Hazard may go higher than forecast in the afternoons with solar warming.Cornice failures can trigger large avalanches.
Confidence
Fair - Timing or intensity of solar radiation is uncertain
Weather Forecast
Synopsis: The weather pattern is generally stable with a weak northwesterly flow. Some convective flurries may happen in the afternoon.Wednesday: Sunny with a few cloudy periods. Freezing level s at valley bottom overnight, rising to 1500m in the afternoon, winds generally light occasionally gusting to moderate from the south west.Thursday: Cloudy with flurries, trace of precipitation. No overnight freeze, freezing levels rise to 1800m in the afternoon, winds generally light, occasionally gusting to moderate from the south west.Friday: Sunny with cloudy periods and some flurries. No overnight freeze and freezing levels may go up to 2100m, winds light to moderate from the south west.
Avalanche Summary
A low probability, but high consequence avalanche problem plagues the Columbia regions. These highly destructive and largely unpredictable avalanches are expected to be isolated, but certainly possible anywhere at any time. Avalanches continue to step down to old, deep weak layers. Numerous size 1.5 and 2 avalanches were reported yesterday as well as a size 3 and a 3.5 avalanche! The smaller avalanches were, for the most part, in the recent storm snow, but the big ones were on south and south west aspects and most likely the result of solar heating and generally rising spring temperatures. Sluffing in steep terrain is being reported on all aspects.
Snowpack Summary
Recent warm temperatures have resulted in good settlement and bonding of the storm snow from earlier in the week. Reports of human-triggered avalanches have diminished. Three persistent weaknesses contribute to a highly variable snow pack with the possibility of triggering deeper instabilities. The mid-March sun crust/surface hoar layer down 50-80cm 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 triggering by light loads, but still has the ability to produce large avalanches, and we continue to see avalanches 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
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.