A recently formed storm slab and touchy persistent weak layers are making for tricky conditions. Conservative terrain selection is critical.Use extra caution on steep south-facing slopes if the sun is out in force strength in the afternoon.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain
Weather Forecast
A mix of sun and cloud is expected for Sunday with light intermittent flurries possible. Freezing levels are expected to fall to valley bottom overnight and climb to around 1000m on Sunday afternoon. Alpine winds are forecast to be light from the northwest. Light snowfall is forecast for Monday but less than 5cm is expected. Freezing levels are expected to reach around 1000m on Monday and alpine winds are forecast to be moderate from the northwest. Similar conditions are expected for Tuesday with light snowfall, cool temperatures, and moderate northwest winds.
Avalanche Summary
On Friday, widespread natural avalanche activity up to size 3 was reported in throughout the region. This includes both storm slabs releasing down 30-60cm and persistent slabs failing down 70-100cm on the early January surface hoar layer. The storm slabs were failing on all aspects from 2000 to 2500m while the persistent slabs were failing on a variety of aspects around 1500-1900m. One persistent slab was also reported from 2300m on a southwest aspect. A skier accidently triggered a size 2 persistent slab avalanche on an east aspect at 1600m on the early January surface hoar layer down 70-80cm. Ski cutting also produced a few size 1-2 storm slab avalanches typically 20-30cm thick but one was 60cm thick. Natural activity is generally not expected on Sunday but could be possible on steep south facing slopes if the sun comes out in full strength in the afternoon. Recently formed storm slabs and a reactive persistent slab are both expected to be sensitive to human-triggering on Sunday.
Snowpack Summary
Recent storm loading brings storm slab thickness to around 20-40cm and up to 50cm in wind loaded areas. The recently destructive persistent weak layer from early January is now typically down 70-110cm in most places and continues to produce hard but sudden results in snowpack tests. The overlying slab remains primed for triggering and is especially touchy at and below treeline. The lower snowpack is well settled and strong below this layer.
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.