The weather might feel like spring, but the snowpack isn't there yet. A dangerous persistent weak layer is in play and it is not typical for our region. Warming will increase the chances of destructive avalanches on this layer.
An ageing wind slab problem exists on north and east aspects at treeline and above. The reactivity of some older slabs could be extended where they overlie a persistent weak layer. Warming may increase the likelihood of triggering slabs in the afternoon.
20 to 70 cm of 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 propagate far and wide. It can be found on all aspects at treeline and is most reactive at this elevation band, but it can be found above and below. Recent avalanches have been triggered on this layer in both the north and south of the region.
Rapid warming on Wednesday will cause surface layers to lose cohesion and shed from steep slopes. This may occur naturally but is even more likely with a human trigger. Larger releases have potential to trigger destructive persistent slab avalanches, especially around treeline.
Wednesday night: Mainly clear. Moderate to strong west winds. Freezing levels remaining near 3300 metres.
Thursday: A mix of sun and cloud. Moderate to strong west winds. Treeline high temperatures around +1 with freezing levels around 2000 metres, falling overnight.
Friday: A mix of sun and cloud. Light north winds. Treeline high temperatures around +1 with freezing levels rising from surface to 2500 metres, remaining elevated overnight.
Saturday: Sunny. Light northeast winds. Treeline high temperatures around +4 with freezing levels to 2700m, remaining elevated overnight.
Several more skier triggered avalanches occurred in the region on Sunday. See reports here, here, and here. Although the St Jacob's slide looks to have been a wind slab, it may have run on the same persistent weak layer associated with other recent avalanches in the region. The Allison Pass avalanches seem more definitively associated with this layer. Each highlights the challenging risk management scenario we're currently in.
Another near miss associated with our late January persistent weak layer occurred in the area of Spectrum Peak, near Rainbow Mountain (Sea to Sky) on Sunday. At least three large avalanches with 1-2 metre crown fractures were remotely triggered by a group of hikers. Fortunately no one was caught. Wind loading at the start zone may account for fracture depths that exceed the typical burial depth of the suspected weak layer.
On Saturday a fatal avalanche occurred in the Duffey Lake area. This avalanche was a very large (size 3), skier triggered avalanche with significant propagation. It was triggered at upper treeline on a northeast aspect and it failed on a shallower layer of surface hoar before stepping down to the crust from late January referred to above and discussed in our snowpack summary. A more detailed report can be found here. This layer was also responsible for several large skier triggered avalanches in the neighbouring Sea to Sky region.
We anticipate a significant increase in the likelihood of avalanches on this layer during the warming pattern from Wednesday onward.
Warming temperatures and light drizzle has likely moistened the surface on all aspects and elevations. A trace of new snow preceded the warming, doing little to bury heavy wind effect and wind slabs resulting from recent south winds. Formation of new surface crust may occur in open areas where radiative cooling occurs overnight. Any crust that does form should deteriorate with daytime warming.
The primary concern in the snowpack at present is the 20 to 70 cm of snow from the past week that is sitting above a variety of surfaces including facets and crust, but also surface hoar in more sheltered areas at mid-elevations. This layer is most prominent at treeline and above. It has been found on all aspects in the Duffey Lake area. This layer is up to about 90 cm deep in the south of the region and has been giving similar, moderate to hard, sudden results in recent snowpack tests.
