South 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, light to moderate southwest wind, alpine temperature +1 C, above freezing layer from 1600 m to 2500 m. 

MONDAY: Mix of sun and cloud, up to 5 cm new snow, moderate to strong west wind, alpine temperature -2 C, the above freezing layer will be pushed out Monday morning, freezing level dropping to 1500 m.

TUESDAY: Mix of sun and cloud, up to 3 cm new snow, light west wind, alpine temperature -5 C, freezing level 1400 m.

WEDNESDAY: Cloudy, up to 5 cm new snow, light west wind, alpine temperature-2 C, freezing level rising to 2100 m. 

On Sunday, at the time of writing, explosives triggered numerous storm slabs up to size 2 below treeline on southeast aspects.

On Saturday, several large natural storm and wind slabs up to size 3 were observed on all aspects in the alpine. Explosives triggered avalanches up to size 2.5. Riders triggered many small wind slabs up to size 1.5 at treeline as well as a large (size 3) wind slab in the north of the region (see this MIN report). Many avalanches released on the weak layer that was buried at the end of January. 

On Friday, a natural avalanche cycle of storm slabs up to size 2 was observed, as well as a large size 3 avalanche on an east aspect. Numerous small rider triggered avalanches and a large (size 2) storm slab triggered by a skier were reported. Explosives triggered numerous large (size 2) avalanches. A deep persistent slab avalanche was triggered by a machine and released on the early December facet/crust layer described in the Snowpack Summary.

Many 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 1600 and 2200 m on all aspects except due south. Read this MIN for a detailed description of a skier-triggered avalanche in the south of the region -- thank you for the detailed report.

The recent storm brought 10 to 30 cm snow. The snow has formed 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 40 to 80 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 100 to 240 cm. The last reported avalanche was on February 4 in the southwest of the region. 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.

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