Wind slabs have been reactive to human triggers in recent days. As you enter wind-affected terrain, watch for wind slabs on all aspects.
A developing persistent slab problem warrants caution on open slopes and convex rolls at treeline where buried surface hoar may be preserved.
Wind slabs will exist on various aspects in exposed terrain and around ridgelines. Last weekend's storm initially came in with southwest winds, but with the intrusion of arctic air, winds switched to the northeast, building slabs in less common locations.
These slabs may have formed on top of old crusts and hard wind-pressed snow and there is uncertainty about how quickly they will bond to old surfaces.
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 storm snow is bonding poorly to this hard interface in many areas, and 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. Over the past few weeks, reactivity on this layer had tapered, but the storm over the weekend 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.
The ridge of high pressure will rebuild on Thursday, with the potential for lingering valley cloud and an alpine temperature inversion on Friday and Saturday.
Wednesday Overnight: Light snowfall. Alpine temperatures around -15 C. Light to moderate northwesterly winds.
Thursday: Cloudy with light snowfall, clearing in the afternoon. Alpine temperatures around -12 C. Light to moderate northerly winds.
Friday: Mainly clear, lingering valley cloud. Freezing level rising to 500 m. Alpine temperatures around -10 C. Light to moderate northwesterly winds.
Saturday: Increasing cloud cover, light snowfall in the afternoon. Freezing level rising to 800 m. Alpine temperatures around -6 C. Moderate to strong southwesterly winds.
In the north of the region, a natural avalanche cycle occurred during the peak of the storm on Saturday afternoon. Reactivity persisted into Sunday, with numerous human-triggered storm and wind slabs reported throughout the day. In the south of the region, wind slabs were reactive to human triggering, with natural loose dry avalanches observed out of steep terrain in the alpine.
As this storm added load to the snowpack, the mid-January weak layer began to "wake up". Two human-triggered avalanches occurred on northerly aspects at or just above treeline and a large natural avalanche also occurred on a north-facing aspect at treeline.
As arctic air invaded the province on Monday, strong northerly winds redistributed the storm snow into wind slabs on southerly aspects. A natural avalanche cycle occurred on southerly aspects as a result of this wind-loading. Several human-trigged wind and storm slab avalanches were also reported throughout the day.
Last weekend's storm brought 20-80 cm of new snow accompanied by southwesterly winds. The north of the region received highest snowfall amounts, with tapering amounts to the south. Recent northeasterly has created a heavily wind-affected surface in exposed areas and redistributed storm snow into wind slabs in lee areas. Below ~1200 m the majority of the precipitation fell as rain, with 10-20 cm of snow now overlying a thick melt-freeze crust.
Up to 80 cm now overlies a weak layer formed in mid-February. This layer is comprised of surface hoar in sheltered areas, a melt-freeze crust at lower elevations, a sun-crust on steep solar aspects, and hard wind-affected snow in the alpine and exposed treeline. Reports suggest that in many areas, the storm snow is bonding poorly to this firm layer.
The mid-January surface hoar/crust layer is now down 50-150 cm in the snowpack. This layer has started to 'wake up' following last weekend's snowfall. See the avalanche summary for more details.
The facet/crust layer that formed in early December is now buried around 150 to 250 cm. This layer is currently considered dormant but could become active later this season.
