Recent winds have been hammering loose snow into wind slabs. As these gain strength, concern lingers over slabs that have formed on a buried layer of surface hoar at mid elevations.
Weather Forecast
Saturday night: Mainly cloudy. Moderate west winds increasing to strong or extreme northwest in the alpine.Sunday: A mix of sun and cloud. Strong northwest winds decreasing over the day. Alpine temperatures dropping from around -2 to -6 with freezing levels around 1400 metres.Monday: Mainly sunny. Light southwest winds. Alpine high temperatures around -8 with freezing levels back to valley bottom.Tuesday: Mainly sunny. Light southwest winds. Alpine high temperatures around -6.
Avalanche Summary
Only one new avalanche was reported on Friday, a small (size 1) persistent slab triggered over our persistent weak layer of concern, down about 30 cm. It was the result of a ski cut in a sheltered meadow on a northeast aspect at 1900 metres. Avalanche observations from the middle of last week were limited to smaller (size 1-1.5) loose dry sluffs and small slabs releasing naturally from steep alpine features.On Monday, explosives triggered size 2 storm slab avalanches and cornices. Our 10-30 cm-deep weak layer of surface hoar was the failure plane.
Snowpack Summary
A new surface crust is now likely to be widespread below about 1500 metres. Above this elevation, 10-30 cm of badly wind-affected snow forms the current snow surface, the product of snowfall this past weekend and more recent strong and shifting winds. Lower down, the recent snow is sitting over a weak layer of surface hoar. This surface hoar is most prominent around treeline elevations (1500-1900 m). On solar aspects it is likely to combined with an underlying sun crust and below treeline with a more widespread temperature crust.In shallow snowpack areas, the base of the snowpack may still be composed of weak faceted grains. In deeper snowpack areas, the middle and lower portions of the snowpack are generally considered to be well-settled and strong.
Problems
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.