Rapid and continuous loading of new snow, strong wind, and rising temperatures are expected to bring a widespread natural avalanche cycle to the region this weekend. Very large avalanches may run to valley bottom. Travel in avalanche terrain is not recommended.
Heavy snowfall over Friday night and Saturday is expected to form a widespread new storm slab problem. Strong winds and rising temperatures will aid slab formation and promote natural avalanche activity.
A weak basal snowpack layer exists in much of the region, particularly in the Spearhead and further north. Although the likelihood of triggering this layer has substantially decreased, the consequences of an avalanche on this layer are severe. This weekend's storm will test this layer with increasing load and avalanche activity in surface layers.
Reports from the Whistler area on Thursday included numerous observations of storm slabs releasing with skier traffic as well as with explosives, generally ranging from size 1.5-2 (large). Crown fractures varied from 20-80 cm, owing to some wind transport of roughly 30 cm of new snow.
Looking forward, the coming storm is expected to form a widespread new storm slab problem as new snow accumulates and forms reactive slabs as a result of wind and rising temperatures.
Increasing load as well as surface avalanches will steadily increase concern for the possibility of very large avalanches releasing on our weak basal snowpack layer.
Friday night: Cloudy with increasing snowfall bringing 10-20 cm of new snow by morning. Winds increasing to strong southerly.
Saturday: Cloudy with continuing snowfall bringing 20-40 cm of new snow, continuing overnight. Strong south or winds shifting southwest. Alpine high temperatures around -5.
Sunday: Cloudy with easing flurries bringing 10-15 cm of new snow and 2-day snow totals to 50-90 cm. Snowfall beginning again overnight. Moderate south winds. Alpine temperatures reaching -1 as freezing levels rise to about 1700 metres.
Monday: Cloudy with easing flurries bringing about 5 cm of new snow. Light south winds. Alpine high temperatures around -2 with freezing levels to 1500 metres.
40-60 cm of new snow is expected to accumulate over the region by the end of the day on Saturday. This will bury recent wind slabs and wind-affected surfaces in exposed areas at all elevations and soft, low density snow in sheltered areas. The new snow will add to just over a meter of storm snow that has fallen in the last week.
Professionals in the region are continuing to track a pair of weak surface hoar layers from mid and late December. Currently 70 to 160 cm below the snow surface, these layers are giving increasingly stubborn results in snowpack tests but may remain sensitive to human triggering in places where the snowpack goes from thick to thin, steep unsupported slopes, and around sheltered shallow, rocky start zones.
The beginning of the season had less precipitation than usual, which led to a layer of sugary faceted grains as well as a hard melt-freeze crust near the base of the snowpack. In a more seasonally normal winter we wouldn't be thinking about this interface, but it has produced large, destructive natural avalanches as recently as January 11th.
Concern for very large avalanches releasing over this weak basal layer will increase as loading from new snow, wind, and rising temperatures strain the snowpack and cause avalanche activity in surface layers.
