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Avalanche Forecast

Archived

Dec 20th, 2021–Dec 21st, 2021

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Kootenay Boundary.

Northwest winds may form wind slabs in areas that are usually more windward, which could catch riders off guard. 

Seek out sheltered terrain where you can avoid these wind slabs and find better riding.

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack. Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Monday Overnight: Mainly clear with light to moderate northwest winds at ridgetop. Alpine temperatures around -15 C.

Tuesday: Cloud cover increasing in the afternoon. Moderate winds shifting southwest. Temperatures rising to -10 C in the alpine. Snowfall beginning overnight with 5-15 cm of accumulation.

Wednesday: A stormy day. Moderate to strong southwest winds will accompany 15-30 cm of new snow accumulation. Freezing levels rising to 500m in the afternoon. Continued snowfall overnight with another 10-30 cm of accumulation.

Thursday: Snowfall continuing into the morning with another 5-15 cm of accumulation. Winds easing into the afternoon into the light to moderate range from the southwest. Some clearing possible for the afternoon. Freezing levels dropping to valley bottom. 

Avalanche Summary

On Sunday, recreationalists in the Kootenay Pass area observed hard results in a compression test on the early December crust. In this area the crust was buried about 1 meter deep. See the MIN report here.

On Saturday, operators reported several natural and human triggered storm slab avalanches up to size 1 in the alpine and treeline.

Snowpack Summary

2-5mm surface hoar growth has been reported on the snow surface throughout the region formed overnight on Sunday. In the Kootenay Pass and Nelson area, this overlies a thin rain crust formed from warm temperatures during Saturday's storm. 

Saturday's storm brought 10-30 cm of new snow to the region. Moderate northwest winds overnight and into today will create pockets of fresh wind slab in the alpine and treeline. 

Below the new snow, 20-60 cm of more consolidated snow overlies a substantial crust that formed in early December. This crust is 10cm thick on average and is present across all aspects to at least 2300m. A layer of weak facets (sugary snow) has been reported above this crust in Kootenay Pass and the Whitewater backcountry, and likely exists in other areas. This problem is particularly hard to predict and tricky to manage. For this reason wide, conservative terrain margins and disciplined backcountry travel techniques will be very important. Get more details and photos in our forecaster blog. 

The lower snowpack is composed of several early season crusts. Snow depths at treeline average 150-200 cm. Below 1800m the snowpack remains relatively shallow with 80-120cm on average. The deepest snowpack can be found in the Kootenay Pass area.

Terrain and Travel

  • Be careful as you transition into wind affected terrain.
  • Be aware of highly variable recent wind loading patterns.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.
  • Avoid rock outcroppings, convexities, and anywhere the snowpack is thin and/or variable.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.