Persistent weak layers are not typical on the South Coast. Backcountry users in the region need to keep in mind the possibility for storm slab avalanches to 'step down' well beyond the depth of new snow.
Confidence
Moderate - Freezing levels are uncertain
Weather Forecast
Saturday: Periods of snow and rain with 15-30 cm accumulating at higher elevations. Strong south winds, gusting to extreme. Freezing level to 1500 metres with alpine temperatures around 0. Sunday: Periods of snow and rain with 25-30 cm accumulating at higher elevations, mostly in the afternoon. Moderate to strong southwest winds. Freezing level to 1500 metres with alpine temperatures around 0. Monday: Approximately 10-15mm of rain over the day. Moderate to strong southwest winds. Freezing level to 1800 metres with alpine temperatures around +2.
Avalanche Summary
No new avalanches have been reported, but it should be noted that stormy weather has been discouraging backcountry travel and obscuring visibility into alpine terrain.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.Looking forward, expect a drastic change in avalanche hazards as you transition between elevation bands. Loose wet avalanche hazards will dominate where rain and wet snow accumulate, while thick, reactive storm slabs can be expected at higher elevations. This increases the need for awareness of overhead hazards. To complicate matters, large persistent slab avalanches may still be possible in isolated areas, and this potential increases while precipitation (especially rain) loads the snowpack.
Snowpack Summary
Another 15-20 cm of new snow fell in the North Shore mountains over Thursday night, bringing the typical storm accumulation in the past week to 130-200 cm. Our most recent winds have been from the south, but previous winds were more variable and formed wind slabs on all aspects below our most recent new snow accumulations. Mid storm layers in the upper snowpack have recently produced moderate, resistant results in snowpack tests. Below the recent storm snow, you'll find the late-February interface. This layer is composed of a sun crust on southerly aspects and surface hoar on shaded aspects. Recent snowpack tests of this layer have produced variable results with occasional sudden fracture character. With this in mind, conservative choices are still recommended until there is higher confidence in this layer becoming unreactive. We could still be in a low probability, high consequence scenario where very large avalanches remain 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.