Periods of sun over the next few days will increase the danger on south facing slopes. Solar radiation could be enough to trigger stubborn persistent slabs or cause cornice failures. Conservative route selection remains crucial at this time.
Confidence
Fair - Timing or intensity of solar radiation is uncertain
Weather Forecast
A ridge of high pressure will build on Monday and should persist until at least Wednesday. A weak disturbance may cross the region on Wednesday resulting in increased cloud cover and isolated light showers. Monday: A mix of sun and cloud, freezing level am: 1100m pm: 1600m, ridgetop wind: light SW-WTuesday: Mostly sunny, freezing level am: 400m pm: 1800m, ridgetop wind: light NWWednesday: A mix of sun and cloud, freezing level am: 600m pm: 1800m, ridgetop wind: light SW-W
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. The most recent was a natural size 3 in the Cariboo region on Thursday. The S Columbia had one step down on Tuesday and the N Columbia on Monday. While these layers are typically dormant, they are expected to become reactive again with the new loading or the upcoming strong solar inputs.Reports from Saturday for the South Columbia include isolated natural and human triggered avalanches up to size 2 in the recent storm snow down 20-30cm. Loose sluffing from steep terrain features was also reported.
Snowpack Summary
New storm snow has formed a soft slab which is typically 20-30cm thick. On south aspects, the storm snow overlies a sun crust, but due to recent warm temperatures, the layers are generally bonding well. In isolated areas, the storm slab is reported to be quite reactive to triggering. Stiffer wind slabs may exist in immediate leeward features on NW through E aspects.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 50-80cm 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 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.