Regions
Northwest Inland.
As snow accumulates on Monday, the Avalanche Danger will rise. Expect reactive new storm slabs to form.
Confidence
Fair - Due to the number of field observations
Weather Forecast
Another pacific frontal system will bring snowfall to the coast on Monday. Lingering flurries are expected on Tuesday before a weak dry ridge of high pressure builds on Wednesday. Sunday night and Monday: Up to 15cm of snow / Strong southwest winds / Freezing level at valley bottom Tuesday: Very light snowfall and broken cloud / Moderate southwest winds / Freezing level at valley bottom Wednesday: Broken cloud / Moderate southwest winds / Freezing level at valley bottom
Avalanche Summary
Several storm slabs to size 2.5 were reported in the wake of Thursday night's storm. With more snow and wind on the way on Sunday night and Monday, I'd expect continued storm slab activity. It's important to keep in mind, a surface avalanche in motion may be what it takes to trigger deeper, more destructive persistent slab avalanches.
Snowpack Summary
Prior to the weekend, moderate to locally heavy snowfall and strong southwest winds built deep storm slabs which may still be reactive in wind-exposed terrain. More snow and wind forecast for Sunday night and Monday will add to this ongoing storm slab problem.About 70cm below the surface, you'll likely find a rain crust which was buried on January 26. Limited observations suggest the overlying slab may have a reasonable bond at this interface.A rain crust and/or surface hoar layer buried mid-January may be found at variable depths (in some areas over 100cm below the surface). This layer has reportedly gained significant strength, although I would keep it on my radar especially at higher elevations or in the far north of the region. Near the base of the snowpack is a crust-facet combination buried in November. This deep persistent weakness was recently reactive, and may still be touchy in the far north of the region. Wherever you are, I'd be cautious of this deep and destructive layer. Possible triggers include thin spot triggering in high elevation terrain, or a cornice fall.
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.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
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.