Storm slabs are primed for people triggering large avalanches, and have the potential to step down to deeper persistent weaknesses resulting in very large and dangerous avalanches.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Friday
Weather Forecast
WEDNESDAY: Overcast with light snow and periods of heavy flurries combined with light southwest winds.THURSDAY: A mix of sun and cloud with flurries during the day, and a stronger system moving in late in the day.FRIDAY: Cloudy with 15-20cm of fresh snow accompanied by moderate SW winds. Alpine temperatures reaching -5 C.
Avalanche Summary
On Sunday and Monday we had reports of natural, skier and snowcat triggered storm and wind slab, and dry loose avalanches up to Size 2. Of note were 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. Subsequent explosives control on Monday produced a Size 3 from the hang fire.
Snowpack Summary
A wide-ranging 65-120 cm of fresh snow has fallen in the past week, which is bonding poorly to widespread faceted snow, thin sun crust on steep southerly aspects, as well as more isolated small buried surface hoar. Moderate to strong southerly winds have formed touchy slabs at all elevations with multiple weaknesses within and under this recent storm snow. 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 solar aspects, and spotty surface hoar on shaded aspects. This layer 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. Its reactivity has been especially prominent over the crust at lower elevations.
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.