Regions
Northwest Coastal.
Recently formed storm slabs will continue to be reactive to human-triggering on Friday and conservative terrain selection remain very important.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain on Friday
Weather Forecast
Unsettled conditions are expected for Friday with periods of light snowfall and sunny breaks. The far south of the region could see 5-10cm of accumulation in the morning. Alpine winds are expected to be moderate from the southeast and freezing levels are forecast to reach around 1000m in the afternoon. Periods of light snowfall are expected on Saturday with moderate southerly winds in the alpine and freezing levels around 700m. A mix of sun and cloud is currently forecast for Sunday morning with a weak storm pulse arriving Sunday afternoon.
Avalanche Summary
On Wednesday, numerous storm slab and wind slab avalanches up to size 2 were reported from the areas north of Stewart. This includes two skier triggered avalanches on northeast aspects at 1400 and 1700m elevations. Explosives triggered numerous slab avalanches primarily on north through east aspects between 1400 and 1600m. On Tuesday, a natural storm cycle up to size 2.5 was reported north of Stewart. Explosives also triggered numerous storm and wind slab avalanches in the same area. No new avalanches have been reported around Terrace but it appears that there haven't been any observers in the field in the last couple days. Recent storm slabs are expected to remain reactive to human triggering on Friday. In the far north of the region, deeply buried persistent weak layers may still be reactive to large triggers such as a cornice fall or a smaller avalanche stepping down.
Snowpack Summary
50-100cm of recent storm snow has accumulated over the past week and overlies a melt freeze crust which extends up to around 2000m elevation. This recent snow is reported to be bonding well to the crust and all the recent avalanche activity has been failing within the storm snow. Moist snow was reported below 1200m on Wednesday, wet snow below 800m, and below threshold below 500m. Recent wind has redistributed the surface snow at higher elevations forming wind slabs in leeward features. The early January surface hoar/facet layer is down 100-200 cm and remains a concern for commercial operations. This layer seems the most reactive in the north of the region and there were several large avalanches reported last week that were releasing on this layer.
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.
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.