A buried weak layer has produced a number of large and surprising natural and skier triggered avalanches this week. Conservative terrain selection is critical with a high consequence problem like this. Check out the latest Forecasters' Blog for more tips on managing this problem.
Wind slabs may remain reactive to human triggers on lee features at upper elevations. If triggered, wind slab avalanches may step down to deeper layers resulting in larger avalanches.
A persistent slab problem associated with a crust buried 50-150 cm deep has produced a number of large and surprising avalanches recently. This layer is most likely to be triggered in windy areas where the depth of the snowpack is highly variable. Avoid likely trigger spots such as steep, rocky or convex slopes with a shallow or thin to thick snowpack.
Friday night: Clear. Light northwest wind. Alpine temperature around -24 C.
Saturday: A mix of sun and cloud. Wind building to moderate northwest to southwest. Alpine high around -16 C.
Sunday: Increasing cloud with flurries starting, up to 5 cm. Strong southwest wind. High of -11 C.
Monday: 15-30 cm new snow. Strong southwest wind easing to moderate. High of -8 C.
There has been an alarming pattern of large, persistent slab avalanches being reported each day this week.
On Friday, we received preliminary reports of a size 3 avalanche on Whale's Back near Nelson and another incident in Kootenay Pass with few details.
On Thursday, evidence of a natural size 2.5 persistent slab avalanche was observed on a northeast aspect at 2000m in the Bonnington Range.
On Wednesday, a skier triggered size 2 storm slab stepped down to the early December crust at Kootenay Pass and a naturally triggered size 3 persistent slab was reported in this MIN post from near Rossland.
On Tuesday, explosive control work at Kootenay Pass produced numerous persistent slab avalanches failing on the early December crust, up to size 3.5.
On Monday, several riders were involved in a human triggered size 2.5 persistent slab avalanche near Nelson. The avalanche occurred on a southeast aspect at approximately 1900 m. The treeline slope was heavily wind effected and the rider triggered the avalanche from a thin spot where the layer was only around 60 cm deep. Here is the MIN report of the incident.
With plenty of soft, loose snow available for transport, moderate winds are expected to form fresh wind slabs in lee terrain features at upper elevations.
A crust formed in early December has been responsible for numerous very large persistent slab avalanches in recent days. The depth of the crust ranges widely from 50-150 cm and is most likely to be triggered in wind affected terrain below ridgetops. '
The lower snowpack is composed of several early-season crusts. It is these basal crusts that are the primary concern for persistent slab avalanches in thin snowpack areas near Rossland.
Nearly all of the recent avalanches have been triggered from shallow spots in the snowpack where the crust is less than 100 cm deep. Avoid steep, rocky slopes with a shallow or thin to thick snowpack.
