North Columbia

Deep and reactive new storm slabs have formed on an unstable snowpack capable of producing very large avalanches. A Special Avalanche Warning is in effect in this region. Copy this address to view details: http://bit.ly/2nSOUyX

High -

Friday: A mix of sun and cloud with gradual clearing over the day. Light north winds. Alpine high temperatures to -12.Saturday: Mainly sunny with valley cloud due to an alpine temperature inversion. Light southwest winds. Alpine high temperatures to around -9, cooler at lower elevations.Sunday: Mainly sunny with valley cloud due to a lingering temperature inversion. Light southwest winds. Alpine high temperatures to -8, cooler at lower elevations.

Wednesday's reports included observations of storm slabs releasing both naturally and with ski cutting from size 1-2. No new persistent slab releases were observed in the North Columbias.On Tuesday, numerous storm slabs were triggered naturally, by explosives, and skiers. They were small to large (size 1 to 2), on all aspects, with depths from 20 to 40 cm. Persistent slab avalanches were large (size 2 to 3), explosives-controlled, 100 to 200 cm deep, on all aspects, and generally releasing on the late-November crust.Monday's reports showed evidence of a natural avalanche cycle from Friday to Sunday, with widespread storm, wind, and persistent slab avalanche activity. The persistent slabs released on all aspects and on all of the layers discussed below. Crown fractures ranged from 100 to 400 cm deep.Expect dangerous snowpack conditions to persist on Friday as our snowpack adjusts to the new load. A stable weather pattern in the forecast suggests we may see a decrease in natural avalanches, but human triggering remains a big concern. Storm and wind slabs have the potential to step down to deeper weak layers, which could produce very large avalanches with high consequences.

25-70 cm of new snow fell during Wednesday's storm. In some areas this buried a new layer of surface hoar present on all aspects and elevations. The new snow adds to 100-200 cm of storm snow from the past two weeks, which sits on an unstable snowpack with four other active weak layers:1) 150 to 270 cm of snow sits on the crust and/or surface hoar layer from mid-January. The crust is reportedly widespread, except for possibly high elevation north aspects. The surface hoar was reported up to tree line elevations and possibly higher.2) The early-January weak layer is 170 to 300 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.3) Another weak layer buried mid-December consists of a facet/surface hoar/crust combination, which is buried 200 cm or more below the surface. It is most problematic at and below tree line.4) A crust/facet layer from late November is yet another failure plane responsible for recent very large avalanches.The wide distribution and ongoing reactivity of these layers suggests that choosing simple terrain free of overhead hazard is the best avoidance strategy.