The region will continue to see substantial snowfall amounts into Friday, which is rapidly loading buried weak layers. Travel in avalanche terrain is not recommended.
Weather Forecast
THURSDAY NIGHT: Cloudy with snowfall, accumulation 10 to 20 cm, moderate to strong southwest winds, alpine temperature -4 C, freezing level 1400 m.FRIDAY: Cloudy with snowfall, accumulation 5 to 10 cm, moderate to strong southwest winds, alpine temperature -3 C, freezing level 1500 m.SATURDAY: Mix of sun and cloud, light southwest winds, alpine temperature -4 C, freezing level 1000 m.SUNDAY: Cloudy with light snowfall, accumulation 5 to 10 cm, light to moderate southwest winds, alpine temperature -3 C, freezing level 1000 m.
Avalanche Summary
Storm slabs were reactive to skier traffic and explosives on Thursday, producing large (size 2) avalanches with depths around 20 to 40 cm. It is possible that a natural avalanche cycle will occur as the storm continues Thursday night and into Friday. Avalanches could run fast and far, well into runout zones.
Snowpack Summary
A strong storm is dumping snow in the region Thursday into Friday. Around 20 to 40 cm of snow has fallen over most of the region up to Thursday evening (almost 70 cm in the far north!), and another 20 to 30 cm is expected Thursday night into Friday. All this snow has likely formed touchy storm slabs. The snow is falling with strong westerly winds, so expect deeper deposits in lee terrain features near ridges.All this snow is loading a weak layer that formed during the dry spell in early December. It is around 80 to 120 cm deep. The layer is composed of weak and sugary faceted snow, feathery surface hoar crystals, and on south aspects also a sun crust. Another similar weak layer is buried around 120 to 150 cm deep. The base of the snowpack has weak faceted layers at alpine and treeline elevations. These basal weak layers have been producing large avalanches as recently as December 31. Human triggering any of these layers is most likely on slopes that didn't previously avalanche and on slopes that have variable snowpack depth, such as rocky alpine features.
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.
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.