Avalanche Forecast
Regions: Sea To Sky.
Although the likelihood of triggering avalanches is decreasing, the persistent slab problem warrants avoiding steep convexities and areas with a shallow, rocky, or variable snowpack.
Confidence
Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.
Weather Forecast
Sunday night: Cloudy with scattered flurries and 3-5 cm of new snow, light south winds, alpine temperatures around -1 C with freezing level around 1700 m.
Monday: Cloudy, isolated flurries with 1-3 cm of accumulation possible overnight, light to moderate southwest winds, alpine high temperature near -2 C with freezing level dropping from 1200 m.
Tuesday: Cloudy, 10-20 cm of new snow, moderate southwest winds, alpine high temperatures near-2 C with freezing level around 1000 m.
Wednesday: Cloudy, 15-25 cm of snow, light to moderate southwest wind, alpine high temperatures near -1 C with freezing level near 1300 m.
Avalanche Summary
Avalanche activity has diminished over the past few days. A few small avalanches, both human and explosive-triggered, were reported Saturday in the recent storm 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, human, and explosive-triggered persistent slab avalanches. Many of these avalanches either failed on the mid-November weak layer or stepped down to it, even scouring the lower snowpack away to reveal ground. A few of these avalanches were remotely triggered. See here for some photos of one of them.
Despite a decrease in avalanche activity, human-triggering large avalanches remains possible at higher elevations. Safe travel requires evaluating snow and terrain carefully.
Snowpack Summary
15-25 cm of new snow fell over the weekend, and winds from the southwest drifted this snow into slabs on leeward features at higher elevations.
Below the recent snow, the upper snowpack consists of around 70 to 120 cm from last weekend's storm. This overlies a variable weak layer of surface hoar and a crust from mid-December (down 70-90 cm), as well as a deeper weak layer of sugary faceted grains and a crust buried in late-November (down 100-200 cm). Both of these persistent weak layers produced many large and destructive avalanches during and in the days after the storm. 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 and propagating results on these layers (check out this MIN report from the Musical Bumps on Saturday). It is atypical for the region, and it is expected to persist for some time.
Terrain and Travel
- Avoid steep convexities or areas with a thin, rocky, or variable snowpack.
- If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
- Uncertainty is best managed through conservative terrain choices at this time.
Avalanche Problems
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: Unlikely - Possible
Expected Size: 2 - 3.5
Wind Slabs
At upper elevations, southwest winds previously drifted the most recent 15-25 cm of snow into wind slabs that may still be reactive to human triggering. Expect this problem 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: Unlikely - Possible
Expected Size: 1 - 1.5