Regions
Kootenay Boundary.
The storm may be over, but we're not out of the woods yet. Forecasted warming will have a continued destabilizing effect on the snowpack with the potential for very large avalanches. Conservative terrain selection remains critical.
Confidence
Poor - Freezing levels are uncertain
Weather Forecast
Synopsis: Unseasonably mild Pacific air moves into the region in the wake of the exiting storm system. The models are currently showing an Above Freezing air mass between 1200m and 3200m with temps between 0c and +4c on Wednesday and Thursday. Wednesday: Freezing Level: Valley Bottom, Above Freezing Layer 1200 to 3200m; Precipitation: Nil; Wind: Treeline: Light, SW | Ridgetop: Moderate, W.Thursday: Freezing Level: Valley Bottom, Above Freezing Layer 1500 to 3200m; Precipitation: Nil; Wind: Treeline: Light, SW | Ridgetop: Moderate, W.Friday: Freezing Level: Valley Bottom; Precipitation: Nil; Wind: Treeline: Light, Variable | Ridgetop: Moderate, NW.
Avalanche Summary
On Monday storm slabs ran on all aspects/elevations to size 2.5. Many of the reported avalanches started in the storm snow and then stepped down to the mid-December crust/ surface hoar interface.
Snowpack Summary
30 - 60 cm fell across the region over the last 48 hours with moderate southwest winds shifting these accumulations into deeper deposits in lee terrain. Warm temps, with the potential for a little rain will likely keep the storm storm slab sensitive to light triggers. The new snow overlies a variety of old surfaces which include heavily wind-affected surfaces in exposed locations, faceted powder and buried surface hoar in sheltered terrain. Up to 100 cm below the surface, you'll likely find a touchy weak layer of surface hoar sitting on a thick rain crust. This widespread persistent weakness exists at all elevation bands, and continues to be the primary layer of concern for the region. With the recent load of storm snow, I expect this layer to remain active with the potential for large and destructive avalanches. At the base of the snowpack, a crust/facet combo appears to have gone dormant for the time being.
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.