Highly unpredictable and practically unsurvivable deep persistent slab avalanches will likely remain a concern for the foreseeable future. Continued conservative terrain selection is crucial for safe travel.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Saturday: Mainly cloudy with sunny periods and isolated flurries bringing up to 10cm of total accumulation. Freezing level around 1500 m and Moderate SW alpine wind. Sunday: Unsettled conditions with snow flurries bringing another 5-10cm to some areas. Freezing level around 1500 m and moderate SW alpine wind. Monday: A mix of sun and cloud with isolated light flurries. Freezing level around 1500 m and light wind.
Avalanche Summary
Reports from Thursday include isolated dry loose sluffs in steep terrain. Recent very large deep persistent slab avalanches in the North Columbias and neighboring regions highlight a low probability, but high consequence avalanche problem. These highly destructive and largely unpredictable slab avalanches are expected to be isolated, but certainly possible anywhere at anytime.
Snowpack Summary
Weaknesses within and under the 40-70 cm of rapidly settling recent storm snow are currently reactive to light triggers, especially on wind-loaded slopes. Three persistent weaknesses contribute to a highly variable, fundamentally unstable, complex snowpack with step-down potential. The mid-March sun crust/surface hoar layer down 50-70cm still has potential for human-triggering in isolated areas. The early-March crust/facet/surface hoar layer down around 80-120cm has become less susceptible to human-triggers, but still has the potential to produce large avalanches, and we continue to see fractures stepping down to this layer. The mid-February crust/facet/surface hoar layer is typically down at least 1.5m and direct triggering has become unlikely. However, large loads like cornices or smaller avalanches stepping down can still trigger this layer and produce very large avalanches.
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.
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.