A week of heavy snowfall and strong winds have left large storm slabs primed for human triggering. Avoid overhead hazards and stick to mellow terrain.
Confidence
High - Due to the number and quality of field observations
Weather Forecast
SUNDAY: Afternoon flurries with 5-10 cm of new snow, moderate southwest winds, alpine temperatures around -7 C.MONDAY: Cloudy with isolated flurries and trace accumulations, strong southwest winds, alpine temperatures around -10 C.TUESDAY: Cloudy with light flurries picking up in the afternoon, strong southwest winds, alpine temperatures around -10 C.
Avalanche Summary
A widespread natural avalanche cycle occurred on Friday night, with many size 2-3 cornice triggered avalanches on north and east aspects. Explosive avalanche control on Saturday produced size 2 storm slabs and ski cutting produced size 1-1.5 avalanches on wind-loaded features. Natural activity is expected to taper off, but storm slabs will remain primed for human triggering.
Snowpack Summary
Another 20 cm of snow fell on Friday, bringing the three day storm total to roughly 80 cm and the weekly total to almost 150 cm. Strong southwest winds have extensively redistributed the snow and formed large brittle cornices. The new snow is rapidly settling into a denser slab which sits over a variety of old surfaces including surface hoar, facets, stiff wind slabs, sun crusts, and a rain crust below 1900 m. Isolated basal weaknesses may still exist in shallow snowpack areas and appear to isolated to north-northeast aspects in the alpine.
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.