Snow, wind, warming and sun-exposure will keep storm slabs touchy, and coupled with the potential for cornice triggers, the likelihood of massive deep persistent slabs is expected to increase.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Sunday
Weather Forecast
SATURDAY: A mix of sun and cloud with localized periods of intense sun-exposure and isolated light flurries. Increasing cloud with snow in the afternoon. Light becoming moderate SW winds. Freezing levels generally remaining in valley bottoms with alpine temperatures around -5 C.SUNDAY: Cloudy with 10-15cm of fresh snow by the morning and continued light flurries throughout the day. Light to moderate SW winds and freezing levels rising to 1500m for southern areas, but remaining near valley bottoms in the north.MONDAY: Mainly cloudy with another 5-10cm by morning and light flurries throughout the day. light to moderate SW winds and freezing levels rising to 2000m.
Avalanche Summary
Reports from Thursday include a few skier-triggered 10-40cm thick Size 1 storm slabs and loose dry sluffs, as well as two 50cm thick explosives-triggered Size 2's. A cornice also collapse naturally, but didn't trigger a slab on the slope below. Early reports from Friday morning suggested that a natural avalanche cycle was underway in response to lheavy loading from snow and wind.On Sunday and Monday we had reports of two very large natural deep persistent slab avalanches in the Selkirk mountains near Trout Lake. One was a 250 cm deep Size 3 on a north aspect on Sunday and the other was a 300 cm deep Size 4 on a southwest aspect on Monday that filled the creek at the bottom of the valley and ran 100m up the other side, both of which released on deeply buried old facets.
Snowpack Summary
Around 20-25cm of fresh snow has added to the 60-100cm of settled storm snow from the past week, which is bonding poorly to faceted snow, thin sun crust on steep southerly aspects, as well as more isolated small buried surface hoar. Southerly winds have formed touchy slabs with multiple weaknesses within and under this recent storm snow at all elevations . A persistent weakness buried mid-February is now down about 90-135cm and composed of a thick rain crust up to about 1800 m, sun crusts on steep southerly aspects, and spotty surface hoar on shaded aspects. This and deeper persistent weaknesses has seen a recent increase in more sudden snowpack test results and has been identified as a failure plane in a number of recent avalanches.
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.