Periods of heavy snow and strong winds will create dangerous avalanche conditions on Friday. Keep slope angles low and overhead hazards at a distance.
Beneath the snow from last weekend's storm, a layer of surface hoar and facets may linger as a persistent slab problem in isolated areas. A crust/facet combo from early November is more widespread and can be found closer to the ground. Easy-to-trigger avalanches in the new storm snow have the potential to step-down to these deeper layers.
Periods of heavy snowfall combined with strong winds are expected to form reactive storm slabs. These slabs will be more sensitive to triggering and larger in size in areas where the wind has transported the snow into deeper, more cohesive drifts. Natural triggers from heavy snowfall, wind, and cornice falls are a concern with this rapid load.
A powerful storm moves in overnight and continues into the weekend.
Thursday night: Overcast, 15-25 cm of snow above 1100 m, strong southwest winds.
Friday: Overcast, 20-30 cm of snow above 1200 m, strong southwest winds with extreme gusts, freezing level rising through the day to around 1500 m.
Saturday: Overcast, 45-65 cm of snow above 1500 m, strong southwest winds with extreme gusts.
Sunday: Mostly cloudy, 20-30 cm of snow above 1400 m, strong southwest winds with extreme gusts.
Several small wet loose avalanches (up to size 1.5) ran during the peak warming on Wednesday, primarily on steep south-facing slopes. Near Shames, several small wind slabs (up to size 1.5) released naturally in loaded alpine features.
In the aftermath of last weekend's storm, a series of notable avalanches have been reported further north in the region. These were very large deep persistent slab avalanches (size 3-3.5) that were both explosive-triggered and natural releases that ran on weak snow near the base of the snowpack. The extent of this snowpack structure in the region is not well-known.
Observations in this region are very limited. If you're out in the field, please consider sharing a photo or description via the Mountain Information Network (MIN).
A powerful storm is forecast to make landfall Thursday night. Snow totals ranging from 35-55 cm are expected to accumulate by Friday afternoon, with higher amounts forecast for the southern half of the region. Strong southwest winds will rapidly load the new snow into reactive slabs. The new snow is accumulating above a melt freeze crust and hard, wind-affected snow and may not bond well.
There is high uncertainty about the distribution and sensitivity of persistent weak layers in the snowpack, which is compounded by the rapid loading expected from the incoming storm. A layer of buried surface hoar and facets that formed in late November may still linger in isolated areas. A widespread crust/facet layer from early November still warrants concern. Further north in the region, we're learning that weak snow developing near the ground has produced very large deep persistent slab avalanches. The bottom line is that easy-to-trigger storm slabs have the potential to step-down to deeper layers and trigger very destructive avalanches.
The total snowpack depth is roughly 150 to 200 cm at upper treeline and alpine elevations.
