South Columbia

Forecast strong winds, heavy snow, and rising freezing levels will push the avalanche danger to HIGH. Some areas may not see HIGH danger until Wednesday.

Moderate - Timing, track, or intensity of incoming weather system is uncertain

Some areas may have some warm air that is close to zero up as high as 2500 metres Monday night. Cooler air in the valleys should keep the snow line down closer to 1200 metres. I expect this will be variable across this region. Expect variable snowfall amounts across the region overnight; 15-20 cm in the north and western upslope areas of the Monashees, and likely 5-10 in the southern selkirks. The storm is forecast to come from the west with strong southwest winds and rapid warming Strong southwest winds on Tuesday with freezing levels around 1200 metres and another 5-10 cm of new snow. The storm will continue on Wednesday with strong southwest winds, freezing levels rising to 1500 metres and another 15-20 cm of new snow. Thursday should be slightly cooler with moderate southwest winds and 3-5 cm of snow.

Windslab and loose snow avalanches have been likely to be triggered in isolated terrain by the additional load of a skier or rider. Conditions are expected to change overnight as a new storm moves into the region and new storm slabs develop. I expect this will result in a direct action cycle of storm slab avalanches. As the load increases over the next few days we will be looking closely at recent persistent weak layers and discussing the likelihood of these layers failing under the new load of storm snow combined with wind and warm temperatures.

The new storm is arriving overnight. Storm slabs are expected to develop above existing weak layers. I have left the previous snowpack description that describes what the new snow will be landing on. Recent cold and clear weather has promoted both faceting of surface snow as well as the growth of surface hoar that has been reported to be up to 10mm in size in some areas. Reports also exist of a sun crust beginning to form on steep solar aspects. Below the surface, up to 30 cm of low density snow fell earlier this week. During and in the days after the storm, moderate to strong southwest and then northerly winds shifted this snow into reactive wind slabs on leeward slopes. The mid-December facet/surface hoar persistent weakness can be found buried 70-120 cm deep and is generally considered dormant. However, we are still receiving occasional reports of sudden results in snowpack tests, suggesting that it has to potential to propagate into a large avalanche if triggered, particularly in shallow spots where this layer is closer to the snow surface. The lower snowpack is well bonded and features a thick rain crust near the ground.