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Confidence
Fair - Due to the number of field observations
Weather Forecast
A dry ridge of high pressure will develop on Saturday before another pacific frontal system makes its way inland bringing up to 15cm of snow for Sunday. On Monday, light snowfall is possible; however, a general clearing pattern is expected for early next week. Freezing levels are expected to sit at about 800m on Saturday, and then rise to about 1200m on Sunday and Monday. Winds should remain southwesterly throughout the period, increasing from light on Saturday to Strong on Sunday and Monday.
Avalanche Summary
At the time of publishing this bulletin, no new avalanche activity had been reported. That said, on Saturday I'm sure there was a round of fast sluffing in steep, sheltered terrain and wind slab activity on leeward alpine slopes. I'm confident the new snow will remain sensitive to human triggering for some time due to the presence of underlying surface hoar. Small avalanches have the potential to step down and trigger a persistent slab which would greatly increase the consequences of a release.
Snowpack Summary
Up to 25cm of new snow fell on Friday night. The new snow, which has been distributed into deeper deposits in higher wind-exposed terrain, overlies widespread 5-15mm surface hoar, a sun crust on steep sun exposed slopes, and/or wind affected surfaces in the alpine. The problematic mid-December surface hoar/crust layer is typically down 50-80cm below the surface and remains sensitive to human triggering is some areas. The reactivity of this persistent weak layer appears to be quite variable but still has the potential for large, destructive avalanches in some areas. The layer appears to be the most reactive at and below treeline.
Problems
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.
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 Dry
Loose Dry avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.