Reactive wind slabs will likely form throughout Sunday. Diligently watch for changing conditions and rising hazard as the day progress.
Strong winds and expected snowfalls (up to 15 cm) will likely reload previously scoured slopes and build fresh and reactive wind slabs on any exposed terrain, lee features, and around ridgelines. Uncertainty remains in how well the developing slabs will bond to underlying varied snow surfaces.
There are two prominent weak layers in the upper meter of the snowpack.
An upper layer exists down ~50-100 cm and is comprised of surface hoar in sheltered areas, a crust at lower elevations and on solar aspects, and hard wind-affected snow in the alpine and exposed treeline. Be particularly cautious in sheltered areas where surface hoar may be preserved, or south-facing aspects where past wind-loading had occurred and a sun crust may be buried.
The lower layer may be found 100-150 cm deep in the snowpack. This layer consists of faceted snow, a sun crust on steep solar aspect, and most notably large surface hoar crystals in sheltered terrain. This layer 'woke up' during the last storm and should be treated with caution with this incoming one. The most reactivity on this layer has been seen between 1600-2200 m in open areas in the trees, in cut blocks, and on steep convexities. Terrain features to be suspect of include the lee side of ridges, openings in trees, cut blocks, and burns, especially on northerly aspects where the layer may not be capped by a firm crust.
A shift in the weather pattern is expected early Sunday as a series of weather systems will move through the region. Moderate to heavy snowfalls, rising freezing levels, and warmers temperatures are expected until mid-week.
Saturday night: Intermittent flurries up to 5 cm. Freezing level returning to valley bottom. Alpine temperatures around -10 C. Moderate southwesterly winds gusting 50 km/h.
Sunday: Snow 5-10 cm. Freezing level rising to 1000 m. Alpine temperatures around -6 C. Strong southwesterly winds.
Monday: Snow 20-25 cm. Freezing level rising to 1000 m. Alpine temperatures around -4 C. Moderate southwesterly winds gusting 40 km/h.
Tuesday: Snow 5-10 cm. Freezing level rising to 1300 m. Alpine temperatures around -2 C. Strong southerly winds gusting 65 km/h.
Wind slabs are still showing signs of instability on isolated features as human-triggered and natural avalanches have been reported thought out the region.
A few skiers were caught by surprise when triggering size 1-1.5 wind slab avalanche on various alpine and treeline slopes. Increased winds on Friday naturally triggered numerous wind slab avalanches in the eastern part of the region.
In the neighbouring region of South Columbia, two large persistent slab avalanches (size 3) were reported this week, one near London Ridge, and one from the Valhallas. They both occurred naturally on a south-westerly alpine slopes. These evidences are showing that persistent weak layers had begun to "wake up".
Last week's extensive northerly winds have created heavily wind-affected surfaces in exposed areas and wind slabs in lee areas. Continued cold temperatures have transformed surface conditions since the latest snow (30-100 cm) about a week ago. A thin sun crust was observed on steep south-facing slopes at lower elevations. Large feathery surface hoar crystals have also been observed in sheltered areas on all elevations. Below ~1200 m, 15-25 cm of snow is now overlying a thick melt-freeze crust.
Two weak layers exist within the upper snowpack: the mid-February layer (down 120 cm) and the mid-January layer (down 50-150 cm). They both consist of surface hoar / melt-freeze crust. Although reactivity of these layers had tapered off lately, they should be treated with caution with this incoming series of snowfalls as they may produce large, unexpected avalanches.
