Make conservative decisions with the temperatures in mind, pay attention to the daylight and leave plenty of time to get home safely.
Shifting wind direction will likely result in soft wind slab formation on new aspects in atypical reverse loading patterns. Expect wind loaded pockets in lee terrain features such as just below ridge crests and roll-overs. Watch for drifted or stiffer areas of snow, or cracking under your skis.
A layer of weak facets (sugary snow) can be found down 70-150 cm near a thick crust layer that formed in early December. Even though it is a deeply buried, it is still possible to trigger avalanches on this persistent weak layer, especially at treeline elevations.
The best strategy with a persistent avalanche problem is conservative terrain management - avoid likely trigger spots such as steep convex slopes, and areas where the snowpack tapers rapidly from thick to thin. Learn more about this problem here in a recent forecaster blog.
Arctic air maintains cold and clear conditions across the Columbias.
Monday night: Partly cloudy. No significant precipitation. Light westerly wind at 2000m, moderate northwest upper level winds. Alpine temperatures around -23 C.
Tuesday Partly cloudy with isolated flurries. Light wind at 2000m, strong northwest upper level winds. Alpine high -21 C.
Wednesday: Partly cloudy. Light wind at 2000m, strong northwest upper level winds. Alpine high -23 C.
Thursday: Flurries bringing up to 5 cm overnight then clearing. Light southwest wind at 2000m, moderate northwest upper level winds. Alpine high of -20 C.
Over the weekend, storm/wind slabs were reactive naturally and to skier traffic up to size 1.5 on mostly northeast but occasionally southeast aspects in the alpine. At least one wind slab was reported to have run on a recently buried freezing rain crust that is present in the south of the region.
The heavy snowfall this week produced a natural slab cycle to size 2 throughout the region on Wednesday and Thursday in the storm snow, and on the recently buried surface hoar. Slabs were also very easily triggered by skiers and riders, observed to size 2 on all aspects. A sympathetic size 1 was observed in a below treeline feature, failing on the surface hoar.
Last Thursday, a skier triggered persistent slab avalanche failed on the early December crust/facet interface that has been lurking in the snowpack with unpredictable results. The slab was 80cm deep, and occurred on a southwest facing unsupported treeline slope. This indicates that this layer is still a concern for human triggering - conservative terrain choices are the best defence against this tricky problem.
If head into the mountains, please submit your findings and photos through a Mountain Information Network report.
50-90 cm of fresh storm snow has accumulated over the past week, with highest amounts in the south of the region. In the alpine and exposed treeline elevations it has been redistributed into deeper deposits in wind loaded features by southwest winds. At lower elevations the settling storm snow may sit over a weak surface hoar layer on sheltered slopes at treeline and below. This layer was still reactive in snowpack tests on Sunday in the Goat Range area.
The crust formed by the early December rain event sits 70-150cm deep and is found up to 2400m in the South Columbia's. In areas where the crust is buried deeper than one meter, it has begun to decompose and shows limited reactivity. In many areas, the snow above is well bonded to the crust. However in some areas around treeline and below, weak faceted grains have been observed above this crust - creating a weak interface that has proven to remain reactive to human triggers.
