Danger may 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 - Forecast snowfall amounts are uncertain
Weather Forecast
Synopsis: A Pacific frontal system approaching the coast will spread clouds and precipitation inland over the next few days.Thursday: Cloudy with flurries, some parts of the region may receive 10 to 20 cm of precipitation. No overnight freeze, freezing levels up to 1400m, Winds moderate, gusting to strong from the south west.Friday: Cloudy with flurries, up to 20 cm of precipitation possible. No overnight freeze and freezing levels may go up to 1600m. Winds light, gusting to moderate from the south west.Saturday: Cloudy with flurries. 5 to 10 cm of precipitation in the forecast for the region, daytime freezing level around 1500m, overnight freezing levels will drop close to valley bottom, winds, light to moderate from the south west.
Avalanche Summary
Reports from yesterday of numerous 1 to 2.5 avalanches on north and east aspects, as well as cornice failures triggering avalanches. Neighboring regions have reported large avalanches on north and east aspects. There is concern that weak layers will become more reactive with rising temperatures and solar warming. This spring, a low probability, high consequence avalanche problem plagues the Columbia regions. Highly destructive and largely unpredictable avalanches are possible right now.
Snowpack Summary
The last storm produced 30-40cm of new snow in parts of the forecast region. Reports suggest there has been moist snow up to 1800m on all aspects and perhaps higher on south facing slopes. Stiff wind slabs exist in lee terrain on NW through E aspects at tree line and into the alpine.Three persistent weaknesses now contribute to a highly variable, complex snowpack.A mid-March sun crust/surface hoar layer down 50-80cm has potential for human-triggering in select locations. Newly formed wind slabs may have been formed on this layer. An 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 have reports of avalanches stepping down to this layer. A mid-February crust/facet/surface hoar layer that we've been watching is now down at least 1.5m and although direct triggering has become less likely, a large load like a cornice failure or smaller avalanches gaining mass could trigger this layer and produce very a large and destructive avalanche.
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.