Persistent weak layers are not typical on the South Coast. Backcountry users in the region need to keep in mind the potential for storm slab avalanches to 'step down' well beyond the depth of new snow.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Friday
Weather Forecast
Thursday night: Periods of snow and wet snow bringing 20-40 cm of new snow to higher elevations. Strong to extreme southwest winds. Freezing level around 1500 metres with alpine temperatures around 0. Friday: Continuing snowfall and wet snow bringing 10-20 cm of new snow to higher elevations. Moderate to strong southwest winds. Freezing level to 1400 metres with alpine temperatures around 0. Saturday: Another pulse of wet snow and rain bringing up to 20 cm of new snow to higher elevations. Strong to extreme south winds. Freezing level to 1400 metres with alpine temperatures around 0. Sunday: Continuing wet flurries and rain bringing up to 20 cm of new snow to higher elevations. Moderate to strong southwest winds. Freezing level to 1500 metres and alpine temperatures around +1. Forecast snowfall amounts are subject to freezing levels. Wet snow can generally be expected to accumulate 300-400 metres below the freezing level.
Avalanche Summary
On Wednesday, explosives triggered two size 1 storm slabs which released down 10-25 cm. These were described as having minimal propagation and minimal entrainment. On Monday, a report includes whumphing and a ski cut triggering a size 2 slab which released on the crust layer. Explosives also triggered numerous storm slabs 20-30 cm thick and one stepped down to the crust layer down 40-60 cm. On Thursday, recently formed storm slabs may still be reactive to human triggering, especially in wind loaded terrain. Large persistent slab avalanches have generally become unlikely but still may be possible in isolated areas. Natural sluffing is possible from steep sun exposed slopes.
Snowpack Summary
Another 15-30 cm of snow fell in the North Shore mountains on Wednesday, bringing the typical storm accumulation in the past week to 115-230 cm. Recent winds have been from a variety of directions and wind slabs should be expected on all aspects; some may exist below less wind affected new snow accumulations. This recent snow overlies the late-February interface that is composed of a sun crust on southerly aspects and surface hoar on shaded aspects. An observation on Wednesday suggests this layer has gained a lot of strength since the weekend. However, conservative choices are still recommended until we are sure this layer has become unreactive. We could be in a low probability, high consequence scenario where very large avalanche are still possible in isolated areas. Substantial snowfall and warming in the forecast are two factors that may promote increasing reactivity at this interface over the weekend. the Below this interface, the mid and lower snowpack appear to be well settled and strong.
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.