A persistent weakness in the snowpack has recently resulted in some very large avalanches and concerning burials. Conservative terrain selection remains critical.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
THURSDAY: Mainly cloudy, light southwesterly winds, and freezing levels in valley bottoms with alpine temperatures reaching -10 C.FRIDAY: Mainly cloudy, light southwesterly winds, and freezing levels in valley bottoms with alpine temperatures reaching -5 C.SATURDAY: Mainly cloudy, moderate to strong southwesterly winds, and freezing levels rising as high as 2200 m.
Avalanche Summary
Over the last 5 days there were several very concerning large persistent slab avalanches that were human triggered. At least 10 close calls were reported to us and several of these avalanches involved multiple people being buried. Four of these avalanches were in the Allen Creek riding area and one was in the Monashees near Blue River. These avalanches all released on or stepped down to the mid-December weak layer down 1-1.5 m in the snowpack. Check out the MIN posts and a great video on the Frozen Pirate Snow Services Facebook page for more details. Also reported on Tuesday were several natural slab avalanches up to Size 2 triggered by direct sun exposure on steep south facing slopesLooking forward, the recent storm snow may remain reactive to human triggering in wind loaded terrain and steep unsupported or convex features where buried surface hoar may remain reactive. Even if surface instabilities gain strength, deeper destructive persistent slab avalanches failing on the mid-December weak layer will remain a serious concern for the foreseeable future.
Snowpack Summary
40-60 cm of recent storm snow typically overlies the variable mid-January interface which consists wind affected surfaces in exposed terrain, surface hoar up to 7 mm in sheltered areas, and/or widespread faceted old snow. The bond between the new snow and the old surface is generally gaining strength with colder temperatures that followed the storm but weaknesses are lingering, especially where surface hoar is preserved. Strong southwest winds during the storm have formed wind slabs in leeward terrain features. Isolated moderate outflow winds may have more recently resulted in reverse wind loading. The mid-December surface hoar/facet layer lies anywhere from 50 cm down in shallow snowpack areas to 1.5 m down in deeper snowpack areas. This layer woke up during the recent storm and has remained reactive to human triggers. Several large human-triggered avalanches including events with multiple burials have occurred on this layer. I would expect continued reactivity at this interface for the foreseeable future.
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.