There has been a noticeable increase in persistent slab avalanche activity over the last two days. This suggests the load sitting over persistent weak layers has reached a critical level. Adopt a conservative approach in the coming days.
Confidence
Moderate - Freezing levels are uncertain on Wednesday
Weather Forecast
MONDAY: Mix of sun and cloud / Light northwest wind / Alpine temperature -9 TUESDAY: Mix of sun and cloud / Light to moderate northwest wind / Alpine temperature -4, possible temperature inversion WEDNESDAY: Cloudy with sunny breaks / Light west wind / Alpine temperature -1, temperature inversion
Avalanche Summary
Recent reports show numerous natural and human triggered loose dry avalanches in steep terrain to size 1.5 and natural storm slab avalanches to size 2 in the alpine and at tree line. There have been several reports of persistent slab avalanches naturally, skier and remotely triggered to size 2 on north and east aspects between 1500 and 2200m. Consider the potential to trigger persistent slabs on top of the mid December persistent weak layer as the temperature rises and the storm snow begins to settle and consolidate more rapidly in the coming days.
Snowpack Summary
40-70cm of low density new snow from last week 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 mid-December, 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.