Recent storm snow has been reactive on steep slopes. The presence of buried weak layers increases the potential size of avalanches.
Confidence
Moderate - Intensity of incoming weather systems is uncertain
Weather Forecast
Friday: 5-10 cm new snow. Treeline temperature around -7C. Westerly ridgetop winds gusting to 50 km/h.Saturday: 2-5 cm new snow. Treeline temperature around -12C. Light variable winds.Sunday: Sunny but cold. Treeline temperatures around -17C. Winds light northeasterly.
Avalanche Summary
On Wednesday, several avalanches were reported in recent storm snow up to size 2.5. Previously, a very large, widespread natural avalanche cycle up to size 4 was observed last week. Slopes of all aspects and elevations ran full path and reached valley floor. In some cases, mature timber was destroyed. Many of these failed on persistent weak layers mentioned in the snowpack discussion. Although natural avalanche activity on these layers has tapered-off, human triggering of very large avalanches remains a very real possibility.
Snowpack Summary
25cm of recent storm snow is settling into a slab in the upper snowpack. Wind data has been sparse, although I suspect strong winds have shifted these accumulations into deeper, reactive slabs in wind-exposed terrain. These accumulations overlie a mix of older wind slabs in exposed higher elevation terrain, a sun crust on steep solar aspects and surface hoar on sheltered slopes.Below the snow surface, several persistent weak layers make up a troublesome snowpack. In the top 1-1.5 m of the snowpack, two surface hoar layers buried in January can be found. Expect to find at least one of these layers on all aspects and elevations.Deeper in the snowpack (120-200 cm deep) is a facet/crust/surface hoar layer from December, most prevalent at and below treeline.Near the base of the snowpack is a crust/facet combo, most likely to be triggered from thin spots in the alpine.All of these layers have produced large avalanches recently. The wide distribution and ongoing reactivity of these layers suggests that avoidance through choosing simple terrain is the best strategy.
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.