Avalanche Forecast

Sunday night: Cloudy with isolated flurries and a trace of new snow, increasing a bit by morning. Light variable winds, becoming moderate southwest in the alpine.

Monday: Cloudy with scattered flurries bringing up to 5 cm of new snow, easing over the day and overnight. Light to moderate south winds shifting northwest by evening and increasing. Alpine high temperatures around -13.

Tuesday: Cloudy. Light west or northwest winds, increasing to moderate or strong in the alpine, continuing overnight. Alpine high temperatures around -11.

Wednesday: Cloudy with isolated flurries and a trace of new snow. Moderate to strong northwest winds easing over the day. Alpine high temperatures around -11.

A bout of sustained strong east and northeast winds prompted widespread wind slab formation and reactivity on Friday and to a slightly lesser extent on Saturday. Many 20-50 cm-deep slabs released naturally, with skier triggering and with explosives on Friday, some with remarkably wide propagation. Having formed over weak, faceted grains, these wind slab hazards may persist for quite a bit longer than usual.

We also have many reports in from last week of persistent slab avalanches triggered by skiers at elevations around treeline (1800-2000 m) where a persistent weak layer exists as surface hoar. Check out the 7-day MIN for more details. Persistent slab avalanche activity has been on the decline more recently, but subtle factors like diminishing slab properties and savvy terrain use may account for this trend. Professionals in the region continue to highlight persistent slabs as an ongoing concern.

Another explosives-triggered persistent slab in the Bonningtons last Tuesday stepped down to our early December crust layer to produce a 200 cm-deep, size 3.5 (very large!) avalanche. A similar avalanche (size 3.5, step down to December crust) occurred naturally in the same area in the first week of February.

Snow from last week has seen a great deal of wind effect at upper elevations, forming reactive slabs over weak, faceted surface snow that developed during the cold snap. It may contain a roughly 50 cm-deep freezing rain crust (from January 31) or surface hoar at lower elevations. It continues to settle over another weak layer buried 60-80 cm deep (from January 24). 

This persistent weak layer may exist as facets or a crust but it has been most reactive where it exists as large surface hoar crystals in steep, sheltered areas at treeline and below. This layer is slow to gain strength and requires careful terrain selection, even as its likelihood of triggering gradually diminishes.

There are several more layers of surface hoar in the mid snowpack, with the most notable one down about 80-120 cm, buried in early January. These older layers (or the crust described below) may take precedence over the layers described above in shallower snowpack areas from the Nelson Range through to the Okanagan.

Another widespread crust from early December is surrounded by weak faceted grains and buried deep within the snowpack. It may be possible to trigger this layer with a large load in a shallow rocky start zone.

• Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
• Avoid open slopes and convex rolls at and below treeline where buried surface hoar may be preserved.
• If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
• Use caution on large alpine slopes, especially around thin areas that may propagate to deeper instabilities.