Regions
Kootenay Boundary.
The cooling has started, but it may take a couple of days to recover from the very warm air and high freezing levels.
Confidence
Fair - Timing or intensity of solar radiation is uncertain
Weather Forecast
The freezing level is expected to drop overnight down slightly below 1000 metres and then rise again on Wednesday up to 1500 metres. Expect valley cloud in most areas with clear skies above 1500 metres combined with light winds on Wednesday. The freezing level should drop down to valley bottoms by Thursday morning and then rise up to about 1000 metres during the day. Valley cloud may persist with clear skies and light winds above. Southwest winds should develop on Friday as cloud moves into the region ahead of the next Pacific front.
Avalanche Summary
On Monday there were several size 2.0-2.5 loose wet and wet slab avalanches released with explosives at Kootenay Pass, and a size 3.0 natural avalanche in the backcountry near Whitewater ski resort on a Southeast aspect with a very wide propagation that probably released on the mid-January persistent weak layer during a period of strong solar radiation. Moist or wet loose snow avalanches have been reported from several areas up to size 2.0 over the past few days during the warming event. On Saturday a skier triggered a size 2 persistent slab which failed on the mid-December layer. This was a surprising result because it was on a well-supported, concave terrain feature (which had also been heavily skied).
Snowpack Summary
The recent snow overlies various surfaces including surface hoar, a thin crust, or a settled slab. Warm temperatures are affecting the surface layers and turning them moist or wet. Continued warming has increased the likelihood of triggering the persistent weak layer of mid-Jan surface hoar layer which is down about 20-60 cm. These large, well-preserved surface hoar crystals sit on a thin crust up to about 1900m on north aspects and all the way to ridgeline on south aspects. A deeper crust/surface hoar combo buried in mid-December is typically down 80-120cm and has produced some large avalanches recently.
Problems
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
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.