Regions
Northwest Coastal.
The likelihood of triggering an avalanche will increase with the snow and wind forecast for Monday.
Weather Forecast
MONDAY: snow starting overnight with 10-15cm expected by the end of the day, light to moderate southwesterly winds, 1000m freezing level. TUESDAY: light to moderate snowfall continues, moderate to strong westerly winds, 800m freezing level. WEDNESDAY: flurries, light and variable winds, 800m freezing level.
Avalanche Summary
New snow may cover lingering wind slabs that are still be reactive to human triggering on Monday. Several natural and artificially triggered cornice collapses in the last week have caused large avalanches in the alpine. On Thursday, a natural size 3.5 persistent slab avalanche was reported in Bear Pass. This avalanche occurred on a northwest aspect at 1350m elevation and released on the late-February surface hoar layer.
Snowpack Summary
Ongoing light snowfall and southerly wind have formed hard or soft wind slabs at treeline and in the alpine. Moist snow can be found at lower elevations and on solar aspects. A layer of surface hoar or facets down 70-100cm can be found in areas north of Stewart sitting below Marchs accumulated storm snow. Recent avalanche activity suggest that we may be approaching a critical load on this interface. An older crust/facet layer buried in early February can now be found around 80-120 cm below the surface although this layer has been dormant recently. Cornices are reported to be large and fragile.
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.
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.
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.