Although the likelihood of avalanches is gradually decreasing, the potential to trigger large persistent slabs avalanches warrants conservative terrain choices.
Weather Forecast
THURSDAY NIGHT: Lingering flurries with 2-5 cm of snow, strong wind from the southwest, freezing level rapidly dropping, alpine temperatures drop to -10 C.FRIDAY: Cloudy with isolated flurries in the morning then clearing in the afternoon, moderate wind from the west, alpine temperatures around -10 C.SATURDAY: Clear in the morning then increasing cloud throughout the day, light wind from the south, alpine high temperatures around -8 C.SUNDAY: 5-12 cm of snow, light wind from the southwest, alpine high temperatures around -6 C.
Avalanche Summary
The storm snow is still reactive, especially in wind-loaded areas. Storm slabs have resulted in skier and explosive triggered avalanches (size 1-2), and natural avalanches (size 2-3.5) on a daily basis for the past week. Many of these avalanches were triggered from a distance (remote triggered). A recent remotely triggered size 2.5 avalanche occurred on Wednesday in the Valhalla Range on a north aspect around 2200 m. Most of the persistent slab activity on Wednesday was reported on north aspects between 2000-2300 m.
Snowpack Summary
Approximately 70-120 cm of recent storm snow sits on a weak layer of facets (sugary snow), surface hoar (feathery crystals) and a sun crust (on south aspects). Strong winds have promoted widespread slab formation, particularly with the most recent new snow available for transport.Another weak layer of surface hoar and sun crust that formed in mid November is up to 120 cm below the surface. This layer is most likely to be a problem in areas where the surface hoar sits on the sun crust. This combination is most likely found on steep, south facing slopes at treeline. The potential may exist for smaller avalanches to step down and trigger this deeper layer, resulting in large avalanches.At the base of the snowpack is a crust that formed near the end of October. Concern for this layer is dwindling but it may still be worth considering in places such as steep, rocky, alpine terrain, especially where the snowpack is shallow.
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.
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.