Avalanche Forecast

Archived

Jan 30th, 2020–Jan 31st, 2020

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

South Rockies.

Increasingly reactive slabs will develop through Friday from a combination of new snow, strong winds, and rising freezing levels.

Confidence

Moderate - Uncertainty is due to the track & intensity of the incoming weather system.

Weather Forecast

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.

Avalanche Summary

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.

Snowpack Summary

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.

Terrain and Travel

Storm slab size and sensitivity to triggering will likely increase through the day.
Watch for wind-loaded pockets especially around ridgecrest and in extreme terrain.
Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.
Be aware of the potential for large avalanches due to the presence of a deep persistent slab.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.