New snow (and maybe rain at lower elevations), gusty winds, and rising temperatures are priming the snowpack for a natural avalanche cycle. Hazard will rise through Friday, peaking later in the day.
A weak layer that formed as a crust in mid-November persists in the region. This layer my awaken as rapid loading from new snow, wind, or smaller storm slab avalanches stresses deeper instabilities, resulting in larger and more destructive avalanches.
Rapid loading from snow and gusty winds will produce touchy and reactive storm slabs. The likelihood of triggering an avalanche will be increase towards the end of the day as new snow accumulates and is redistributed by wind.
Thursday Night: Snow, 10-20 cm. Alpine temperature -7 C. Southwest wind 35 gusting to 65 km/hr.
Friday: Snow, 15-40 cm. Alpine temperature -3 C. Southwest wind 20-40 gusting to 65 km/hr. Freezing level rising to 1600 m, potentially producing wet snow and rain at lower elevations.
Saturday: Continued snow and flurries, 10-30 cm. Alpine temperature -1 C. Southwest wind 30-40 gusting to 75 km/hr. Freezing level 1500 m.
Sunday: Flurries, 5-15 cm. Alpine temperature -6 C. Southwest wind 20 gusting to 60 km/hr. Freezing level dropping to valley bottom.
Natural wind slab avalanches (size 1.5) were reported Wednesday morning, with crowns 50-60 cm deep, these avalanches were suspected to have failed overnight Tuesday as strong winds reached 90 km/hr.
On Wednesday, two large natural avalanches in the Morrissey area were reported to have failed on a deeper instability, a crust from mid-November. A cornice failure triggered a size 2.5 avalanche, and one size 2 slab avalanche failed naturally, both avalanches had crown depths 70-100 cm, were on east-northeast aspects, and suspected to have been triggered by strong winds. This layer has been trending towards unreactive recently, but this report suggest that triggering is still possible with large loads or in areas with a thin and variable snowpack.
Explosives testing was able to trigger a handful of small (size 1) slab avalanches on Tuesday from steep alpine terrain.
Prior to this weather system, wind had impacted and redistributed loose snow, and formed wind slabs in exposed alpine terrain and around ridge features. In sheltered areas, 20-40 cm of recent snow was gradually settling. New snow will fall on a variable and wind-affected surface.
Crust layers from November and October can be found 40-100 cm below the surface. These layers produced large avalanches with explosive triggers around Dec 13-14, but since then have appeared to gain strength, but may become overloaded with incoming snow.
Snowpack depths range between 60-130 cm at higher elevations and taper rapidly below treeline.
