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 light convective flurries and light easterly winds. Alpine temperatures around -15 C.FRIDAY: Cloudy with 5-10cm of fresh snow accompanied by light to moderate southerly winds. Alpine temperatures reaching -10 C.SATURDAY: Cloudy with 3-5cm of fresh snow accompanied by light to moderate southerly winds. Alpine temperatures reaching -5 C.
Avalanche Summary
No new avalanches were reported on Tuesday. Reports from Monday include several natural avalanches up to Size 1.5 out of extreme terrain after brief sun-exposure, and one natural Size 2.5 on a northeast aspect. On Sunday we had one report of a natural cornice fall size 2.0 and a skier accidentally triggered a size 1.0 avalanche. Storm slab avalanches were widespread on Saturday up to size 2.5.
Snowpack Summary
45-70 cm of new snow fell in the past week, which is bonding poorly to faceted snow, thin sun crust on steep southerly aspects, as well as surface hoar in sheltered areas in some parts of the region. The snowfall was accompanied by moderate to strong southerly winds, which promoted the formation of touchy storm slabs at upper elevations. About 60-120 cm below the surface you'll likely find the mid-February persistent weakness, which is composed of a thick rain crust up to about 1700 m, sun crusts on steep solar aspects, and spotty surface hoar on shaded aspects. Recent reports have suggested greater reactivity on this layer at lower elevations where the rain crust is more prominent. Professionals in the region have also warned that a 'threshold load' has been reached above this interface, particularly where it presents as a crust. Some lingering surface hoar layers from January are now down a metre or slightly more and the weak mid-December facets are down about 1.5 metres. These deep persistent weaknesses still have the potential to react to human triggers or smaller storm slab avalanches.
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.