's snowfall will accumulate with strong southwest wind, forming wind slabs in exposed terrain. These will overly previous storm layers within the top 40 cm of the snowpack. The freezing level is expected to reach around 1500 m, so rain followed by a cooling trend will limit avalanche concerns below that elevation.
This snow will continue to load a couple weak layers in the snowpack. The shallower layer, being around 30 to 70 cm deep, includes feathery surface hoar crystals. There is uncertainty in the distribution of this layer, but the most likely place to find it would be in terrain features sheltered from the wind around treeline and lower alpine elevations. The more widespread layer in the snowpack is a melt-freeze crust from early December, which may have a weak layer of sugary faceted grains above it. This layer is generally found around 40 to 80 cm deep and has produced varying test results in the region. This layer should be treated as suspect were you find faceted grains above the crust.
The remainder of the snowpack is well-settled in the south of the region. In the north, another melt-freeze crust with associated faceted grains around it lies near the ground. The most recent avalanche activity observed on this layer was on December 1st when a cornice fall triggered a size 3 avalanche. More recent snowpack tests in the Cayoosh zone found this layer down 100+ cm and unreactive. While our uncertainty around the reactivity of this crust is keeping it on the radar, it does not appear to be an active problem at this time.
Terrain and Travel
- Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
- Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
- If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
Problems
Wind Slabs
Around 20 cm of snow with strong southwest wind will form new wind slabs in exposed terrain. Use particular caution immediately adjacent to ridges, where the slabs will likely be thickest and touchiest. The storm snow may still be touchy in sheltered terrain features, so check for the bond of the snow prior to committing to avalanche terrain.
Aspects: North, North East, East, South East, South, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Persistent Slabs
A couple problematic layers exist around 40 to 80 cm deep, including weak layers of surface hoar and faceted grains above a hard melt-freeze crust. There has not been notable avalanche activity on these layers but Saturday night's snow accumulation will increase the likelihood of triggering them.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 20th, 2020 4:00PM
