Regions
Kootenay Boundary.
Avalanches are expected to be easily triggered during and after the storm on Wednesday.
Confidence
Fair - Forecast snowfall amounts are uncertain on Wednesday
Weather Forecast
20-30 cm new snow is expected to fall overnight Tuesday and through Wednesday. Ridgetop winds around 60 km/h from the SW overnight Tuesday should become northwest 30-40 by midday Wednesday. Freezing levels are expected to be around 1300 m on Wednesday. On Thursday, lingering flurries are possible, with generally light west or northwest winds and freezing levels at valley bottom. On Friday, conditions should be dry with some clear spells. Northwest winds may pick up in speed as colder weather descends from the north.
Avalanche Summary
Widespread natural and remotely triggered storm slab avalanches to size 2.5 were reported from across this region on Sunday. They occurred on a variety of aspects and elevations between 1700 and 2200 m. The recently buried crust/surface hoar interface was the culprit in most of these avalanches, although in one event near Rossland, a remotely triggered storm slab in motion stepped down to the November crust/facet interface near the ground on a planar slope at 2100m. On Monday, a skier-triggered an avalanche remotely from 10 m away on an east aspect at 2050 m, while avalanche control produced avalanches up to size 2.5 on the recent rain crust.
Snowpack Summary
Up to 50 cm of recent storm snow overlies a prominent surface hoar layer above a thick rain crust that extends as high as 2400 m. This crust/surface hoar interface was very reactive during the storm over the weekend, and still has the potential to produce further avalanches with additional load from new snow or wind, or in response to human triggers. High elevation north aspect slopes do not have the rain crust, but are still reported to have buried touchy surface hoar. On these slopes, deeper facets or buried surface hoar from earlier in the month may also be lurking. At the base of the snowpack, a crust/facet combo has become less likely to trigger, although a recent avalanche that occurred on this layer near Rossland indicates this layer should remain on our radar on steep, smooth terrain at treeline and in the alpine.
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.
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.