UPDATED DANGER RATINGS. More than 40 cm new snow fell in the southern half of the region and around Whistler. Alpine Danger updated to HIGH.
Confidence
Low - Intensity of incoming weather systems is uncertain
Weather Forecast
A deep low (978mb) west of Haida Gwaii will send a strong pulse of moisture Wednesday evening. Cooler air accompanies this low, keeping freezing levels steady near 1000 m throughout the period. Precipitation amounts vary between the models but expect 20-30 cm of new snow by Thursday morning (possibly more over southern sections like Diamond Head / Red Heather) and strong winds from the southeast. Freezing levels will start to drop Friday as the low moves south bringing another 10-20 cm and light northerly winds.
Avalanche Summary
Natural activity to size 1.5 was observed Monday on east slopes near Brandywine / Metal Dome (failing on layers within the storm snow) and a remote-triggered 1.5 near ridgeline in the same zone. Explosives control in the alpine showed numerous soft slab and cornice releases up to size 2.0 on Wednesday, again, so far, very few releases on the mid-November crust. Storm slabs will continue to build and remain touchy through the stormy forecast period.
Snowpack Summary
Snow depths are variable across the region with the snow line sitting around 1400 m and approaching threshold for avalanches in open areas below treeline. Average snowpack depths at treeline are 80-120 cm with the upper 40-60 cm sitting on a thick mid-November crust and solid mid-lower snowpack. In the alpine, snowpack depths are 150 cm or more with the mid-November crust down 80-100 cm. Available reports suggest the new snow is sticking well to the crust so far. Recent storm snow accumulations near Whistler were ~35cm (with more to the south and less to the north). Strong winds combined with the new snow will likely build touchy slabs on leeward slopes and terrain features at higher elevations. During the storm the snow may be reactive on steep slopes: Watch for weak layers within the storm snow and dig down to test for these layers before committing to a slope. Lots of great posts to the Mountain Information Network... please keep 'em coming!
Problems
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.