Winter continues at higher elevations, watch for wind slabs and cornices that may be large and fragile.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Thursday
Weather Forecast
Overnight: Moderate southeast winds, 3-5 cm of new snow, and freezing down to 800 metres. Thursday: 5-8 cm of new snow with moderate southeast winds and daytime freezing up to 1600 metres. Friday: Another 3-5 cm of new snow in the morning with light westerly winds and daytime freezing up to 1400 metres. Saturday: A few more cm of snow with light winds and a chance of scattered or broken cloud in the afternoon. Daytime freezing up to 1400 metres.
Avalanche Summary
Several natural cornice releases were reported up to size 2.5 on Tuesday. Explosive control produced cornice falls up to size 2.0 on Monday. Natural cornice falls up to size 2.5 were reported on Sunday from North aspects in the alpine between 2300-2400 metres.
Snowpack Summary
Recent moderate to strong winds redistributed new snow from the past few days into wind slabs on a range of aspects in the alpine and at treeline. These wind slabs have developed above a mix of old surfaces that include melt-freeze crusts below about 1900 metres and on solar aspects in the alpine. Fragile new cornice growth also developed along ridgelines over the course of the week. Below the new snow, a number of storm snow and crust layers that formed over mid to late March appear to be well bonded. The February weak layers are now down about 120-150 cm and the deep mid-December facet layer and November rain crust both still linger near the bottom of the snowpack. These deep weak layers produced large avalanches with cornice falls and other heavy triggers in late March and early April. They remain a concern and may be more likely to fail on southerly aspects during periods of strong solar radiation.
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
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.