Recently formed storm slabs are expected to be reactive to human-triggering on Sunday. Conservative terrain selection is critical and it is best to avoid recently wind loaded areas. If the sun comes out, use extra caution on steep south aspects.
Confidence
Moderate - Due to the number of field observations on Saturday
Weather Forecast
One last pulse of snowfall is expected to bring another 10-20cm to the region Saturday overnight. Unsettled conditions are expected on Sunday as cold Arctic air begins to move south and replace the replace the recent storm system. Lingering flurries are expected in the morning and sunny breaks in the afternoon. Alpine winds are forecast to be light to moderate from the northwest and freezing levels are expected to fall below valley bottom. On Monday, mostly sunny conditions are expected with light alpine winds from the northwest and treeline temperatures below -10C. On Tuesday, a mix of sun and cloud is forecast with light alpine winds from the northwest and treeline temperatures below -15C.
Avalanche Summary
On Friday, explosives triggered three storm slab avalanches up to size 1.5 in the north of the region. These slabs were typically 20-40cm thick and were on northeastern aspects in the alpine. Ski cutting in the Monashees was producing numerous size 1 loose dry avalanches on steep slopes at treeline. On Wednesday, explosives triggered a size 2.5 persistent slab in north of the region on a northeast aspect in the alpine. The slab was 60-100cm thick and released on the crust layer from mid-November. On Sunday, recent storm slabs are expect to remain reactive to human-triggering. These storm slabs should be most reactive on north through east aspects in the alpine as a result of recent wind loading. Persistent slab avalanches on the mid-November crust will remain an ongoing concern and appears to be a low probability, high consequence problem which likely would require a heavy trigger or triggering from a thin area.
Snowpack Summary
Recent snowfall and strong alpine winds from the southwest have formed storm slabs. These slabs are thickest and most reactive in leeward features in wind exposed terrain. In some areas, the recent storm snow may sit over a layer of small surface hoar. There is some uncertainty regarding the distribution of this surface hoar in the region but it was reported in a couple locations in the Monashees west of Revelstoke. The mid-November crust is now down 60-120 cm. Test results on this layer have been highly variable ranging from easy to unreactive. While there has not been much avalanche activity on this layer yet, it has many professionals concerned. It will be important to track how this layer evolves, especially with the upcoming period of cold weather. Below the crust the snowpack is generally well settled. Snowpack depths vary greatly throughout the region with an average of 110-190 cm at treeline elevations. Watch out for stumps, rocks, and open creeks below treeline.
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.