Regions
Kootenay Boundary.
Strong winds from the West are expected to keep loading lee slopes tomorrow and possibly trigger large avalanches.
Confidence
Fair - Freezing levels are uncertain on Tuesday
Weather Forecast
Tonight and Sunday: The tail of the system exiting is expected to leave more precipitation tonight. Light to moderate amounts are forecasted for tonight (10 mm in water equivalent) and none during the day tomorrow with strong winds from the W. Freezing levels should fall to valley bottom tonight. There could be some clearings in the morning tomorrow. Monday: Another warm front is pushing in bringing light to moderate amount of precipitation during the night between Sunday and Monday, strong Westerlies and freezing levels rising to 1800 m. Tuesday: A ridge building over the regions should bring dryer and warm temperatures.
Avalanche Summary
A natural avalanche cycle happened today with the new snow and warming up of temperatures. Several natural avalanches up to size 2.5 occurred yesterday and today on all elevation and all aspect. Yesterday, multiple skier triggered avalanches up to size 1.5 were also reported yesterday in the storm snow on E and NE aspects. Yesterday, explosive control in the southern part and central part of the region produced up to size 2.5 avalanches on SE and SW aspects that propagated quite wide.
Snowpack Summary
Windloading lee of strong westerly winds in the alpine and at treeline will keep avalanche conditions dangerous tomorrow. The new windslabs will most likely be very touchy and could possibly be triggered naturally. The ~30 cm of snow from the last storm is expected to have to potential to sluff in steep sheltered terrain. Under the new snow, a surface hoar layer is sitting on a sun crust on S facing aspects in some locations and could be buried around 40 cm. The persistent weak layers (PWL) are a concern to avalanche professionals as they have been producing sudden planar results and show good propagation potential when tested in the last week. The end of November facet/crust or surface hoar layer down 90 cm (or 50 cm where snowpack is very thin) and the depth hoar or basal facetting are the most worrisome layers. The additional load from the strong winds tomorrow could make them reactive again.
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.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.