New storm snow will add to the complexity of an already tricky snowpack. Looks like a good day to make conservative terrain choices.
Confidence
Moderate - Intensity of incoming weather systems is uncertain on Tuesday
Weather Forecast
Tuesday Night: 10-15cm of new snow / Strong southwest winds / Alpine temperature of -9Wednesday: Generally overcast skies / Moderate northwest winds / Alpine temperature of -13Thursday: Mix of sun and cloud with scattered flurries / Light and variable winds / Alpine temperature of -17Friday: Overcast skies / Moderate northwest winds / Alpine temperature of -10
Avalanche Summary
On Sunday, a remotely-triggered (from a distance) size 1.5, slab avalanche was observed on a southerly aspect at 1700 m. A weak layer buried in early January was the suspected failure plane. A very large, widespread avalanche cycle up to size 4 was observed last Thursday and Friday. 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.Looking forward, expect a new round of storm slab activity in response to new snow and wind on Tuesday night. Storm loading has the potential to re-activate deeper, more destructive persistent weak layers.
Snowpack Summary
By Wednesday morning I would expect up to 30cm of new snow. Forecast strong ridgetop winds are expected to redistribute the new snow into much deeper, reactive slabs in wind-exposed terrain.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.