Yellow means caution ! Wind slabs are still reactive to human triggering on all aspects, and buried weak layers remain suspicious on open slopes. Make sure to read the section Avalanche Problems.
Wind slabs exist on various aspects in any exposed terrain and around ridgelines. Shifting winds (initially from the southwest, then switching to the northeast) are 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 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. 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 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. 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.
A strong ridge of high pressure will keep the region dry and cold until the weekend, with the potential for lingering valley clouds and an alpine temperature inversion. The next storm is expected late Saturday night as a shift in the weather pattern will occur.
Thursday night: Clear skies. Patchy valley cloud. Alpine temperatures around -20 C. Light northerly winds.
Friday: Mix of sun and clouds. Lingering valley cloud. Freezing level rising to 500 m. Alpine temperatures around -12 C. Light to moderate westerly winds.
Saturday: Increasing cloudiness. Flurries up to 5 cm in the afternoon. Freezing level rising to 800 m. Alpine temperatures around -8 C. Moderate to strong southwesterly winds.
Sunday: Snow 5-10 cm. Freezing level rising to 1100 m. Alpine temperatures around -6 C. Moderate to strong southwesterly winds.
On Wednesday, a skier accidentally triggered a size 1 storm avalanche and took a ride on a previously skied slope in open trees just west of Revelstoke. See this MIN report for photos and details. Earlier this week, shifting arctic winds have redistributed the storm snow into wind slabs and a natural avalanche cycle occurred on southerly aspects. Several human-triggered wind and storm slab avalanches were also reported since then.
In the neighbouring region of South Columbia, two large persistent slab avalanches (size 3) were reported, one near London Ridge, and one from the Valhallas. They both occurred naturally on a south-westerly alpine slopes. These evidences are showing that, with added load to the snowpack from the last weekend’s storm, the mid-January weak layer began to "wake up".
Last weekend's storm brought 30-100 cm of new snow accompanied by southwesterly winds. 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 120 cm now overlies a weak layer formed in mid-February. This layer consists 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. There has been reactivity on this layer during last weekend's snowfall. See the avalanche summary for more details.
