North Columbia

The snowpack cannot be trusted right now. Large, human-triggered avalanches on a buried weak layer remain likely. Signs of the problem are becoming less obvious while the consequences are becoming more serious. Keep it conservative on Sunday.

Saturday night: Decreasing cloud, isolated flurries with trace accumulations, light southwest winds, alpine temperature -9 C.

Sunday: Mix of sun and cloud, light west winds, alpine high temperature -6 C.

Monday: Mostly clear, light southwest winds, alpine high temperature -5 C, freezing level 1000 m.

Tuesday: Increasing cloud, 5-10 cm of snow, moderate southwest winds, alpine high temperature -4 C, freezing level 1200 m.

Over the past several days, a natural wind slab avalanche cycle was observed. These avalanches were small to large (size 1-2.5) breaking 25-50 cm deep on leeward features at upper elevations. In several cases, cornice falls triggered these wind slabs. If triggered, wind slabs or cornice falls could step down to the buried surface hoar layer, forming large avalanches. Use added caution on sun-exposed slopes when the sun is shining, as it could rapidly deteriorate the storm snow and cornices and cause loose or slab avalanches.

Consistent reports of avalanches releasing on the February 22 surface hoar have been submitted over the past week. Human-triggered and remotely-triggered avalanches have been reported by nearly every operation region-wide, even as professionals tiptoe around and avoid suspect terrain features. The problem is touchy but tricky. In many instances, slopes were ski cut with no results, only to have the third or fourth skier in the group trigger the slab.

Avalanche size has increased over the past week, with more of the activity releasing size 2+. Observations extend to all aspects and elevations, but north to east aspects near treeline have been a hot-spot for reactivity. Avalanches are also occurring at unusually high elevations for surface hoar, with several cases of wind slabs stepping down in the alpine. This combination has produced very large (size 3-4) avalanches.

Incremental snowfall and wind have formed reactive wind slabs in the alpine and open areas at treeline that remain possible to human trigger. Cornices are large and looming after a week of successive storms. 

A widespread weak layer of surface hoar is buried 50-90 cm deep. On solar aspects, this layer may sit over a crust. Incremental loading, strong winds, and mild temperatures have increased the depth and slab properties of the snow above the weak layer, creating the potential for larger avalanches with serious consequences. Riders will be capable of triggering this layer for some time until there is a substantial change in the snowpack. Read more about surface hoar on our forecaster blog.

• Be aware of the potential for large avalanches due to the presence of buried surface hoar.
• Use conservative route selection and resist venturing out into complex terrain.
• Stay off recently wind loaded slopes until they have had a chance to stabilize.
• Brief periods of sun could quickly initiate natural avalanche activity.