Storm slabs may continue to be reactive to human triggers, and deep persistent weak layers continue to be a concern.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
Tonight: Overcast with a chance of flurries combined with moderate westerly winds and freezing at valley bottoms. Wednesday: Mix of sun and cloud with moderate southwest winds and daytime freezing levels at 1200 metres. Thursday: Temperatures trending colder as the arctic air descends, combined with light southeast winds and 3-5 cm of new snow. Friday: A few more cm of new snow, light winds, and alpine temperatures near -20.
Avalanche Summary
A natural cycle during the storm up to size 1.5 was reported on Tuesday from the Limestone area. Cornice growth was reported to be significant and may have triggered at least one of the storm slabs. Also keep in mind that the deep persistent slab problem is a low probability/high consequence scenario that warrants extra caution around large open slopes, especially in shallow snowpack areas. There was a report today of a size 3.5 avalanche at Mt Harmer in the Lizard/Flathead region that released on or stepped down to the deep weak layer near the ground.
Snowpack Summary
On Limestone ridge today there was 15 cm of new storm snow at 2100 metres elevation. The height of snow on the ground was 160 cm and the foot penetration was 20 cm. The December facets were down 70 cm at this location and produced hard sudden collapse compression test scores in snow profile tests. This elevation is above the extent of the recent rain crust that extends up to about 1900 metres. A deeply buried weak layer of facets (sugary snow) continues to be a concern in most areas of the region. If triggered this deep weak layer may result in very large avalanches down to near the ground.
Problems
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.
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.