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 -12 C, freezing level 1300 m dropping to below valley bottom.FRIDAY: Partly cloudy, light northerly winds, alpine temperature near -13 C, freezing level below valley bottom.SATURDAY: Mostly sunny, light to moderate westerly winds, alpine temperature near -10 C, freezing level below valley bottom.

Storm slab avalanches were reported on Tuesday, being triggered naturally, by explosives, and skiers. They were small to large (size 1 to 2), on all aspects, and depths from 20 to 40 cm. Persistent slab avalanches were large (size 2 to 3), explosively controlled, 100 to 200 cm deep, on all aspects, and generally releasing on the late-November crust (described in the section below).Evidence of the natural 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 layers discussed 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 and strong winds. 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.

Up to 80 cm of snow could accumulate in the region by Thursday afternoon. This overlies 100-200 cm of storm snow from the past two weeks that has formed widespread storm slabs as well as wind slabs in lee features. All of this sits on an unstable snowpack. There are four active weak layers that we are monitoring:1) 80 to 150 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 is 5 to 20 mm in size and was reported up to tree line elevations and possibly higher.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 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.