Fresh snow rests on a reactive persistent slab and requires wide terrain margins and disciplined decision-making. Don't let blue skies and powder fever lure you into consequential terrain. Triggering large avalanches is likely.
A layer of weak facets (sugary snow) can be found down 70-160 cm near a crust layer that formed in early December. Recent snow and wind is overloading this weak layer, which could produce very large, destructive avalanches. Cornices could also reach their breaking point, triggering very large persistent slab avalanches on slopes below.
20-30 cm of new snow has formed a fresh storm slab problem across elevations and aspects that may remain reactive to human triggering. Areas in the south of the region received greater accumulations, ranging from 30-50 cm. Strong south winds during the storm likely built larger, more sensitive slabs in wind-drifted areas at upper elevations.
Sunday ushers in Dr. Jekyll's benign weather following Mr. Hyde's Saturday storm
Saturday night: Decreasing cloudiness, 5-10 cm of snow in southern parts of the region, moderate west winds, treeline temperatures cooling from -6 C to -10 C, freezing level dropping to valley bottom.
Sunday: Clearing in the afternoon, isolated flurries with trace accumulations, winds becoming northeast and light, high treeline temperatures near -11 C, freezing level at valley bottom.
Monday: Mainly sunny, no new snow expected, light north winds, high treeline temperatures near-9 C, freezing level at valley bottom.
Tuesday: Increasing cloudiness, no new snow expected, winds becoming southwest and increasing to strong, high treeline temperatures near -8 C, freezing level at valley bottom.
On Saturday, operators reported numerous small to very large (size 1-3) avalanches releasing naturally in the storm snow across elevations and aspects. Although natural avalanche activity is tapering, avalanche conditions may remain primed for human-triggering on Sunday. Avalanches in the new snow have the potential to step-down to recently reactive weak layers, producing very large, destructive avalanches. Cornices may also reach their breaking point, triggering persistent slab avalanches on slopes below.
On Thursday, an operator in the north of the region observed a widespread, large natural avalanche cycle, releasing on the early December crust on southerly aspects at upper elevations. In the neighbouring Sea-to-Sky region, we received reports of two large (size 2-3) persistent slab avalanches on northwest and southwest aspects between 1700-1900 m. Notably, skiers remote-triggered these avalanches while traveling uphill. Several small (size 1-1.5) sympathetic avalanches also released on nearby terrain features during these close calls. There are valuable accounts and photos in this MIN report and this MIN report.
Saturday's powerful storm brought 20-30 cm of new snow, with favored areas in the south of the region seeing 40-50 cm. Gradual warming and extreme southwest winds likely accelerated and compounded slab formation. Storm slabs may be poorly bonded to previous snow surfaces, which include hard wind-scoured surfaces, cold light snow, and possible surface hoar in sheltered areas. Stay alert to changing slab conditions as you move into wind-exposed terrain, and monitor the bond of the new snow to the old snow surface.
A culprit weak layer of facets (sugary snow) can be found down 70-160 cm deep over a widespread crust that formed in early December. On Thursday, this persistent slab problem produced a natural avalanche cycle in the north of the region and surprised several recreationists in the neighbouring Sea-to-Sky region with large, remotely-triggered avalanches. This problem is particularly hard to predict and tricky to manage. Given the rapid load from new snow and wind along with recent avalanche activity, this persistent slab problem requires wide, conservative terrain margins and disciplined backcountry travel techniques. Get more details and photos in our forecaster blog.
