Northwest Coastal

It's about to get a lot milder and a lot snowier for the south of the region. Expect fresh storm slabs to grow in size and sensitivity through the weekend. As the snow piles up, there is increased potential for large avalanches on deep weak layers near the base of the snowpack.

The incoming low pressure system will primarily impact the far south of the region. Heaviest snowfall will be in the Kitimat area, with the Terrace region also receiving generous amounts, with less further inland and in the north. The following snowfall amounts focus around Shames.

Friday night: Increasing cloud with flurries starting, accumulating 5 to 10 cm of new snow. Winds moderate building to strong south to southeast winds. Alpine temperatures around -20 C.

Saturday: Cloudy with flurries bringing 20-30 cm of new snow, and as much as 40 cm near Kitimat. Strong southwest to southeast, approaching extreme at ridgetop. Alpine high around -15 C.

Sunday: 10-20 cm overnight with 10-20 cm through the day. Strong south to southwest winds, approaching extreme at ridgetop. Alpine high around -10 C.

Monday: 10-20 cm overnight, tapering to 5-10 cm through the day. Light to moderate south to southwest winds. Alpine high around -7 C.

Widespread storm slab avalanche activity is expected throughout the storm this weekend. 

During the cold snap, avalanche activity was predominantly windslab related amid the strong outflows. Sizes dwindled to the 1-1.5 range in the latter part of the week.

Reports of deep persistent slab avalanches have been trickling in since the previous snowfall and wind event. They are associated with a November crust layer near the base of the snowpack, producing very large avalanches (size 3+) with crown depths of around 2 m. Observations are typically in alpine terrain, on lee or cross-loaded slopes. This may be a preview of what is to come as a significant new snow load and a large upswing in temperatures stress these layers further.

Steady snowfall over the weekend will bring storm totals of 30 cm Saturday, 50 to 70 cm Sunday, reaching 70 to 100 cm by Monday. The initially light, fluffy snow will be buried by incrementally warmer, denser snow towards the end of the storm. As the snow piles up, it is being blown around by strong southerly winds. At alpine and treeline elevations, fat pockets of wind loaded snow are expected in lee features such as below ridgecrests and roll-overs.

The new snow is falling on extensively wind affected and faceted surfaces at all elevations. Typically, the longer a snow surface sits exposed before being buried, the less eager it is to bond to new snow. After the clear skies, frigid temperatures and extreme outflows last week, we suspect that there will be plenty of avalanche activity through the storm at the new snow interface.

A deep crust from mid November lurks at the base of the snowpack. A couple of recent large avalanches have run on this layer on lee and cross-loaded features high in the alpine. As this storm applies significant new snow loads to the snowpack, these deep layers will feel the stress and we may see some very large avalanches in high places.

• Watch for fresh storm slabs building throughout the day.
• Use increased caution at all elevations. Storm snow is forming touchy slabs.
• Avoid freshly wind loaded terrain features.
• Storm slabs in motion may step down to deeply buried weak layers resulting in very large avalanches.