Avalanche Forecast
Issued: Dec 28th, 2019 4:30PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Persistent Slabs and Loose Wet.
astclair,
Avalanche Canada
The persistent slab is evolving to a low probability/high consequence scenario where you may not observe any signs of unstable snow before making a dangerous decision. Managing reactive wind slabs adds further complexity to choosing safe terrain on Sunday.
Summary
Confidence
Weather Forecast
Saturday night: Cloudy with scattered flurries continuing overnight bringing 3-5 cm of new snow, light to moderate southwest winds, alpine high temperatures around -4 C and freezing level near 1000 m.
Sunday: Cloudy with isolated flurries and 1-3 cm of new snow. Light south winds. Alpine high temperatures around -1 C with freezing levels rising to 1800 m.
Monday: Mix of sun and cloud, isolated flurries with 1-3 cm of accumulation possible overnight, light to moderate southwest winds, alpine high temperature near -2 C with freezing levels dropping from 1200 m.
Tuesday: Cloudy, 10-20 cm of new snow, light to moderate southwest winds, alpine high temperatures near-2 C with freezing level around 1100 m.
Avalanche Summary
A couple small avalanches, both human and explosive-triggered, have been reported in the new snow. These avalanches released on leeward aspects (north to northwest facing slopes) near tree line.
Reports from earlier in the week captured widespread large to very large (size 2-3) natural persistent slab avalanches, along with numerous explosive-triggered size 2-3 avalanches, targeted in the Whistler area. Many more size 2-3 avalanches were triggered by explosives and by skiers on Saturday and Sunday.
Of the avalanches mentioned above, many either failed on the mid-November weak layer or stepped down to it, even scouring the lower snowpack away to reveal ground. Some of the avalanches were remotely triggered. See here for some photos of one of them.
Human-triggering large avalanches remains a real possibility at higher elevations. Very cautious route-finding and conservative decision-making is currently required for safe travel at higher elevations.
Snowpack Summary
15-25 cm of new snow from the last 48 hours fell on a mix of crusts or old snow surfaces that are not likely to bond well, and moderate to strong southwest winds have drifted the new snow into reactive slabs on leeward features.
Below this recent snow, the upper snowpack consists of around 70 to 120 cm from last weekend's storm, which overlies a variable weak layer of surface hoar and a crust as well as a deeper (100-200 cm) weak layer of sugary faceted grains and a crust buried in mid-November. Both of these persistent weak layers produced many large and destructive avalanches during and in the days after the storm, often with light triggers and even remote triggers. Avalanche activity on these layers has been on a downward trend, but our fundamentally unstable snowpack structure remains a serious concern in the region. Snowpack tests continue to produce sudden results on these layers.
Managing your risk through conservative terrain choices along with selective avoidance of high-consequence avalanche terrain is strongly advised until the snowpack gains strength. It is atypical for the region and is expected to persist for some time.
Terrain and Travel
- In times of uncertainty conservative terrain choices are our best defense.
- Persistent slabs have potential to pull back to lower angle terrain.
- Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
- If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
Problems
Wind Slabs
15-25 cm of recent snow combined with sustained southwest winds are expected to form reactive new wind slabs that will need to be managed on Sunday. Expect this problem to increase with elevation and to be most pronounced in the immediate lee of wind-exposed terrain features. Wind slab releases currently carry a serious risk of triggering a deeper weak layer, especially in shallow, rocky areas.
Aspects: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Persistent Slabs
A pair of touchy weak layers are buried deep in the snowpack near and above tree line, and the consequences for triggering either one are severe. These layers have been responsible for many very large avalanches in the region over the past two weeks. Shallower but more reactive wind slabs may also step down to one of these deeper layers and create a very large, very destructive avalanche.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Loose Wet
Warming temperatures and light rain at lower elevations is likely to produce wet loose activity in the surface snow. These point releases are expected to be small but could have greater consequences in areas where terrain traps exist.
Aspects: All aspects.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Dec 29th, 2019 5:00PM
