Touchy avalanche conditions with all the recent snow. Use a cautious approach in the mountains this weekend.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
Saturday: Increasing cloud and wind with flurries in the afternoon and freezing level remaining in valley bottoms. Sunday: 5-10cm of fresh snow possible by Sunday morning, with another 5-10cm throughout the day. Freezing level near valley bottom with moderate to strong southwesterly ridgetop winds. Monday: Another 2-5cm of snow with cooler temperatures and light westerly winds.
Avalanche Summary
Reports from Thursday include a remotely triggered Size 1.5 slab avalanche from a snowmobiler on a sheltered north facing slope below treeline that ran on surface hoar buried early-December. Other reports included more evidence of a large natural avalanche cycle at alpine and treeline elevations on Tuesday and Wednesday.
Snowpack Summary
Continued snowfall brings total treeline snowpack depths to over a metre, with even more in the alpine, but it diminishes quickly below around 1700m. Recent rain created a near surface crust as high as 1800m (or higher in the southern part of the region). Above that elevation, storm and wind slabs are bonding poorly to the old snow surface buried early December, which is probably a crust and/or a combination of facets and buried surface hoar now down around 40-60cm in sheltered treeline areas. The buried surface hoar may be well preserved in sheltered areas, creating particularly touchy slabs with the potential for remote triggering, extensive releases and prolonged sensitivity to triggering. Below that a thick weak layer of facets may be lurking around the thick mid-November crust.
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.
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.