The Winds of Change đŸŒŹïž

Date
jeudi 18 janvier 2024
Auteur
Tyson Rettie, Avalanche Forecaster

Last week saw arctic air push its way south from the territories into Alberta and British Columbia. By Thursday, it had made its way to the US border and most of BC was experiencing temperatures below -20 °C, with many areas seeing temperatures at valley bottoms below -30 °C. As the arctic front passed over the area, it brought light to moderate precipitation, along with northerly winds exceeding 60 km/h in the alpine. Once the arctic air mass was firmly in place, northerly winds continued in many regions.

With the exception of Vancouver Island and the southwest coast, where conditions will be notably different, this weekend looks set to bring a change.  The weather looks more seasonal (warmer and wetter), but what does this mean for those heading into the mountains? You may encounter wind slabs on slopes that are often scoured, and also soft loose new snow that is not sticking well to the old surface layers. Since there are a variety of conditions you may encounter, it will be important to be alert for signs of instability. 

A small slope in a clearing between trees cracks in front of a pair of skis.

Image from MIN: wind slab triggered in open trees near Revelstoke on Thursday.

The most concerning slopes will be ones where a layer of soft facets is sandwiched between new wind slabs and the old, hard, wind-affected snow. If you've taken an AST 1, you may recall your instructor expressing concern for a slab over a weak layer on a firm sliding surface


A skier performs a Rutschblock test and triggers a weak later between two harder layers.

Image from the archives: a weak layer fails between two hard layers in a snowpack test.

If you're thinking of getting out there, here are some tips to manage this scenario:

  • Avoid making assumptions based on previous experience in the terrain.
  • Constantly assess conditions as you move through the mountains.
  • Look for signs of instability. Sometimes layers of large facets can collapse and make a whumpf,  just like surface hoar.
  • Get your shovel out and take a look at the layering of the upper snowpack.

But wait, I saved the best piece of information for last. In sheltered terrain, where a slab is less likely to be found, the skiing and sledding is going to be awesome! It shouldn’t be too hard to find great riding this weekend if you plan for the conditions.