Regions
Kootenay Boundary.
The new storm snow will add to a touchy slab that overlies the weak layer from early January. Tricky conditions are expected and conservative route selection is critical. If you get more than 25cm new snow, treat the local alpine hazard as HIGH.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
A Pacific storm system will bring snowfall to the region starting Tuesday evening. 15-20cm of new snow is expected between Tuesday evening and Wednesday evening. Freezing levels are expected to peak around 1500m and alpine winds should be moderate to strong from the southwest. There is some uncertainty for a second storm pulse on Wednesday night into Thursday. One model is showing the storm ending on Wednesday while another is showing an additional 10-15cm by midday Thursday. Things will start to dry out sometime on Thursday and sunny breaks are possible in the afternoon. Freezing levels are expected to be around 600m and winds should be light from the northwest after the storm ends. On Friday, a ridge of high pressure should result in dry, sunny conditions with light alpine winds.
Avalanche Summary
No new avalanches were reported on Sunday or Monday. On Friday and Saturday, skiers triggered a few storm slabs size 1-2 in the Nelson area. A thin storm slab overlying the recently buried surface hoar was the suspected culprit in all of these avalanches. Storm slab avalanches are expected to increase in size and reactivity with the upcoming new snowfall.
Snowpack Summary
The new snowfall will sit on top of 20-40 cm of snow that fell last week. In some areas this old snow exists as low-density powder, while in other places warm temperatures and wind have encouraged the formation of a cohesive slab. This old storm slab is resting on a widespread layer of surface hoar which overlies a melt-freeze crust on solar aspects or facets in shaded areas. This critical layer, which was buried at the beginning of January, is one to watch as the overlying slabs get deeper over the next couple days. About 60-80 cm below the surface, you might find a rain crust from mid-December which co-exists with facets in some areas. Recent snowpack tests suggest that it could still be capable of producing human triggered avalanches. The snowpack below this layer is generally strong and well-settled.
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.