Danger will increase on south facing slopes during afternoon warming. 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 keep the Cariboos sunny and dry for Tuesday. Unsettled conditions are expected for Wednesday and Thursday resulting in a mix of sun and cloud with localized convective flurries.Tuesday: Sunny, freezing levels am: surface pm: 1400m, ridgetop wind: light W-NWWednesday: A mix of sun and cloud, light scattered flurries possible, freezing levels am: 400m pm: 1500m, ridgetop wind: light SW-WThursday: A mix of sun and cloud, light scattered flurries possible, freezing levels am: 1000m pm: 1800m, ridgetop wind: increasing to moderate S-SW during the day
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 were natural size 3 avalanches in the Cariboo region on Sunday and on Thursday. The S Columbia had one step down last Tuesday and the N Columbia last Monday. While these layers are typically dormant, they may become reactive again with strong solar radiation input.Reports from Sunday include a natural size 3 stepping down to the early-February layer on a NW aspect and a natural size 2.5 storm slab avalanche. Also reported from the weekend was a remotely triggering size 2 slab avalanche that released down 100cm.
Snowpack Summary
The weekend storm snow typically produced 30-40cm of new snow. Reports suggest moist snow up to 1800m on all aspects and higher on south facing slopes. Stiffer wind slabs may exist in immediate leeward features on NW through E aspects in exposed terrain in the alpine.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.