Cornice falls have been responsible for most of the avalanches reported in the past few days. Storm slabs or cornices may trigger persistent weak layers resulting in large avalanches.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Thursday
Weather Forecast
Overnight: 5-10 cm of new snow with moderate southerly winds and freezing down to 1200 metres. Thursday: Another 5-10 cm during the day with light-moderate southerly winds and daytime freezing up to 1800 metres. Friday: 3-5 cm of new snow with light southeast winds and daytime freezing levels up to 2000 metres. Broken or scattered cloud in the afternoon. Saturday: Overcast with flurries and light winds as a cooling trend starts to lower freezing levels.
Avalanche Summary
On Tuesday several natural cornice falls up to size 3.5 were reported, and several natural cornice falls were reported on Monday up to size 3.0.
Snowpack Summary
New snow is forecast to develop new storm slabs in the alpine and at treeline. These new storm slabs are developing above a mix of old surfaces, that include melt-freeze crusts on solar aspects in the alpine, and all aspects at treeline. In some areas the new snow may hide some lingering wind slabs that are left from the last stormy period. The biggest problem lately has been large and fragile cornices, that have regularly fallen off naturally with loading from wind, or due to warming from direct sun or daytime heating. The February weak layers are 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 have been released with large triggers like cornice falls, and they may be more likely to fail on southerly aspects during periods of strong solar radiation or from heavy loading from storm snow. Expect cornices to experience new fragile growth during the next few stormy days.
Problems
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.
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.