Blustery winds and convective storms keep building wind slabs in the alpine. With wintery snow up high and Spring crusts down in the valley, the snowpack and conditions are quite variable.
Weather Forecast
Mixed weather for Friday, with sun and cloud, isolated flurries, freezing levels rising to 1400m, and moderate west winds. Tonight and into Saturday we should see another 10cm of snow with moderate to strong SW winds. Freezing levels will remain below 1500m.
Snowpack Summary
New snow from convective flurries overlies a series of crusts on most aspects up to 1900m. Reports of strong to extreme winds in the high alpine will have formed surface wind slabs. Cold, dry snow can be found on northerly slopes while solar aspects rapidly become moist with sunshine. Huge cornices line the upper ridges.
Avalanche Summary
Numerous natural avalanches to size 2.5 were observed in the highway corridor yesterday. All were failing within the recent storm snow and had shallow deposits. Neighbours surrounding Rogers Pass are still observing large natural avalanches, often triggered by large cornices.
Confidence
Due to the number of field observations
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.