The sun will be out in force on Monday. As it rapidly warms the snowpack, there will be an increasing likelihood of highly destructive persistent slab avalanches that run full path. Cornice falls are likely triggers.
Weather Forecast
Monday: A mix of sun and cloud. Light northwest winds. Freezing level to 1700 metres with alpine temperatures around -5.Tuesday: Cloudy with sunny periods. Light southwest winds. Freezing level to 1900 metres with alpine temperatures around -3.Wednesday: Mainly cloudy. Light to moderate south winds. Freezing level to 2300 metres with alpine temperatures around 0 to +1.
Avalanche Summary
Reports from Saturday include numerous observations of storm slabs releasing from Size 1.5-2. Ski cutting, skier traffic, and explosives account for the majority of these occurrences, however one Size 3 storm slab was reported to have run naturally. One highly notable observation details a Size 3.5-4 deep persistent slab that was triggered by a large cornice fall. This slide ran all the way to ground and reached the end of its historical runout. It featured a crown fracture of up to 4 metres at its deepest. North to west aspects showed the most activity in reports and all activity occurred above about 2000 metres.Reports from Friday showed several recent (12-36 hour old) persistent slab releases to Size 3 with natural storm slab and cornice triggers. Two of these occurred on southeast aspects. Improved visibility also revealed evidence of numerous storm slabs and wind slabs having released from Size 2-3 over the same time period.Looking forward, anticipate a stark change in the snowpack from morning to afternoon as solar warming breaks down surface crusts and promotes instability in a wide range of avalanche problems over the course of each day.
Snowpack Summary
10-30 cm of new snow now overlies a crust below about 2100 metres (higher on solar aspects) and moist snow below about 1800 metres. Below the new snow interface, storms over the past week brought 40-60 cm of snow to the region. Several other crusts as well as moist snow are likely to exist within this storm snow, mainly at lower elevations and on solar aspects. Moderate to strong southwest winds during and since the storm formed wind slabs on leeward slopes as well as fragile cornices along ridgelines. 90-130 cm of accumulated snow now overlies a more widespread rain crust below 2000 m and sun crust on solar aspects at higher elevations. At higher elevations, the February weak layers are down 160 cm to over 2 metres and the deep mid-December facet layer and November rain crust both still linger near the bottom of the snowpack. These layers were active during a storm in mid-March and produced some very large avalanches. Occasional deep releases continue to be reported and these deeply buried weaknesses remain a serious concern as solar radiation and warming temperatures begin to penetrate the snowpack at increasingly higher elevations.
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.