Huge changes have impacted the snowpack in the last week and it needs time adjust to the new loads. A careful, cautious approach to assess backcountry conditions needs to be followed to stay safe the next few days.
Weather Forecast
Today will bring a reprieve from the stormy onslaught. Expect a mix of sun and cloud with flurries, an alpine high of -11*C, and freezing levels staying below 800m. Winds will back off to moderate from the west. More of the same on Monday, with freezing levels rising to 1100m.
Snowpack Summary
An intense front finished off a week of storms yesterday, with heavy rain/snow and extreme SW winds (100+ km/h). A rain crust will be prevalent below ~1600m, and widespread wind and storm slabs have formed at tree-line and alpine elevations. Below this, warm, rapidly settling storm slabs are widespread at all elevations and aspects.
Avalanche Summary
Natural and artillery controlled avalanches to size 4 were observed in the highway corridor yesterday. The biggest avalanche was Macdonald West Shoulder failing simultaneously in 4 start zones, ripping through mature timber and running to the valley floor. We did not patrol the backcountry yesterday, but suspect the same results there.
Confidence
Due to the number of field observations
Problems
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.