Regions
Northwest Coastal.
Stormy conditions on Tuesday are expected to form new wind slabs and storm slabs at higher elevations. Use a conservative approach to travel and watch for changing conditions throughout the day as the storm progresses.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Stormy conditions are expected Monday night through Wednesday morning. 15-25cm is expected between Monday night and Tuesday afternoon with strong southwest winds in the alpine and freezing levels around 800-1000m. Another 5-15cm is expected Tuesday overnight before a weak ridge of high pressure dries things out on Wednesday. A mix of sun and cloud is expected for Wednesday with freezing levels around 700m and light alpine wind. There is uncertainty for Thursday with one model showing a weak storm pulse and another showing dry conditions persisting.
Avalanche Summary
No new slab avalanches were reported on Saturday or Sunday. Ski cutting on Sunday produced only loose sluffing. On Friday, a skier triggered a size 1.5 wind slab on a northwest aspect at 1600m elevation. The slab was 10-20cm thick and was triggered on a thin, rocky, unsupported slope. Explosives also triggered several size 1.5 cornices on west aspects around 1500m elevation. As the storm progresses on Tuesday, wind slabs will likely form first and if enough snow accumulates, a more widespread storm slab problem could develop. In the far north, 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
Up to 30cm of 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 soft 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.