Regions
Northwest Coastal.
Storm slabs continue to develop at higher elevations and will be reactive to human-triggering on Wednesday. Conservative terrain choices remain critical.Use extra caution on steep south facing slopes in the afternoon if the sun is out in full force.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Another 5-15cm is expected Tuesday night before things dry out on Wednesday morning. Sunny breaks are possible on Wednesday and alpine winds are expected to be light from the east. Freezing levels are forecast to be 700-1000m. The next storm pulse is expected to arrive Wednesday evening. There is some model uncertainty regarding precipitation amounts and the region could see 5-25cm by Thursday afternoon. Freezing levels are expected to be around 700m on Thursday with light southwest winds. Light snowfall is expected to continue on Friday.
Avalanche Summary
On Monday, several natural wind slab avalanches size 1.5-2 were reported around Terrace. These were mainly on northeast aspects and the average slab thickness was 35cm. A size 1 storm slab was also skier triggered in the Terrace area on a steep convex roll. North of Stewart, a natural cornice failure triggered a size 2.5 deep persistent slab avalanche on a southeast aspect at 2500m. This occurred in steep unskiable terrain. Explosives in the same area triggered several wind slab avalanches size 1-1.5 as well as one size 2.5. These were mainly on north through east aspects between 1400 and 1600m, and slab thickness varied from 15-60cm. Storm slabs and wind slabs are expected to remain sensitive to human-triggering on Wednesday. If the sun comes out, natural avalanches are possible on steep south facing terrain features. 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. Moist snow was reported below 1100m on Sunday and wet snow below 800m. 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 70-200 cm and remains a concern for commercial operations. This layer seems the most reactive in the north of the region and there have been several large avalanches reported in the past week 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.