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 1600 metres with alpine temperatures around -5.Tuesday: A mix of sun and cloud. Light southwest winds. Freezing level to 1900 metres with alpine temperatures around -2.Wednesday: Mainly cloudy. Light to moderate southwest winds. Freezing level to 2300 metres with alpine temperatures around 0 to +1.
Avalanche Summary
One report from Saturday details a remotely triggered Size 1 wind slab avalanche that occurred on an east aspect at 2200 metres. This avalanche was noted to have released over a crust layer.Reports from Friday showed evidence of a natural, solar-triggered avalanche cycle in deeper snowpack areas of the region producing storm slabs releases to Size 2. (Two Size 2.5 and 3 slides were also reported.) One approximately 24 hour old Size 3 deep persistent slab release was also reported west of Invermere. This avalanche was triggered by a natural cornice fall and is suspected to have failed on the November crust.Reports from Thursday include observations of storm slabs releasing to Size 2.5 with explosives triggers. Explosives triggered cornices reached Size 3. Several slabs and/or cornice releases were noted triggering persistent slabs which then ran to ground. North aspects saw the majority of this activity.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
10-15 cm of new snow now overlies temperature crusts below about 1900 metres and sun crust all the way into the alpine on solar aspects. This precipitation fell as rain below about 1600 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. At higher elevations, the February crust/facet layer is now down around 130-150 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.
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.
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.