A complex and unpredictable snowpack exists, especially in the northern half of this region. Natural and human triggered, large avalanches continue to occur.
Confidence
Fair - Timing of incoming weather systems is uncertain
Weather Forecast
The next Pacific frontal system will reach the coast tomorrow. The front will affect most of the Interior through the forecast period. Wednesday will see light convective flurries, moderate ridgetop winds from the SW and freezing levels near 1700 m. For Thursday, precipitation amounts expected are 10- 20 mm. Ridgetop winds will be strong from the SW and freezing levels rising to 2500 m. Warm air will remain over the region on Friday with freezing levels rising to 3000 m. Skies will show a mix of sun and cloud and ridgetop winds will be moderate strong from the SW.
Avalanche Summary
On Tuesday afternoon we received a report of a human triggered persistent slab avalanche size 2.5 on a northerly aspect at 2000 m. On Monday, numerous natural and human triggered slab avalanches were reported up to size 2.5 with the majority failing in the recent storm snow and the deeper mid-March interface. There were several remotely triggered avalanches with the farthest being triggered from 150 m away. On Wednesday, I suspect natural avalanche activity to taper off; however, remote human triggering remains a major concern up to size 2.5. Avalanches that fail on the deeper weak interfaces will be large and destructive.
Snowpack Summary
At higher elevations, 40-60 cm of dense storm snow sits over the mid-March interface which had included crusts, moist snow, wind affected surfaces, and/or old wind slabs. This interface has recently come alive and is reactive, especially in the North (Revelstoke and surrounding area). Recent snowfall amounts taper off towards the south of the region. Strong SW winds had redistributed the new snow into wind slabs on leeward features. Digging deeper, 50-80 cm is the mid-February facet/crust interface. There is some concern for this layer to wake-up in the South Columbia region and storm slab avalanches have the potential to step down to it. Snow surfaces are wet to around 1700 m and moist to around 2200 m. The snow surface is undergoing melt-freeze cycles as freezing levels drop overnight.
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.