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 west winds. Freezing level to 1500 metres with alpine temperatures around -6.Tuesday: Cloudy with sunny periods. Light south winds. Freezing level to 1900 metres with alpine temperatures of -4.Wednesday: Mainly cloudy. Light to moderate south winds. Freezing level to 2100 metres with alpine temperatures around 0.
Avalanche Summary
Reports from Saturday included observations of several Size 1 storm slabs releasing with ski cutting as well as one roughly 12 hour old natural Size 1.5 slab release in the Rogers Pass area. Evidence of an older (about 24 hours) Size 3 slab avalanche was also reported. It released from steep alpine terrain.Reports from Friday showed several deep persistent slab releases to Size 3.5 having occurred within the previous 24 hours with natural, cornice fall, and rider triggers. One of these featured a crown fracture of up to 3 metres in depth. Numerous wind slabs were also observed releasing naturally to Size 2.5 while natural loose wet and loose dry slides ranged from Size 1.5-2.Reports from Thursday included numerous observations of storm slabs releasing naturally from Size 1-2.5 throughout the region. Natural triggers (including natural cornice fall) featured prominently in reports. Crown fractures generally ranged from 30-40 cm.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
10-20 cm of new snow now overlies a crust below about 2200 metres on (higher on solar aspects) and moist snow below about 1700 metres. Below the new snow interface, storms over the past week brought 40-60 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 160-200 cm 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 were also reported in late March 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.