Regions
Kootenay Boundary.
A strong Southwest flow is expected to bring strong winds and rapidly rising freezing levels. Expect avalanche danger to increase as the storm develops.
Confidence
Fair - Intensity of incoming weather systems is uncertain
Weather Forecast
A strong southwest flow will bring moderate to locally heavy precipitation from Tuesday to Thursday with some weather models calling for up to 50mm of rain or snow (depending on elevation) over the forecast period. Freezing levels are expected to peak at about 2300m on Tuesday and Wednesday, and then drop to about 1900m on Thursday. Strong southwest winds are forecast for the entire period.
Avalanche Summary
The new storm slab is the immediate concern as it has produced a few avalanches to size 2 in northeast-facing alpine terrain in the southwest corner of the region. Avoid freshly wind loaded features at higher elevations. The more complex problem is the lingering persistent weak layer of buried hard rain crusts with weak facets or surface hoar at the interface. Rising freezing levels during the forecast storm combined with the chance of heavy loading from rain or wet snow may be enough to weaken the bond at the crust.
Snowpack Summary
The developing storm slab is variable across the region. In the Monashee range near Big and Little White we have reports of a 25 cm storm slab that has been transported by the wind at higher elevations and it is sitting on a weak layer of facets or surface hoar. This storm slab is giving easy results from light forces in snow profile tests. The new storm slab is reported to be thinner near Nelson where it is sitting on a thin re-frozen rain crust. In the Kootenay Pass area the new storm slab is about 20 cm and may be sitting on surface hoar, and then another layer of about 20 cm (40 cm total) above the hard November 29th crust. The November 20th crust may be anywhere from about 40 cms above the ground in deeper snowpack areas to on the ground or non-existent in shallower snowpack areas. The November 20th crust continues to give moderate planar results in snow profile tests in some areas; that means the right combination of load and terrain features may result in an avalanche down to this buried layer.
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.