Avalanche Forecast

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

THURSDAY: Cloudy with snowfall, intense early morning, accumulation 20 to 40 cm, moderate westerly winds, alpine temperature dropping to -10 C, freezing level 1500 m dropping to below valley bottom.FRIDAY: Partly cloudy with possible valley cloud, light to moderate northerly winds, alpine temperature near -14 C, freezing level below valley bottom.SATURDAY: Partly cloudy, light to moderate northwesterly winds, alpine temperature near -10 C, freezing level below valley bottom.

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, they were between 100 and 250 cm deep, on all aspects, and triggered naturally and by explosives.Evidence of the avalanche cycle from Friday to Sunday was observed, with widespread avalanche activity, including storm, wind, and persistent slabs.  The persistent slabs released on all of the weak layers described below, they were between 100 and 400 cm deep, and on all aspects.Looking forward, dangerous snowpack conditions will persist in the region on Thursday with substantial amounts of new snowfall.  This trend will likely continue until a more stable weather pattern governs and we see a decrease in avalanche observations. All of our buried weak layers continue to produce large, destructive avalanches from natural and human triggers. Storm and wind slabs have the potential to step down to deeper weak layers, which could produce very large avalanches with high consequences.

Between 50 and 100 cm of snow may accumulate by Thursday afternoon, with the most in the west of the region. This overlies 100-200 cm of snow from the past two weeks that has formed storm and wind slabs, which sits on an unstable snowpack. There are four active weak layers that we are monitoring:1) 80 to 150 cm of storm 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 is 5 to 20 mm in size and was reported up to tree line elevations and possibly higher. This layer is the primary weak layer of concern and remains within the depth range of human triggering.2) The early-January persistent weak layer is 120 to 170 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 150 to 200 cm 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.