Increasingly reactive slabs will develop through Friday from a combination of new snow, strong winds, and rising freezing levels.
An increasingly reactive storm slab will develop as snowfall accumulates and is impacted by wind. Tread carefully around open and ridge features at treeline and above, new snow and storm slabs may hide older, stiff wind slabs. Rain and rising freezing levels will encourage loose-wet avalanches at lower elevations.
Facetted, weak snow at the base of the snowpack is a cause for concern with forecasted weather. Rapid loading from new snow and wind or rapid warming from rising freezing levels or rain can stress previously inactive weak layers in the snowpack. If this layer "wakes up" with increasing load and temperatures, it will produce large and destructive avalanches.
Thursday night: Mostly cloudy with isolated flurries. Alpine low temperature -8 C. Moderate to strong west-southwest wind.
Friday: Flurries and snow, 5-20 cm. Alpine high temperature +1 C. Moderate to strong west-southwest wind. Freezing level rising to 2000 m.
Saturday: Snow, 10-20 cm. Alpine high temperature -1 C. Moderate southwest wind gusting to extreme. Freezing level dropping to 1600 m.
Sunday: Mix of sun, cloud, and isolated flurries. Alpine high temperature -8 C. Light southwest wind with strong gusts. Freezing level dropping to valley bottom.
Several recent natural, small to large (size 1-2.5) wind slab avalanches were reported on northwest through northeast aspects at upper elevations on Monday.
On January 16th, a natural, size 2 wind slab stepped down to the deep persistent basal facets below a rock band, details here.
Southwesterly winds have scoured exposed areas at treeline and above and developed stiff windslabs previously wind affected surfaces. With strong winds, slabs have formed further down into lee terrain features and cornices have grown. A crust can be found up to 1700 m and higher on solar aspects in the alpine due to previous warming and sun exposure.
A well consolidated mid-pack overlies a generally weak basal snowpack. The bottom 10-20 cm of the snowpack consists of facets and deteriorating crusts. Although inherently weak, the benign weather pattern early this week likely promoted a decreasing trend in reactivity for this avalanche problem. Areas that are most likely to harbor this problem are shallow, rocky start zones.
