Forecast new snow and wind will continue to develop storm slabs over surface hoar and crusts. Expect remote triggering and wide fracture propagations resulting in large avalanches. Conservative terrain selection is essential.
Confidence
Moderate - Intensity of incoming weather systems is uncertain on Saturday
Weather Forecast
5-10 cm forecast overnight with freezing levels remaining above 1500 metres and strong southwest winds. Another 5-10 cm during the day on Saturday with strong southerly winds and freezing levels creeping up to 1800 metres. There is a chance of sunny periods in the late afternoon when a brief break between storm pulses moves across the region. Warm, wet, and windy on Sunday. A weak ridge should drop freezing levels to valley bottoms by Monday morning, before broken or scattered cloud allows the sun to bring the freezing level back up to 1500 metres during the day.
Avalanche Summary
Reports from Thursday that several natural size 2.0 slab avalanches and one size 3.0 storm slab were suspected to have released on the late February surface hoar layer. There was also a report of a size 3.0 avalanche remotely triggered by a skier that was 20 metres away. On Wednesday, there were reports of a few natural slab avalanches up to size 2.5 failing on the late Feb. sun crust/surface hoar combo. These slides were from S-E aspects at treeline and showed impressive propagation. Reports from Tuesday include more evidence of the widespread natural avalanche cycle early Monday, with avalanches up to Size 2.5.
Snowpack Summary
30-60cm thick storm slabs are bonding poorly to a crust on previously sun-exposed slopes and surface hoar (February 27th or late February) on shady and sheltered slopes. Thicker and touchier wind slabs are lurking throughout exposed terrain at and above treeline. A couple of sun crusts might exist in the upper 50-70cm on southerly aspects. The surface hoar and/or crust layer which was buried February 10 is now down around a metre and is still producing isolated hard sudden results in snowpack tests. Large cornices will be getting weak with warmer temperatures.
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.