North Columbia

It is uncertain how reactive the recently buried weak layer is to human triggers. Once triggered, it will likely result in a large avalanche that can have serious consequences. This uncertainty is best managed with conservative terrain choices. 

SUNDAY NIGHT: Clear with increasing clouds, trace of new snow, moderate to strong southwest wind, alpine temperature 0 C, above freezing layer from 1800 to 2200 m. 

MONDAY: Mostly cloudy with snowfall, 5 to 10 cm new snow, moderate to strong west wind, alpine temperature -4 C, above freezing layer will be pushed out Monday morning, freezing level dropping to 1500 m.

TUESDAY: Mostly cloudy, up to 3 cm new snow, light to moderate southwest wind, alpine temperature -5 C, freezing level 1400 m. 

WEDNESDAY: Cloudy with snowfall, 5 to 10 cm new snow, light southwest wind, alpine temperature -2 C, freezing level rising to 2000 m. 

On Saturday, numerous natural storm and wind slab avalanches up to size 2.5 at all elevations and aspects were reported. Explosives triggered several avalanches up to size 2.5. Skiers triggered a few slab avalanches up to size 2. A large (size 2) avalanche was triggered by skiers from a distance, which likely released on the buried weak layer (see this MIN report). A few small slab avalanches (up to size 1.5) released on that same layer naturally. 

On Friday, a natural avalanche cycle of small slabs up to size 1.5 was observed, as well as a few large (size 2.5 to 3) storm slab avalanches on northeast and south aspects at treeline and large (up to size 2.5) wind slab avalanches on north aspects in the alpine. Skiers triggered small storm/wind slab avalanches, and explosives triggered numerous large (up to size 2.5) avalanches. 

A handful more storm and wind slabs were triggered by riders on Thursday to add to the many avalanches observed in the previous few days. Most of the storm slabs released on the surface hoar layer described in the Snowpack Summary, often occurring between around 1700 and 2200 m on east, north, and west aspects. Check out some good example photos here. 

The recent storm brought 20 to 40 cm with locally higher amounts. The snow has formed new storm slabs in areas sheltered from the wind and wind slabs in exposed terrain at higher elevations from strong south to southwest wind. 

The snow loaded a weak layer of surface hoar crystals 5 to 15 mm in size. Warm temperatures overnight Sunday to Monday will keep the likelihood of triggering a slab increased. The layer may be around 60 to 100 cm deep, which is a prime depth for human triggering. The layer is most prominent in areas sheltered from the wind. Example terrain features to treat as suspect include the lee side of protected ridges, openings in the trees, cut blocks, and burns. The layer may not exist on steep sun-exposed slopes, where a melt-freeze crust may be found instead.

The facet/crust layer that formed in early December is buried around 120 to 250 cm. The last reported avalanche was on January 31 from a large explosive, and before that on January 23. Although unlikely, the layer could be triggered from a large load like a cornice fall or a shallower slab avalanche could step down to this layer. Humans are unlikely to trigger it, with perhaps the exception in thin, shallow snowpack areas. Check out the forecaster blog for more information.

• Be aware of the potential for larger than expected storm slabs due to the presence of buried surface hoar.
• Shooting cracks, whumphs and recent avalanches are strong indicators of an unstable snowpack.
• Uncertainty is best managed through conservative terrain choices at this time.