There is more than what can be seen at the surface ! Choose low consequence slopes, as wind slabs and persistent slabs remain a concern for riders.
Fresh small pockets of wind slabs may develop with the expected winds and snowfalls on northerly alpine and treeline features. Lingering wind slabs still exist on various aspects in any exposed terrain and around ridgelines. Although their reactivity has gone down lately, they have built in less common locations, which can still catch riders by surprise.
There are two prominent weak layers in the upper meter of the snowpack.
An upper layer exists down ~30-70 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. Reports suggest that reactivity on this layer may persist. Be particularly cautious in sheltered areas where surface hoar may be preserved, or south-facing aspects where recent wind-loading has occurred, and a sun crust may be buried.
The lower layer may be found roughly a meter 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. The last storm caused this layer to 'wake up' on northerly aspects around treeline. 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 cm. Freezing level rising to 1000 m. Alpine temperatures around -6 C. Moderate to strong southwesterly winds up to 60 km/h.
Monday: Snow 20-30 cm. Freezing level rising to 1200 m. Alpine temperatures around -2 C. Moderate to strong southwesterly winds.
Tuesday: Snow 5-10 cm. Freezing level rising to 1600 m. Alpine temperatures around -2 C. Moderate to strong southwesterly winds.
A large (size 2) persistent slab avalanche caught a skier by surprise Friday in the Valhallas, while skiing a south-facing alpine slope, previously wind-loaded. Recent wind transport induced a significant cornice fall, which naturally triggered a large slab (size 2.5) at upper treeline in the Selkirks.
During the last week, very large persistent slabs (size 3) were observed, such as this one near London Ridge and in the Valhallas. They both occurred naturally on 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 (20-80 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, 10-20 cm of snow is now overlying a thick melt-freeze crust.
Two weak layers exist within the upper snowpack: the mid-February layer (down 50 to 100 cm) and the mid-January layer (down 80 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.
