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Avalanche Forecast

Archived

Jan 20th, 2020–Jan 21st, 2020

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
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 unlikely, human triggered possible.

Regions

South Rockies.

Freezing levels are forecast to hover around 1400 m for the next couple of days before spiking into the alpine on Thursday. Until then, windslabs remain the main concern at upper elevations.

Confidence

Moderate - Uncertainty is due to the fact that deep persistent slabs are particularly difficult to forecast. Uncertainty is due to the limited number of field observations.

Weather Forecast

Monday night: Clear. Moderate southwest winds. Alpine low -7 C. Freezing levels dropping to valley bottom.

Tuesday: Mostly cloudy with isolated flurries bringing a trace of new snow. Moderate southwest winds. Alpine high -4 C. Freezing level around 1400 m.

Wednesday: Broken cloud with isolated flurries bringing a trace of new snow. Moderate west to southwest winds. Alpine high -3 C. Freezing level around 1400 m.

Thursday: Mix of sun and cloud with isolated flurries bringing a trace of new snow. Moderate west to southwest winds, becoming strong at ridgetop. Alpine high 0 C. Freezing level around 1800 m.

Avalanche Summary

On Monday we received reports of natural and explosive triggered windslab avalanches size 1.5-2.

On Thursday a natural size 2 wind slab stepped down to the deep persistent basal facets below a rock band, details here. 

Snowpack Summary

A crust can be found on solar aspects at lower elevations due to daytime warming on Monday. Recent snow has been loaded into pockets of soft windslab in alpine lees, and is settling in the mild alpine temperatures. At lower elevations it may remain unconsolidated. It sits over widespread windslab in exposed areas at all elevations.

A well consolidated mid-snowpack overlies a generally weak basal snowpack. The bottom 10-20 cm of the snowpack consists of facets and deteriorating crusts. There is potential for this deep persistent layer to reawaken in the warm alpine temperatures later in the week.

Terrain and Travel

  • In times of uncertainty conservative terrain choices are our best defense.
  • Be careful with wind slabs, especially in steep, unsupported and/or convex terrain features.
  • Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.