Avalanche hazard is more complicated than meets the eye. Assess wind-exposed areas, and carefully monitor the weak layer beneath the storm snow. Stay disciplined with terrain margins around steep roll-overs and wind-drifted slopes.
30-50 cm of recent snow has been redistributed into wind slabs at upper elevations. Strong winds have shifted from the southwest to the northwest making it possible to trigger these wind slabs on a range of aspects. Cornices are large and looming and may trigger avalanches on slopes below.
30-70 of recent snow sits over a facet-crust weak layer combination that is possible to human trigger. Recent observations demonstrate the potential for this problem to be triggered remotely and release sympathetic avalanches in adjacent terrain.
Tuesday night: Clear, light northwest winds, treeline low temperatures near -12 C, freezing level at valley bottom.
Wednesday: Mainly cloudy, scattered flurries in the afternoon with up to 5 cm of snow, light southwest winds increasing to moderate from the west in the alpine, treeline high temperatures near -10 C, freezing level rising to 100 m.
Thursday: Cloudy, 5-10 cm of snow, light west winds gusting moderate at ridgetops, treeline high temperatures rising to -4 C in the late afternoon, freezing level rising to 900 m.
Friday: Cloudy, 10-20 cm of snow, strong southwest winds with extreme gusts, treeline high temperatures near -5 C, freezing level rising to 1200 m.
It may be possible to trigger large avalanches in specific terrain on Wednesday. Cornices could be reaching their breaking point and could act as triggers on slopes below.
On Tuesday, we received report of a large (size 2.5) human-triggered avalanche near Rainbow Mountain that caught and carried a group of five skiers. The avalanche released on north aspect at 1900 m. It broke 40 cm deep and ran down to the melt-freeze crust that formed in mid-January. The avalanche propagated across adjacent roll-over features and triggered a sympathetic slide on a small feature 200 m away.
On Monday, operators reported numerous large (size 2-2.5) human and explosive-triggered avalanches breaking 20-50 cm deep in the recent storm snow. In a few areas, these slabs broke 70-100 cm deep. In the backcountry, a group remotely triggered an avalanche from Panorama Ridge on a northeast aspect in the alpine. Near Gin Peak, a skier was caught and carried in a small avalanche that released on a steep, convex feature at treeline. On Sunday, a group also travelling near Gin Peak reported triggering a small (size 1.5) avalanche. The avalanche was triggered remotely from below and broke 50 cm deep.
Last week, explosive control work near Whistler produced a cornice fall that triggered a size 2.5 deep persistent slab on the weak faceted crystals above the early December crust. The slab broke 60-130cm deep. This indicates that while the weak layer is unlikely to be affected by skiers and riders, it could still be triggered by very large loads.
Over the weekend, the mountains picked up 30-50 cm of new snow. Strong to extreme southwest winds during the storm have since become moderate from the northwest and have redistributed the recent snow into wind slabs across a range of aspects at upper elevations. Monitor for changing slab conditions as you move into wind-exposed terrain.
The recent snow rests on a problematic layer combination. Weak sugary facets formed during a prolonged dry period over a thick melt-freeze crust. In sheltered areas at upper elevations, surface hoar may also be found on the crust. This late January facet-crust layer has demonstrated continuing reactivity in the aftermath of the weekend storm. Travelers have reported remote-triggered avalanche activity, widespread whumpfing on this layer (see this report from Paul Ridge), and reactivity in snowpack tests (see this report from Gin Peak).
Deeper in the snowpack, it is possible to find another crust layer buried down 100-200 cm with facets above it from December. This layer is most prominent between 1700-2100 m and is currently classified as dormant; although large loads such as a cornice failure or avalanches in motion may still be able to trigger avalanches on this layer.
