Avalanche Forecast
Issued: Dec 27th, 2019 4:30PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
astclair,
Avalanche Canada
The persistent slab problem is evolving into a low probability/high consequence scenario where you may not observe any indication of unstable snow before making a dangerous decision. The formation of new wind slabs will add a layer of complexity to terrain selection on Saturday.
Summary
Confidence
Weather Forecast
Friday night: Cloudy, light to moderate west winds, temperatures near -5 C, isolated flurries with trace accumulations.
Saturday: Cloudy with scattered flurries bringing 3-8 cm of new snow, continuing overnight, moderate southwest winds, becoming strong at ridgetop, alpine high temperatures around -4 C.
Sunday: Mostly cloudy with isolated flurries and a trace of new snow. Light southeast winds. Alpine high temperatures around -3 C with freezing levels rising to 1300 metres.
Monday: Mix of sun and cloud, light to moderate southwest winds, alpine high temperature near -2 C with freezing levels near 1000 m.
Avalanche Summary
Reports from Monday through Wednesday have shown more observations of widespread large to very large (size 2-3) natural avalanches that have run recently, along with numerous (but diminishing) new explosives-triggered size 2-3 persistent slab releases, 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 described in our Snowpack Summary 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 of large avalanches remains a real possibility at higher elevations. Very cautious route-finding and conservative decision making is currently required for safe travel in higher elevation avalanche terrain.
Snowpack Summary
15-20 cm of new snow from the last 24 hours fell on a mix of crusts or old snow surfaces that are not likely to bond well, with moderate west winds actively drifting the new snow into cohesive slabs on leeward features.
Below this recent snow, the upper snowpack consists of around 70 to 120 cm of snow from last weekend's storm.
This storm snow overlies a variable weak layer of surface hoar and rime crust as well as a deeper (100-200 cm) weak layer of sugary faceted grains and hard melt-freeze 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. It is atypical for the region and is expected to persist for some time.
Managing your risk through conservative terrain choices along with selective avoidance of high-consequence avalanche terrain is strongly advised until the snowpack gains strength.
Terrain and Travel
- Uncertainty is best managed through conservative terrain choices at this time.
- 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
15-20 cm of new snow combined with sustained winds is expected to form touchy new wind slabs that will need to be managed on Saturday. 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 from the alpine down into the upper treeline 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
Valid until: Dec 28th, 2019 5:00PM
