North Columbia

Sunday's storm will bring snow and strong winds, which will load a highly complex snowpack. Three weak layers are reactive to natural and human triggers. The easy solution is to choose simple terrain without overhead hazard as the snowpack adjusts.

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

SUNDAY: Cloudy with snowfall, accumulation 10-20 cm, strong to extreme southwesterly winds, alpine temperature -8 C, freezing level near 900 m.MONDAY: Mostly cloudy with light snowfall, accumulation 2-5 cm, light to moderate southwesterly winds, alpine temperature near -9 C, freezing level near 800 m.TUESDAY: Partly cloudy with intermittent snowfall, accumulation 1-3 cm, light to moderate southwesterly winds, alpine temperature near -10 C, freezing level near 600 m.

On Friday, there were several reports of small to large (size 1 to 3) storm slab, wind slab, and persistent slab avalanches, triggered naturally and by skiers and snowmobilers. Similar avalanches were reported on Wednesday and Thursday. These avalanches have been releasing on all three persistent weak layers described in the Snowpack Discussion. Expect similar avalanches to release on all aspects and at all elevations with new load.

The current snowpack is complex, we now have three active weak layers that we are monitoring.30-50 cm of storm snow and new forecasted snow for Sunday sits on a newly formed crust and/or surface hoar layer (mid-January). The crust is reportedly widespread, except for possibly at high elevations on north aspects. The mid-January surface hoar is 5 to 20 mm in size and was reported up to 2100 m and possibly higher. The new snow is falling with strong southwesterly winds, producing wind slabs in lee features. Deeper in the snowpack, the early-January persistent weak layer is 50 to 90 cm below the surface. It is composed of surface hoar on sheltered slopes as well as sun crust on steep solar aspects and is found at all elevation bands. Snowpack tests show sudden fracture characters with moderate loads and high propagation potential, and signs of instability such as whumpfs and cracking. Another weak layer buried mid-December consisting of a facet/surface hoar/crust combination is buried 90 to 150 cm deep. It is most problematic at and below tree line.A rain crust buried in November is 100 to 200 cm deep and is likely dormant for the time being.