South Columbia

Reactive new storm slabs have formed over a snowpack that is 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. Light north winds. Freezing level returning to valley bottom with alpine high temperatures to around -12.Saturday: Mainly sunny with lingering valley cloud due to an alpine temperature inversion. Light northwest winds. Alpine high temperatures to around -9. Cooler at lower elevations.Sunday: Mainly sunny with some valley cloud due to a lingering alpine temperature inversion. Light southwest winds. Alpine high temperatures around -8. Temperature inversion breaking down overnight.

Wednesday's reports included observations of storm slabs releasing naturally as well as with ski cutting and explosives generally from size 1-2, with a couple of outliers at sizes 2.5 and 3. No new persistent slab releases were reported in the South Columbias.Many avalanches were reported in the region on Tuesday. Storm and wind slabs were 20 to 50 cm deep, but as deep as 100 cm in wind-loaded features, on all aspects, at all elevations, and triggered naturally, by explosives, and skiers. Persistent slab avalanches often released on the mid-December layer, between 100 and 250 cm deep.Monday's reports showed widespread avalanche activity involving storm, wind, and persistent slabs. The persistent slabs released on all of the weak layers described below, between 100 and 400 cm deep, and on all aspects.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 also have the potential to step down to deeper weak layers, which could produce very large avalanches with high consequences.

Around 30-40 cm of snow fell during Wednesday's storm. 130-240 cm storm snow totals from the past two weeks have now consolidated into a deep slab over four active weak layers:1) 130 to 240 cm of storm snow sits on the crust and/or surface hoar layer from mid-January. The crust is widespread, with the exception of high elevation north aspects. The surface hoar was reported up to tree line and possibly higher. This layer is our primary weak layer of concern, partially for remaining nearest to the depth range of human triggering.2) The early-January persistent weak layer is 160 to 260 cm deep. 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 170 to 280 cm deep. 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.