A storm slab sitting on a touchy weak layer means conditions will remain tricky over the next few days. Continue to make conservative decisions and have a safe holiday.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Conditions on Wednesday will be unsettled as the storm is replaced by a ridge of high pressure that will establish itself for Thursday and Friday. The freezing levels on early Wednesday are expected to drop before the precipitation ends so we may see some snow at lower elevations. By Wednesday afternoon the precipitation should have ended but scattered flurries are possible. Freezing levels should drop to around 800m and alpine winds should taper off to light from the NW. Thursday is expected to be mostly sunny with freezing levels around 700m and light alpine winds. Friday will be similar with a mix of sun and cloud, freezing levels around 800m, and light alpine winds.
Avalanche Summary
On Monday, reported avalanche activity was limited to one natural cornice release that did not trigger a slab as well as loose sluffing from steep terrain. On Sunday, explosive control produced storm slabs size 1-2. On Saturday, reports of natural storm slab activity up to size 2 at treeline and in the alpine. Ski cutting and explosive control also produced numerous storm slab avalanches, some of which stepped down to an old crust layer in the middle of the snowpack. The deepest slab released was reported to be around 1m thick. I expect with the new storm loading on Tuesday and Wednesday morning that we will see new storm slab activity throughout the region.
Snowpack Summary
The new snowfall is sitting on the storm snow from the weekend which is typically 40-60 cm deep and sits above the mid-December crust/facet/surface hoar layer. This layer extends at least up to treeline and possibly lower parts of the alpine. Strong winds over the weekend created wind slabs which may still be a concern and current strong winds will create new wind slabs which will likely be touchy. The November crust is deeply buried near the ground and while triggering this layer is generally unlikely, the likelihood of triggering does increase during the storm with the additional weight of the new snow.
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.