Storm slabs are highly reactive to human-triggering and conservative terrain selection remains critical for the next few days. Extra caution is required on south-facing slopes in the afternoon if the sun pokes out.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Saturday
Weather Forecast
FRIDAY: A mix of sun and cloud in the morning with light precipitation starting late in the day. Freezing levels reach 1800-2000 m with moderate to strong southwesterly ridgetop winds. SATURDAY: Cloudy with flurries bringing another 3-5cm, followed by sunny breaks. Freezing levels steady around 1800-2000 m and moderate to strong southwesterly ridgetop winds. SUNDAY: A mixed bag sun, cloud, and flurries. The freezing level is around 1800 m and winds ease to light.
Avalanche Summary
Reports from Tuesday and Wednesday include a few explosives and rider triggered storm slab avalanches up to Size 2. Wind and storm slabs should increase in size and likelihood throughout the week with continued light snow and moderate ridge winds. Late last week there were a couple reports of failing cornices triggering persistent slabs up to 60 cm deep on slopes below.
Snowpack Summary
15-30 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. Thicker and touchier fresh wind slabs are lurking in exposed lee and cross-loaded terrain. The surface hoar and/or crust layer buried in mid-February is now down 40-70cm. This layer was less reactive over the weekend 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
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.