A persistent weak layer of buried surface hoar is the sliding layer for human triggered avalanches. This problem results in the avalanche danger staying up at Considerable for the forecast period.
Confidence
Fair - Forecast snowfall amounts are uncertain
Weather Forecast
5-10 cm of dry light snow is expected overnight combined with light Northwest winds, alpine temperatures around -12 and freezing levels at valley bottoms. Cloudy with flurries or light snowfall during the day Sunday with accumulations around 3-5 cm. Northeast winds moderate during the day becoming strong Sunday night. Mostly clear with moderate Northeast winds and alpine temperatures near -20 on Monday. Continued cold and clear on Tuesday.
Avalanche Summary
A natural size 3.0 deep persistent slab avalanche was reported that occurred on Friday. The avalanche may have released near the ground (seen from a distance). Skier controlled avalanches were reported up to size 1.5 failing on the mid-December surface hoar layer that is down 30-70 cm.
Snowpack Summary
Some new windslab has formed at higher elevations that is 20-40 cm thick, easy to trigger, and may step down to the mid-December surface hoar. The touchy mid-December surface hoar layer is now buried below a 50-90 cm consolidated slab that developed during last weeks storm. Below 2100 m this slab sits on a thick, solid crust/ surface hoar combination and has been acting as a perfect sliding layer. A hard rain crust with facets from early November is buried over 1 m down and is currently unreactive, however; triggering from shallow rocky and unsupported terrain remains a concern.
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.