Lingering storm slabs are expected to remain reactive to human triggering on Monday. A persistent weakness in the snowpack has also recently resulted in some large avalanches and concerning burials. Conservative terrain selection remains critical.
Confidence
Moderate - Due to the number of field observations on Sunday
Weather Forecast
A mix of sun and cloud is expected on Monday. Alpine wind is expected to be light from the north. Afternoon freezing levels are forecast to be around 500 m with treeline temperatures around -8C. A mix of sun and cloud is expected for Tuesday and Wednesday. Alpine wind is forecast to remain light from the north and treeline temperatures are expected to be around -10C.
Avalanche Summary
On Saturday, two human triggered persistent slab avalanches were reported in the Allen Creek area. These both released on the mid-December layer and resulted in large avalanches, one of which buried two snowboarders. A third rider triggered avalanche occurred in Allen Creek on Friday and five snowmobilers were buried, two of them fully buried. Check out the MIN posts and a great video on the Frozen Pirate Snow Services Facebook page for more details. Also reported on Saturday were several size 2-3 persistent slab avalanches which were triggered with explosives on all aspects in the alpine. On Friday, three large remotely triggered avalanches occurred. Two of these released on the mid-January layer below the recent storm snow. The third released on mid-December layer and was triggered from 100 m away. On Monday, the recent storm snow is expected to remain reactive to human triggering in wind loaded terrain and steep unsupported or convex features. The mid-December weak layer is a real concern for the region and persistent slab avalanches remain possible. It may be possible to trigger this layer in shallow snowpack areas or smaller storm slab avalanches could step down.
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. Strong southwest winds during the storm have redistributed the new snow and developed wind slabs in leeward terrain features. In deeper snowpack parts of the region, the mid-December facet layer typically lies around 1.5 m below the surface. In shallower areas, particularly those in the southeast (Allen Creek), North (Sugar Bowl) and likely some western areas around Quesnel and Barkerville, the snowpack is likely a lot more suspect, with a thicker, more pronounced facet layer buried only 50-60 cm below the surface. This layer woke up during the recent storm and has remained reactive to human triggers several days after the storm ended. Several large human triggered avalanches including events with multiple burials have occurred on this layer over the weekend.
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.