Regions
Kootenay Boundary.
Additional snow and wind Wednesday night will continue to create touchy storm slabs reactive to human triggers. If triggered, these slabs have the potential to step down to deeper weak layers buried in the snowpack, resulting in large avalanches.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
WEDNESDAY NIGHT: 10-20cm of new snow / Moderate to strong southerly ridgetop winds / Freezing level around 1000m.THURSDAY: Mostly cloudy with isolated flurries / Light to moderate southerly ridgetop winds / Freezing level dropping to 800m.FRIDAY: Mostly cloudy with flurries; 3-5cm / Light to moderate southwesterly ridgetop winds / Freezing level dropping to valley bottom with an alpine high of -8.SATURDAY: 5-15 cm of new snow / Moderate southerly ridgetop winds / Alpine high of -4.
Avalanche Summary
On Tuesday, a skier triggered size 2 persistent slab avalanche was reported in the Rossland area. No further information was reported but it is suspected to have failed on the surface hoar layer buried in early January which was down 75cm. The slope that slid was low angle and the slab propagated across the entire slope. Additionally, a snowcat northwest of Nelson triggered two size 2.5 persistent slab avalanches on the same early January surface hoar layer which was down around 80 cm at 2250m. Forecast new snow and wind Wednesday night will promote ongoing storm slab avalanche activity, especially in higher elevation, lee terrain.
Snowpack Summary
20-25cm of new snow brings recent storm snow total to approximately 45-60 cm which has been redistributed by strong southerly winds. This overlies the most recent crust and surface hoar (weak feathery crystals) layer that was buried mid-January. This crust can be found on solar aspects while the surface hoar has been buried on shaded aspects above 1600m. Below this layer lies a second crust/surface hoar interface buried early-January that is now 70-100 cm below the surface. The mid-December surface hoar layer is buried 100-130 cm below the surface. This spooky layer still produces "sudden" test results and is most pronounced at treeline, but is also present below treeline . Two laminated crusts created by rain events in late November lay just below the mid-December interface, and may co-exist with facets.
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.