Storm slabs continue to be a concern. Wide fracture propagations are possible where the storm snow is sitting on a weak buried layer of surface hoar.
Confidence
Moderate - Freezing levels are uncertain
Weather Forecast
Freezing level dropping down close to valley bottoms overnight with continued light precipitation. Light precipitation and cloudy skies on Monday combined with light southerly winds and freezing levels climbing up to about 1700 metres. A spring diurnal freeze/thaw cycle setting up for Tuesday and Wednesday with freezing levels dropping to valley bottoms overnight and then rising up to 1500 metres during the day. The next major storm pulse developing on Wednesday night.
Avalanche Summary
Natural wind slabs and storm slabs up to size 2.5 were reported on Saturday. Natural and explosives controlled avalanches up to size 2.5 were reported on Friday. One size 3.0 avalanche released sympathetically to a cornice falling 100 metres away. No new natural avalanches reported on Thursday. One skier remote size 2.0 storm slab avalanche was reported from an east aspect in the alpine.
Snowpack Summary
Strong southerly winds have developed wind slabs in the alpine and at treeline. The 20-40 cm of recent storm snow is bonding poorly to a melt-freeze crust on previously sun-exposed slopes and lower elevation terrain, and/or a layer of surface hoar on shady and sheltered slopes at treeline elevations. The surface hoar and/or crust layer buried in mid-February is now down 50-80cm. This layer was less reactive over the past week with cooler temperatures. Large weak cornices remain concerning and have recently triggered persistent slabs on slopes below. The early January surface hoar/facet layer is typically down 70-120cm. Triggering an avalanche on this layer has become unlikely but it still has isolated potential to produce very large avalanches with a heavy trigger. In general, the lower snowpack is well settled and strong, apart from some thin snowpack areas where basal facets exist.
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.
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.