Conditions will deteriorate Sunday as temperatures rise and winds pick up. Storm slabs are expected to become touchier through the day. If you go into the backcountry, minimize your exposure to avalanche terrain.
A persistent weak layer buried 70 to 100 cm deep has produced recent large avalanches and is expected to become increasingly reactive this weekend. This layer has been most sensitive around treeline, but may also extend into alpine terrain and/or down into the trees. Avalanches failing on this interface will be large and destructive.
Between rising temperatures, ramping winds, and continued snowfall, the 20-60 cm of recent low density snow is expected to rapidly settle and develop into a slab overtop of weak, sugary facets on Sunday. Storm slab size and sensitivity to triggering will likely increase throughout the day.
A warm front will bring rising temperatures, wind and steady snowfall to the region on Sunday and Monday. Highest snowfall amounts will be in the Monashees along the southern border of the region.
SATURDAY NIGHT: 5-10 cm of new snow, freezing level 1000 m, strong southwest winds.
SUNDAY: 5-10 cm of new snow at upper elevations with rain in the valleys, freezing level rising to 1500 m, strong southwest winds.
MONDAY: 20-30 cm of new snow at upper elevations with rain in the valleys, freezing level holding at 1500 m, strong to extreme southwest wind.
TUESDAY: 5-10 cm of new snow, freezing level 1000 m, light northwest wind.
On Friday storm slabs began to form and were sensitive to human triggering, this MIN from Friday is a great example of that activity.
In the neighbouring Cariboo region on Wednesday a surprisingly large human triggered avalanche was reported on a northwest facing feature at treeline. A similarly sized skier triggered avalanche was also reported near Revelstoke Wednesday on a steep west facing slope around 1600 m. These are great examples of the strange conditions created by reverse wind loading and may be a portent of things to come as the weekend trends warm and stormy.
The second week of February produced fewer persistent slab avalanches than the first week of February. But, the late January persistent weak layer is still showing signs of instability and a conservative approach is critical as this storm begins to add weight to it. This old MIN report from the Gorge is a helpful example of the lower elevation slopes where the surface hoar has been preserved and may reactivate this weekend.
The first wave of this storm has produced 20-30 cm of low density snow which will likely experience rapid settlement Sunday as temperatures rise and 10-20 cm of heavier snow falls overtop over the day Sunday. At upper elevations, all of this recent snow will be easily blown around by strong to extreme westerly winds, forming thick, reactive slabs in lee features.
Below sits the old, extensively wind affected and faceted snow that sat on the surface during the extended cold, windy drought period. There is uncertainty around the strength of this interface with the new snow, which will surely be tested by the changes brought on by this active weather.
We've now got 70 to 100 cm of snow above the late January persistent weak layer which is composed of surface hoar. This layer is most prevalent at treeline and in "treeline-like" features such as cutblocks. On steep south-facing terrain, this layer may consist of facets on a sun crust, which has produced very large low probability/high consequence avalanches.
