There is uncertainty as to the intensity of a storm impacting the region Thursday night into Friday. Danger ratings and problems reflect the upper end of forecast precipitation which is concentrated in the south of the region.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain on Friday
Weather Forecast
FRIDAY: Flurries or periods of snow, accumulation 5-10cm overnight Thursday into Friday / Light south southeast wind / Alpine temperature -10 SATURDAY: Flurries, accumulation 5-10cm / Light to moderate southwest wind / Alpine temperature -12 SUNDAY: Mix of sun and cloud / Light northwest wind / Alpine temperature -11
Avalanche Summary
Recent reports are limited to both natural and human triggered loose dry avalanches in steep terrain to size 1 in areas that have not been affected by wind. A report from just northeast of Nakusp on Monday shows a size 2 persistent slab release on a north aspect at 1800 metres. Another report from Sunday showed a skier-triggered size 1 persistent slab releasing from a north aspect at 2200 metres. Expect continued potential for triggering loose dry avalanches in steep terrain and consider the potential to trigger persistent slabs as the load continues to increase on top of the mid December persistent weak layer.
Snowpack Summary
15-30cm of low density new snow has buried a recently formed layer of weak feathery surface hoar and/or sugary facets. A layer buried mid-December that consists of surface hoar, sun crust and/or sugary facets is now down approximately 60-100cm.The bond at this old snow interface is of critical importance in areas where the overlying snow has consolidated into a slab. The most concerning areas are those that saw pronounced surface hoar development before the storm, such as sheltered areas at and below treeline, sheltered northerly aspects in the alpine, or anywhere the surface hoar formed on top of a sun crust. Recent snowpack tests show wide ranging reactivity on this weak layer but suggest that it may be most reactive on northerly aspects.A rain crust that formed in late November is now buried 100-140cm deep. Recent evidence from the North Columbia region suggests it may be reactive in steep, variably loaded terrain features in the alpine.
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.