The recent loading of new snow and rain combined with strong wind will build reactive storm and wind slabs on Wednesday. The fresh snow will load buried weak layers and could produce very large slab avalanches in the Northern half of the region.
A couple of problematic layers may be found around 100 to 200 cm deep. The layers are primarily in the north of the region (e.g. Duffey, Hurley) and include weak layers of surface hoar and faceted grains above a hard melt-freeze crust. The likelihood of triggering a persistent slab has reduced, but the consequence of doing so remains high.
New and reactive storm slabs will continue to build by Wednesday. The new snow/ rain load will destabilize the snowpack and natural avalanches are likely. New wind slabs are expected to form at higher elevations due to strong southerly wind.
This problem may not be prevalent at lower elevations where the precipitation fell as rain in the South. Lower elevations may develop a crust as freezing levels drop.
The storm will start to diminish overnight as a ridge of high pressure builds on Wednesday bringing dryer and cooler conditions.
Tuesday Night: Snow/ rain 20-30 cm (up to 40 mm) with the freezing level dropping to 800 m. Ridgetop winds strong and gusty from the southwest.
Wednesday: Mix of sun and cloud with more cloud cover in the South. Alpine temperatures -6 and freezing level 800 m. Ridgetop wind continues to blow strong from the southwest-west.
Thursday: Mix of sun and cloud. Alpine temperatures -4. Freezing level 1700 m and ridgetop wind strong and gusty from the West.
Friday: Snow 10 cm. Alpine temperatures -2 and freezing level 1300 m. Ridgetop wind strong from the southwest.
On Tuesday, no new avalanches were reported at the time of publishing this forecast.
Natural and rider triggered avalanches are likely on Wednesday with the new storm snow and strong southwest wind building reactive slabs, especially where they sit above a buried layer of surface hoar.
Two large avalanches were observed near Duffey Lake last Saturday, described here and here. It is suspected that both avalanches were triggered naturally and on the weak layers described in the snowpack summary. Also, a notable skier-triggered avalanche occurred near Whistler, not far from this region (see the Sea to Sky bulletin for more info).
Up to 20 cm of new snow accompanied by strong wind has fallen in the Northern part of the region with another 15-25 cm expected by Wednesday morning. At this point, most of the precipitation has fallen as rain on the Coquihalla. On Wednesday, new and reactive storm and wind slabs will likely exist at upper elevations, especially in sheltered areas where the new snow may overlie the recently buried surface hoar. Lower elevation terrain may see a firm crust as the temperatures drop.
A melt-freeze crust from early December is found around 100 to 200 cm deep in the snowpack. This crust may have surface hoar or sugary faceted grains sitting above it. This persistent weak layer is most prevalent in the North (e.g., Duffey Lake, Hurley) and large avalanches have failed on this interface in the past week. This interface doesn’t seem to be a concern in the South (Coquihalla) and little information is known for the North Cascades or far South in Manning Park.
