Layer of ice crystals at and above treeline being tested by snow slab recently deposited by storm

Photo: Avalanche Canada

Two different rider-triggered avalanches contributed to this slide in the West Kootenay on Feb. 4.

Avalanche danger is rated ‘considerable’ this weekend at the treeline and into the alpine throughout the West Kootenay backcountry, according to Avalanche Canada.

The national service issued its predictions late Thursday night on the weekend’s white water wonderland — the same weekend as The Ymir Climber at whitewater Ski Resort 10 kilometres north of Nelson.

“Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast,” the report stated.

The uncertainty lies within a layer of surface hoar — ice crystals 30 to 55 centimetres deep — buried at the end of January. It has been most reactive where a slab of new snow now exists above it.

The buried surface hoar has been reported to be as large as 20 millimetres. On shaded slopes it's sitting on 10 to 20 cm. of loose, “sugary facets,” but on sunny slopes it's sitting on a thin, hard crust, the report detailed.

“This layer has been the culprit for many natural and human-triggered avalanches this week, and is not expected to break down and bond anytime soon,” the report related. “Human triggered avalanches may become more widespread as any lower density snow also starts to settle into a slab.”

On Wednesday, west of Creston, explosive avalanche control produced several large slab avalanches (up to size 2.5) in southeast through southwest facing treeline terrain. Also, numerous small (up to size 1.5) loose avalanches were reported on steep slopes facing the sun.

In some parts of the region, alpine and treeline wind has formed deeper, denser deposits of snow in leeward terrain, and potentially scoured some windward ridgetops.

“Looking forward, we expect that human triggered avalanches failing on a buried layer of surface hoar will remain likely for the next few days,” the report predicted.

The lower snowpack is strong and bonded.

Photo: Contributed