South Columbia

Snowfall should taper Tuesday night, but strong to extreme winds are expected to continue to produce natural avalanches Wednesday. Seek out conservative terrain free of overhead hazard.

Moderate - Timing, track, or intensity of incoming weather system is uncertain on Wednesday

A series of powerful pacific frontal systems are taking aim at BC this week, the Interior Ranges are poised to receive significant snow and wind. TUESDAY NIGHT: Freezing level around 900 m, moderate wind at treeline, strong to extreme southwest wind in the alpine switching to northwest/west pre-dawn, 5 to 20 cm of snow.WEDNESDAY: Scattered cloud cover in the morning building to overcast throughout the day, freezing level beginning around valley bottom rising to 1200 m, west/northwest wind, moderate at treeline, strong in the alpine, trace of snow with convective locally heavy flurries possible. 5 to 10 cm Wednesday night.THURSDAY: Overcast, freezing level around 1500 m, moderate to strong southwest wind at treeline, strong to extreme westerly wind in the alpine, 5 to 10 cm of snow.FRIDAY: Overcast clearing to a few clouds in the evening, freezing level beginning around 1500 m potentially rising has high as 2000 m, light southerly wind at treeline, strong southwest wind in the alpine, trace of precipitation possible.

On Monday avalanche control work produced storm slab and loose dry avalanches to size 1.5 on north through east facing terrain. Isolated pockets of wind slab were also observed above 2000 m.A MIN report from Saturday in the Keefer lake area described a size 2 slab that released naturally from a steeper, open feature below treeline. The failure plane of the slab is uncertain but it suspected to be one of our mid-November surface hoar layers, possibly where it is combined with an underlying crust. The report also describes another snowmobile-triggered avalanche that occurred in the Kaslo area. Details on the incident are not complete but suggest that at least one individual was partly buried.

As of Tuesday afternoon the storm has produced around 20 cm of storm snow which lies on the December 9th weak layer. This layer consists of facets, surface hoar, and a crust on solar aspects. By Wednesday morning there could be 40 cm of storm snow on this interface out of the wind, and easily double that in lee alpine features. Lower in the snowpack, a pair of weak layers buried in mid-November are 40 to 80 cm below the surface. These layers present as surface hoar and a suncrust. They are most prevalent at treeline but may be found in sheltered alpine terrain too. While not recently reactive, this storm could push them to their breaking point. The most likely place to see an avalanche failing on this interface would be on south aspects at treeline where the surface hoar formed above a sun crust.At the base of the snowpack is a crust that formed near the end of October. Concern for this layer is dwindling but it may still be worth considering in places such as steep, rocky, alpine terrain, especially where the snowpack is shallow. It would likely take a large trigger such as a cornice fall to produce an avalanche on this layer.