Wind slabs may remain sensitive to human triggers Sunday. The new snow will be prone to solar-triggering if the sun comes out.
Recent snow and wind have formed wind slabs in immediate lees of ridge crests and roll-overs at upper elevations. They may remain sensitive to triggering especially where they sit over a crust.
Cornices are often fragile this time of year, especially when the sun is out. Cornice falls are dangerous in their own right and they can also trigger slabs on slopes below.
A ridge of high pressure sets up over the province. Freezing levels are forecast to steadily rise through the week.
Saturday night: Clearing. Wind easing to light northwest. Alpine temperature around -12. Freezing level valley bottom.
Sunday: Mix of sun and cloud. Light northwest wind. Alpine temperature around -7. Freezing level 1400 m.
Monday: Mix of sun and cloud. Light northeast wind. Alpine temperature around -6. Freezing level 1600 m.
Tuesday: Mix of sun and cloud. Light to moderate northeast wind. Alpine temperature around -6. Freezing level 1600 m.
On Friday and Saturday, most activity was loose sluffing in the new snow. Natural and skier controlled wind slabs were limited to size 1.
On Thursday, a significant natural cornice failure triggered a persistent slab from the slope below size 3. This occurred on a high alpine steep east-facing slope. This is an important reminder to stay well back from ridgetops that may be corniced and avoid slopes with overhead cornice exposure especially under the current conditions where recent rapid growth and daytime warming has made them large and fragile.
10-30 cm of recent snow may have formed isolated pockets of wind slab on leeward slopes and behind terrain features. The new snow sits on a series of melt-freeze crusts on all aspects below 1900 m and southerly aspects to mountain top. Reports indicate snow is bonding well at these interfaces.
Recent warm weather is expected to have promoted bonding around widespread crust layers formed over the last month in the mid to upper snowpack. One widespread crust from mid-March found 30-60 cm deep saw isolated instances of large solar-triggered slab avalanches during the last sunny period. The early November crust is buried down 180-200 cm with faceted crystals above and below it. These persistent weaknesses are definitely worth keeping in mind heading into the next big warmup.
