Storm slabs continue to be a concern for human triggering. Storm slabs in motion may step down to buried persistent weak layers resulting in large and dangerous avalanches.
Confidence
Moderate - Forecast snowfall amounts are uncertain
Weather Forecast
Overnight: Flurries or light snow resulting in 3-5 cm of new snow combined with moderate southwest winds. Tuesday: Mostly sunny with moderate westerly winds and alpine temperatures around -15. Wednesday: overcast with light snow resulting in 3-5 cm of new snow combined with light southwest winds. Thursday: A mix of sun and cloud with increasing cloud and wind late in the day as the next system moves into the region.
Avalanche Summary
On Sunday, natural storm slab avalanches were reported up to size 3.0, and explosives control resulted in storm slab avalanches up to size 3.0.
Snowpack Summary
Just under a week of stormy weather has delivered a wide ranging 25-70 cm of new snow to the region, with the bulk of the new snow arriving over Friday night and blanketing localized pockets with up to 40 cm. The new snow has buried widely reported faceted surface snow as well as surface hoar reported at about 3mm on shaded aspects before the storm. Moderate to strong southerly winds accompanied the new snow, promoting the formation of touchy storm slabs at all elevations. Multiple reactive shears have recently been reported within this recent storm snow. Now roughly 40-90 cm below the surface you'll likely find the mid-February interface which is composed of a thick rain crust up to about 2000 m, sun crusts on steep solar aspects, and spotty surface hoar on shaded aspects. This layer has produced easy results in recent snowpack tests and has proven especially reactive on steep solar aspects. Several deeper weak layers also remain a concern, including the February 3rd surface hoar layer (50-130 cm deep), and the mid-January surface hoar layer in the northern Purcells (about 130 cm deep). Basal facets may still be reactive in shallow, rocky start zones.
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.