Warm and sunny conditions are keeping the danger elevated. Weak cornices, old wind slabs, and loose wet avalanches should be expected on Friday.
The biggest concern remains a weak layer down 20-70 cm which is still capable of producing very large and destructive avalanches.
There may still be old wind slabs lingering at higher elevations but the likelihood of triggering these old slabs continues to diminish with each day of the ongoing warming.
Cornices are expected to become weak with daytime warming and solar radiation.
20 to 70 cm of snow sits over a facet-crust weak layer combination that is still possible to human trigger. It can be found on all aspects at treeline and is most reactive at this elevation band, but it can also be found above and below. While this layer appears most reactive in the north of the region, recent avalanches have been triggered on this layer in both the north and south of the region.
Loose wet avalanches should be expected on steep, sun exposed slopes during the heat of the afternoon.
A ridge of high pressure brings very warm and sunny conditions which are expected to persist until the end of Sunday.
Thursday night: Clearing overnight, moderate NW wind , freezing levels around 2000 dropping to near valley bottom overnight.
Friday: Sunny, light N wind, freezing levels climbing to around 2500 m.
Saturday: Sunny, light variable wind, freezing levels 2500-3000 m with an inversion.
Sunday: A mix of sun and cloud, light to moderate SW wind, freezing levels 2500-3000 m with an inversion.
No new avalanches were reported on Wednesday but snowballing and pinwheels were observed on solar aspects in the alpine and all aspects at elevations below 1900 m suggesting the upper snowpack became weak with the warm temperatures and sun.
On Sunday, several skier-triggered avalanches occurred in the region. 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.
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 weak layer from late-January. 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.
Warming temperatures and recent light rain has moistened the surface on all aspects and elevations. Formation of surface crust began Wednesday night and is expected to continue Thursday night with clear skies in the forecast. Prior to the warming and rain, strong winds had caused extensive wind effect and wind slab formation in exposed terrain at higher elevations.
The primary concern in the snowpack is the late-January weak layer down 20 to 70 cm. This layer is primarily composed of a crust and facets, but also may include surface hoar in more sheltered areas around treeline elevation. The interface is most prominent at treeline and above in the north of the region. 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 but has not produced the same widespread avalanche activity like in the north of the region.
