Regions
Northwest Inland.
Uncertainty about deep persistent slabs warrants a conservative approach to terrain after the storm.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
FRIDAY: Cloudy, light southwest winds, freezing level dropping with alpine temperatures around -5 C.SATURDAY: Cloudy with isolated flurries, moderate southeast winds, alpine temperatures around -8 C.SUNDAY: Cloudy with sunny periods, light southeast winds, alpine temperatures around -8 C.
Avalanche Summary
On Wednesday, several loose wet avalanches were reported from steep rocky terrain at lower elevations where rain fell on snow. Storm slabs sitting on a weak layer of surface hoar and/or a crust are likely easy to trigger at higher elevations. Full depth avalanches continue to be a concern due to the weak faceted snowpack below old hard wind slabs. This deep persistent weak layer may take prolonged warm weather with consistent snowfall to settle and bond.
Snowpack Summary
20-25 cm of new storm snow has been transported by southwest winds and now sits on a mix of old surfaces left behind after the recent cold and windy weather. Expect the new storm snow to bond poorly where it is sitting on a weak layer of buried surface hoar and/or a hard old wind polished surface. The freezing level recently reached 1000 m, and below this level the snowpack is likely moist and will eventually form a hard crust. Most areas continue to have a shallow and weak snowpack that is about 120-150 cm at treeline. In general, the snowpack above treeline has been heavily wind effected; this has resulted in hard wind slabs above weak faceted snow.
Problems
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.
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.