As storm snow settles, persistent slab problems emerge. Sheltered treeline elevations are a primary concern.
Weather Forecast
Monday: A mix of sun and cloud. Light northwest winds. Alpine temperatures around -16.Tuesday: Mainly cloudy. Light southwest winds. Alpine temperatures around -14.Wednesday: Cloudy with isolated flurries bringing a trace of new snow, increasing overnight. Light southwest winds. Alpine temperatures around -16.
Avalanche Summary
Reports from Saturday include observations of a skier triggered Size 1 wind slab on a south aspect at 2200 metres as well as smaller dry loose sluffs from steep, sheltered terrain. Reports from Friday included one skier-triggered Size 2 persistent slab avalanche from a steep, rocky feature on a west aspect at 2100m in the Monashees. The late November crust was indicated as its failure plane. Persistent slab avalanche danger can be expected to persist in areas where our recent snow consolidates into a slab above the December 15 interface. In areas where new snow has not yet consolidated into a slab, expect continued potential for triggering loose dry avalanches in steep terrain.
Snowpack Summary
Last week's stormy weather brought 40-60cm of new snow to the region. Shifting winds have since redistributed this storm snow into wind slabs on a variety of aspects at higher elevations while cold temperatures have worked to inhibit slab formation in wind-sheltered areas. The storm snow lies over a wide variety of old surfaces, including large surface hoar (weak, feather-like crystals), hard crusts formed by sun or wind, and sugary facets. The bond at this old snow interface is of critical importance in areas where the overlying snow has consolidated into a slab. The most concerning areas are those that saw pronounced surface hoar development before the storm, particularly in sheltered areas at and below treeline. Recent snowpack tests show wide ranging reactivity at this interface but suggest that it may be most reactive on northerly aspects.A crust which was formed by rain in late November is another major feature in the snowpack and is down approximately 80-110cm at treeline elevations. Recent snowpack tests as well as an isolated avalanche report from the North Monashees suggests it may be coaxed into reacting in steep, variably loaded terrain features.
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.