Confidence
Poor - Timing, track, or intensity of incoming weather system is uncertain on Thursday
Weather Forecast
Thursday: Snow amounts 10-15 cm. Alpine temperatures -3. Ridgetop winds light-moderate from the South. An above freezing layer between 1500 and 2500m.Friday: Snow amounts tapering off up to 5 cm. Alpine temperatures near -4. Freezing levels 1000 m.Saturday: Mainly dry, moderate to strong northwesterly winds, freezing level at valley bottom.
Avalanche Summary
On Monday, storm slab avalanche activity up to size 2 was observed on most aspects from 1500 m and up. Most avalanches failed down 20 cm, and were said to be touchy with wide propagations on the most recently buried surface hoar layer. With more snow, wind and warming avalanche danger will be on the rise.
Snowpack Summary
Recent winds from the NW have built pockets of new wind slab on opposite slopes. Up to 40 cm of new snow sits on a surface hoar/ stellar / crust layer buried approximately 20 cm down, and has been very touchy to rider triggers. The new storm snow is settling quickly, however storm slabs have been more reactive where there has been wind effect. Buried down 70 cm is the early December surface hoar, facetted snow, and/or a crust interface. Lately this has been more stubborn to rider triggers. The average snowpack depths at treeline range from 100 cm -150 cm.Digging deeper 70-90 cm is the late-November layer, which consists of a sun crust/ facet combo on steep south facing slopes and larger, proud surface hoar crystals in sheltered areas at lower elevations. This layer is generally considered to be unreactive. However, professionals are keeping a close eye on it.At the base of the snowpack the October crust is most predominant on north-facing alpine slopes with smooth ground cover. Triggering this weakness is unlikely, but if triggered the resulting avalanche would be very large and consequential.
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.