New snow amounts within this region will be highly variable on Saturday with more snow in the south. Hazard may be one step lower in areas that receive less than 25 cm of new snow.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Saturday
Weather Forecast
SATURDAY: 20-30 cm of new snow / Strong southwesterly ridgetop winds / Freezing level rising to around 1000m.SUNDAY: 20-30 cm of new snow / Strong southwesterly ridgetop winds / Freezing level rapidly rising to around 1800m.SUNDAY NIGHT: 15-25 cm of new snow / Strong southwesterly ridgetop winds / Freezing level around 1800m.MONDAY: 25-30 cm of new snow / Strong to extreme southwesterly ridgetop winds / Freezing level around 1800m.
Avalanche Summary
Throughout this region there has been evidence of a natural avalanche cycle up to size 3 that occurred within the storm snow on Wednesday. More notable are the numerous explosive triggered, very large avalanches up to size 3.5 that where reported near Whistler on Thursday. These avalanches started in the storm snow and then stepped down to the early January and even the mid-December crusts which resulted in very large avalanches running full path. They occurred on northwesterly aspects below rocky ridgetops at around 1950 m. Although these layers are over 200 cm beneath the snow surface, a large trigger such as a falling cornice or a smaller human triggered avalanche could very likely "step-down" to these layers resulting in large and destructive avalanches.
Snowpack Summary
15-20 cm of new snow on Thursday morning brings recent storm totals to around 60-80 cm which continues to be redistributed by strong southeasterly winds. Last weekend's storm created dense storm slabs lurking in lee and cross-loaded features. Impressive cornice growth has also been observed on the lee side of ridgetops. 200-250 cm below the surface you'll find a crust that was buried early January and is present at all elevations. Below this there is another crust which was buried in mid-December. Recent explosive control work has triggered storm slab avalanches that have then "stepped-down" to both of these crusts, resulting in very large avalanches running full path. Although these layers are over 200 cm beneath the snow surface, a large trigger such as a falling cornice or a smaller human triggered avalanche could very likely "step-down" to these layers resulting in very large and destructive 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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
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.