Cold temperatures will keep persistent slab problems in play for longer than usual. Be especially wary of wind-affected snow at lower elevations.
Weather Forecast
Wednesday: Mainly cloudy with isolated flurries and a trace of new snow. Light west winds. Alpine high temperatures of -17.Thursday: Mainly cloudy with isolated flurries and a trace to 5 cm of new snow. Light west winds. Alpine high temperatures of -17.Friday: Cloudy scattered flurries bringing 2-5 cm of new snow, increasing overnight. Light east winds. Alpine high temperatures of -15.
Avalanche Summary
Recent reports show continued observations of wind slabs releasing to Size 1 with skier traffic and ski cutting, primarily on south aspects around exposed terrain features in the alpine. One naturally triggered Size 1.5 was also observed on Sunday. Skier traffic has also been producing small dry loose sluffs from steep, sheltered terrain. Persistent slab avalanche danger remains a primary concern in areas where last week's storm snow consolidates into a slab above the December 15 interface. In areas where the storm snow has not yet consolidated into a slab, expect continued potential for triggering loose dry avalanches in steep terrain.
Snowpack Summary
Last week's storm brought 40-60cm of new snow to the region. Shifting winds (predominantly from the north) have since redistributed this storm snow into wind slabs on a variety of aspects at higher elevations while cold temperatures have inhibited slab formation elsewhere. Cold has also worked to decrease the cohesion and reactivity of older wind slabs. 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, such as sheltered openings 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 evidence suggests it may be coaxed into reacting in steep, variably loaded terrain features in the alpine.
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.