A persistent weak layer of buried surface hoar is the sliding layer for human triggered avalanches. This problem results in the avalanche danger staying up at Considerable for the forecast period.
Confidence
Fair - Forecast snowfall amounts are uncertain
Weather Forecast
Flurries or light snow is expected to continue overnight and during the day on Sunday with total accumulations around 5 cm combined with light winds and alpine temperatures around -15. Gradually clearing skies during the day on Monday as cold arctic air moves in from the North and alpine temperatures drop down to about -25. Continued cold and clear on Tuesday.
Avalanche Summary
No natural avalanches reported. Explosives control work resulted in avalanches up to size 2.5 that appear to have run on the mid-December surface hoar that is buried down 40-60 cm.
Snowpack Summary
Storm snow totals 30-70 cm above a very touchy surface hoar layer that was buried mid-December. Below 2100 m this storm slab sits on a thick, solid crust/ surface hoar combo and has been acting as the perfect sliding layer. This persistent slab remains touchy to the weight of a skier or rider, especially in wind effected areas. A hard rain crust with facets from early November is buried over 1 m down and has recently been reactive with larger loads (explosives and smaller avalanches) especially in the Northern part of the region where the snowpack is thinner.
Problems
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.
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.