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.
30 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 to propagate far and wide.
This layer is most reactive between 1800 and 2000m, potentially due to the presence of surface hoar on the crust, but this layer be found above and below this elevation band. Avoid avalanche terrain at this elevation and be aware of slopes above that could be remotely triggered.
Daytime warming on Thursday could continue to allow for surface layers to lose cohesion and shed from steep slopes. This may occur naturally on sun-exposed slopes but is even more likely with a human trigger. Larger releases have potential to trigger destructive persistent slab avalanches, especially around lower alpine and treeline.
Wednesday night: Clearing. Light to moderate west winds. Freezing levels remaining near 3000m.
Thursday: Sunny. Light to moderate northwest winds, potentially strong in the alpine. Treeline high temperatures around +1 with freezing levels remaining near 2700 metres, cooler air hanging around 1500 metres.
Friday: Sunny. Light northeast winds. Treeline high temperatures around +2 with freezing levels to 2500 metres, rising overnight.
Saturday: Sunny. Light northeast winds. Treeline high temperatures around +4 with freezing levels to 3000 metres.
Another near miss associated with our late January persistent weak layer occurred in the area of Spectrum Peak, near Rainbow Mountain 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 in the South Coast Inland region a fatal size 3 (very large) skier triggered avalanche was reported in the Duffey Lake area. It ran on the same persistent weak layer from late January mentioned above and described in our snowpack summary. This avalanche was triggered in a upper treeline feature on a northeast aspect and showed significant propagation. Click here to read a more detailed report.
This is the same layer that was responsible for a skier triggered size 2.5 avalanche last week on Rainbow mountain. Several smaller skier triggered avalanches on this layer were also reported throughout the week. Most of the avalanche activity on this layer has taken place between 1800m and 2000m but it can be found above and below this elevation band.
We anticipate a significant increase in the likelihood of avalanches on this layer during the warming pattern that ends Sunday night.
Warming temperatures and light drizzle has likely moistened the surface on all aspects and elevations. A trace to about 5 cm of new snow preceded the warming, doing little to bury heavy wind effect and recent wind slab formations 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.
Our problematic January layer of facets on crust is now buried down 30 to 70 cm. This layer is widespread between 1800 m and 2000 m, but may be found just above or below this elevation band. It has produced several human and remote triggered avalanches in the past few days. Although this layer is widespread, its exact composition and associated reactivity has significant spatial variability. In sheltered terrain at treeline and just above, surface hoar may also be found on this layer. It will certainly promote slab reactivity where it exists.
