North Columbia

Reactive surface instabilities are one side of the story. Warming is pushing the snowpack through a major transition and very large natural avalanches may be the result. Build large safety margins into your terrain selection for the next couple of days.

Wednesday night: Cloudy with continuing flurries bringing 5-10 cm of new snow. Light to moderate southwest winds.

Thursday: Cloudy with light flurries bringing about 5 cm of new snow and 2-day totals to 15-25 cm. Light southwest winds shifting northwest by evening. Treeline high temperatures around -2 with freezing levels to 1500 metres.

Friday: A mix of sun and cloud. Light northwest winds shifting southwest. Treeline high temperatures around -1, alpine reaching 0, cooler at lower elevations under a temperature inversion.

Saturday: Cloudy with scattered flurries bringing about 5 cm of new snow. Moderate southwest winds. Treeline high temperatures around -5.

More natural storm slab and wind slab releases were reported on Tuesday with avalanches reaching size 2 (large) in wind loaded areas. A natural size 3 (very large) wind slab avalanche also crossed the Trans-Canada highway in Glacier National Park. Similar conditions are expected on Thursday.

The persistent early December crust/facet layer has recently been active in this region, producing natural avalanches to size 3. Concern for persistent slab activity like this is being sustained by new snow, wind, and warming.

Up to 10 cm more new snow is expected by Thursday morning, adding to 10-20 cm from Wednesday and a widespread storm slab problem. Melt-freeze or rime crust may be found partway within the new snow in some areas, introducing a new likely failure plane and potentially limiting some wind transport. The new snow has buried a mix of settling storm snow and windslabs from several days of consistent snowfall and southwest wind.

Beneath the new snow, another 50-90 cm of recent storm snow is settling on top of weak, faceted snow that formed during the recent cold snap. As new snow accumulates and temperatures rise, the upper metre of the snowpack may become increasingly upside-down and more likely to avalanche. 

We're actively monitoring the early December crust. In shallow spots it's down 90 cm while in wind-loaded terrain features it can be found as deep as 200 cm below the surface. It consists of faceted grains above a bulletproof crust formed by the Atmospheric Rivers of early December. There has been avalanche activity on this crust over the weekend. Larger avalanches in surface layers as well as natural cornice falls may have potential to step down to this layer in the coming days.

• Choose conservative terrain and watch for clues of instability.
• Storm slabs in motion may step down to deeper layers resulting in large avalanches.
• The likelihood of deep persistent slab avalanches will increase with each day of warm weather.
• Avoid areas with overhead hazard.