Very persistent weak layers buried deep in the snowpack remain a major concern and may 'wake-up' with the weight of the new snow. Conservative terrain selection remains crucial at this time.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
A low pressure system will continue to produce precipitation on Wednesday. Unsettled conditions are expected for Thursday and Friday. Tues. Night: Precipitation: 8-10mm, freezing level: surface, ridgetop wind: light-moderate S-SWWednesday: Precipitation 3-6mm, freezing level am: surface pm: 500-1000m, ridgetop wind: light variableThursday: Precipitation 2-4mm, freezing level am: surface pm: 500-1000m, ridgetop wind: light variableFriday: Scattered precipitation 1-3mm, freezing level am: surface pm: 800-1200m, ridgetop wind: light SW-W
Avalanche Summary
Reports from Monday include a natural size 2 cornice triggered avalanche and sluffing from steep terrain. In the North Columbia region, a very large size 4 avalanche released on a deep weak layer resulting in the destruction of mature timber and 1 fatality.An early report from Tuesday includes two size 1.5 natural avalanches releasing down around 50cm.
Snowpack Summary
The upper snowpack may have various non-persistent weak layers within the recent storm snow. This includes small surface hoar in sheltered areas, thin sun crusts on solar aspects, rain crusts at lower elevations, and/or small facets. These layers are not widespread and are not expected to persist very long but they continue to produce small slab avalanches. Weak cornices and stubborn wind slabs may also be a concern in some areas.There are three persistent weak layers that remain a concern. The mid-March sun crust/surface hoar layer is down 40-70cm. Recent snowpack tests suggest that the reactivity of this layer is becoming more isolated but many areas still have the potential for human-triggering. The early-March crust/facet/surface hoar layer is down around 80-120cm. Human-triggering has become less likely but the layer still has the potential to produce large avalanches and we continue to see activity 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 triggers like cornices or smaller avalanches 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.