The new snow keeps coming! A fresh dump on Thursday will keep avalanche danger elevated in the region.
Confidence
Moderate - Intensity of incoming weather systems is uncertain
Weather Forecast
Thursday: An incoming storm wave will deliver 15-20cm of new snow. Winds will be moderate gusting to strong from the southwest. Alpine temperatures to -8.Friday: Mainly cloudy with isolated flurries and a trace of new snow. Winds light gusting to moderate from the west. Alpine temperatures around -10.Saturday: Mainly cloudy with isolated flurries and a trace of new snow. Winds light from the south. Freezing level dropping to valley bottom and alpine temperatures to -12.
Avalanche Summary
At the time of publishing this bulletin, the only new avalanche observations have been related to sluffing of unconsolidated new snow. However, continued strong ridgetop winds along with new snow on Thursday will promote ongoing wind slab development and activity. For the short term, I also expect loose dry avalanches to be triggered easily in steep sheltered terrain.
Snowpack Summary
Up to 45 cm of new low density snow has fallen since Monday. In higher elevation terrain where the winds were stronger, I would expect new wind slab formation on lee and cross-loaded features. 65-100 cm below the surface you'll find the mid-December interface which consists of a variety of old snow surfaces including wind affected snow, faceted (sugary) crystals, and/or surface hoar crystals. Recent tests suggest the the new snow is well bonded to the interface in some areas. However, a few public and professional reports suggest this layer may be reactive in some areas and worth investigating before pushing into steeper terrain. The thick mid-November crust layer typically sits 1-2 m down in the snowpack. Recent test results on this layer suggest it is generally well bonded to the adjacent snow with the possible exception of shallow snowpack areas. This layer is considered dormant at this time but remains an isolated concern that we will likely be tracking through much of the season.
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.
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.
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.