Regions
Northwest Coastal.
Lingering storm slabs and cornices are expected to become weak with sun exposure and afternoon warming. Use extra caution on sun exposed slopes when the solar radiation is strong.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain
Weather Forecast
Another 5-10cm of snowfall is expected Tuesday overnight with freezing levels around 500m and moderate southwest winds in the alpine. Unsettled conditions are expected for Wednesday as a ridge of high pressure builds over the coast. Cloudy conditions with light flurries are expected for the morning and a mix of sun and cloud is expected for the afternoon. Afternoon freezing levels are expected to be 500-800m and alpine winds are forecast to be light from the northwest. Sunny conditions are expected for Thursday with afternoon freezing levels around 1000m and light outflow winds in the alpine. A mix of sun and cloud is forecast for Friday with freezing levels climbing to around 1500m and moderate outflow winds in the alpine.
Avalanche Summary
On Monday, a ski cut triggered a size 1 cornice release north of Stewart. In the far north, a natural cornice release triggered a size 2.5 persistent slab avalanche on the early March surface hoar layer. Natural sluffing was also reported from steep sun exposed slopes. On Sunday, explosives triggered several winds slabs size 1.5-3 north of Stewart. With ongoing wind loading, wind slabs are expected to be reactive to human triggering on Wednesday. If the sun comes out on Wednesday, cornices and lingering slabs are expected to become weak, and could even fail naturally. Loose sluffing is expected from steep sun exposed slopes.
Snowpack Summary
20-30cm of new snow has accumulated over the last couple days in sheltered areas. Strong southerly winds have redistributed the surface snow forming wind slabs and adding mass to cornices. On south aspects, the recent storm snow may have buried a sun crust. Lower elevations are going through daily melt-freeze cycles. A layer of surface hoar 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
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.
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.