Forecast winds and light snowfall will see wind slabs and cornices continue to grow. The potential for one of these to release and trigger a deeper persistent slab remains a real concern for Saturday.
Weather Forecast
Saturday: Mainly cloudy with scattered flurries bringing up to 5 cm of new snow. Light to moderate southwest winds. Freezing level to 1900 metres with alpine temperatures around -1.Sunday: Mainly cloudy with scattered flurries bringing a trace to 5 cm of new snow. Light west winds. Freezing level to 1500 metres with alpine temperatures around -6.Monday: A mix of sun and cloud. Light variable winds. Freezing level to 1700 metres with alpine temperatures around -5.
Avalanche Summary
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.On Wednesday, a couple of larger storm slab avalanches were reported. They were explosive controlled size 2 and size 3 from South aspects 1800 m and above. There was also one skier controlled size 1.5 slab avalanche that failed on a crust from a South aspect around 2200 m.
Snowpack Summary
10-30 cm of recent snow has fallen at upper elevations and has buried a sun crust on solar aspects. This brings 40-60 cm of accumulated snow over the past week. This recent snow overlies a widespread crust below 2300 m and higher on solar aspects. Alpine wind has recently been strong, mainly from the southwest, and has loaded leeward slopes in exposed terrain at treeline and in the alpine. Large, fragile cornices also exist along ridgelines. At higher elevations, the February crust/facet layer is now down around 130-150 cm and has been reactive with several avalanches recently releasing on it. It is expected to be most reactive in the alpine where the snowpack remains dry. The deep mid-December facet layer and November rain crust both still linger near the bottom of the snowpack and a few avalanches and cornice falls have stepped down to these layers recently resulting in very large avalanches. These layers remain a real concern while touchier wind slabs, storm slabs and cornices continue to present the risk of acting as a trigger for deeper weaknesses.
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.
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.
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.