Wind slabs up to 50 cm deep may remain sensitive to human triggering in the alpine. Persistent slabs may roar back to life over the next few days as we go through a significant warm up. South facing slopes are especially suspect and are best avoided.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain
Weather Forecast
Continued snowfall is expected Saturday night as an unstable air mass fuels convective precipitation. A ridge is expected to build offshore Sunday allowing the entire province to dry out. A more spring-like pattern begins to take shape on Monday with freezing levels creeping towards 2500 m by Tuesday.SATURDAY NIGHT: Overcast, freezing level around 750 m, light to moderate northwest wind, 1 to 5 cm of snow possible. SUNDAY: Broken cloud cover, freezing level beginning around 500 m rising to about 2000 m, light to moderate northwest wind, 1 to 2 cm of snow possible. MONDAY: Scattered cloud cover, freezing level beginning around 1500 m rising to about 2200 m, light west/southwest wind, no significant precipitation expected. TUESDAY: Scattered cloud cover, freezing level beginning around 1500 m rising to about 2500 m, light to moderate south/southwest wind, no significant precipitation expected.
Avalanche Summary
A small human triggered wind slab avalanche was reported from the central portion of the region on an east facing slope at 2500 m on Friday.On Thursday a skier accidently triggered a large persistent slab avalanche on a south facing feature at 2600 m. The size 2 avalanche had a crown 60 cm in depth and failed on the mid March Persistent Weak Layer (PWL). Explosive control work produced cornice failures and storm slabs to size 2.5 on a variety of aspects in the alpine. Loose wet avalanches ran naturally to size 1.5 on steep south facing alpine slopes as well.Reported avalanche activity was limited on Wednesday; a cornice failure produced a size 2 soft slab in steep rocky terrain on an unknown aspect and sluffing was observed in steep terrain to size 1.
Snowpack Summary
Recent wind and cold snow have formed wind slabs 10 to 40 cm in depth in the alpine that sit on a mix of crusts and surface hoar. Test profiles show resistent planar failures at this interface. In wind sheltered terrain Saturday's new snow is coming to rest on a surface crust that exists on all aspects aside from true north below 2000 m. This crust extends up to around 2400 m on south facing slopes. On north facing slopes above 2000 m, temperatures have remained cold and the snowpack is still quite "winter-like." The mid-March Persistent Weak Layer (PWL) is now 40 to 90 cm below the surface. This layer is composed of a crust on all aspects at low elevations. At upper elevations the layer presents as a crust on solar aspects (south and west) and buried surface hoar on polar aspects (north and east). The likelihood of triggering a deeper persistent slab avalanche is steadily decreasing but the consequence of doing so remains high. Solar aspects are expected to be the most suspect as the freezing level begins to rise over the next few days.Deeper persistent weak layers from January and December are still being reported by professional observers, but are generally considered dormant.
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.
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.