Regions
Northwest Coastal.
Extra caution is needed in areas in the northern part of the region where buried persistent weaknesses remain active.
Confidence
Moderate - Intensity of incoming weather systems is uncertain on Monday
Weather Forecast
10-15cm of new snow is expected to fall between Sunday night and Monday afternoon. A mix of sun and cloud is forecast for Tuesday while another 5-10cm of snow is possible on Wednesday. Ridgetop winds should be light to moderate and easterly on Monday, light on Tuesday, and moderate and southerly on Wednesday. The freezing level should sit at about 1300m for the forecast period.
Avalanche Summary
On Saturday a natural cornice fall triggered a size 2.5 slab in northeast facing alpine terrain. Although observations were limited due to weather, the slab is thought to have failed on a crust from late February. No other recent avalanches have been reported.
Snowpack Summary
A dusting of new snow overlies wind slabs in exposed high elevation terrain and settled dry (and faceting) powder on shaded and sheltered slopes. Lower elevations and south aspects have been going through daily melt-freeze cycles. In these areas, new accumulations likely overlie a crust or moist snow. A layer of surface hoar or melt-freeze crust buried on March 3 is down 70-100cm and has been recently reactive in the areas north of Stewart. An older crust/facet layer buried in early February can now be found down over a meter but this layer has been dormant for several weeks.
Problems
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.
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.