Storm slabs are primed for people triggering large avalanches, and have the potential to step down to deeper persistent weaknesses resulting in very large and dangerous avalanches.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Friday
Weather Forecast
THURSDAY: A mix of sun and cloud with increasing cloud and wind late in the day as the next system moves into the region.FRIDAY: Cloudy with 10-20cm of fresh snow accompanied by light to moderate SW winds. Alpine temperatures reaching -5 C.SATURDAY: A mix of sun and cloud with flurries bringing another 2-5 cm. Light to moderate SW winds and alpine temperatures around -5 C.
Avalanche Summary
Reports from Tuesday include one natural Size 2.5 on a southeast facing alpine slope in response to intense cross-loading as well as numerous other natural dry loose and soft slab avalanches up to Size 1 on lee aspects. Explosives control produced wind slab and persistent slab avalanches up to Size 3. One of which was 100-300cm deep, failing on the deep persistent facet/crust weakness buried in November.
Snowpack Summary
A wide ranging 50-100 cm of fresh snow has fallen in the past week, which is bonding poorly to weak faceted snow and small surface hoar. Southerly winds have formed touchy slabs at all elevations with multiple weaknesses within and under this recent storm snow. The persistent weakness buried mid-February is now down 60-120 cm and composed of a thick rain crust as high as about 2000 m, sun crusts on steep southerly 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 southerly aspects. Several deeper persistent weaknesses also remain a concern, including surface hoar buried early-February (80-140 cm deep), and mid-January (over a metre deep primarily in the northern Purcells ). 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.