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Avalanche Forecast

Archived

Jan 17th, 2020–Jan 18th, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
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.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

South Rockies.

There is potential for deep persistent weak layers to become active as temperatures rise in the alpine this weekend. Avoid windslabs, rocky outcroppings and areas of thin snow cover where triggering these deep avalanches is most likely.

Confidence

Moderate - Uncertainty is due to whether buried persistent weak layers become active, triggering avalanches, with the arrival of the forecast weather.

Weather Forecast

Friday night: Mostly clear. Moderate west winds. Alpine low around -16.

Saturday: Increasing cloud with isolated flurries bringing a trace of new snow. Moderate west to southwest winds, becoming strong at ridgetop. Alpine high around -8.

Sunday: Mix of sun and cloud. Moderate southwest winds, strong at ridgetop. Alpine high around -3.

Monday: Mix of sun and cloud. Moderate southwest winds, strong at ridgetop. Alpine high around 0.

Avalanche Summary

Avalanche reports from Thursday were predominantly skier triggered windslabs up to size 1.5. Our field team also reported a natural size 2 windslab avalanche that stepped down to the deep persistent basal facets below a rock band. This is exactly the type of terrain where deep persistent slab avalanches are typically triggered and may be a preview of what is to come over the weekend...

On Sunday, the combination of the warming alpine temperatures and the sun coming out will likely result in increased sensitivity to triggering in recent snow, especially in steep south facing terrain. This could be just the trigger for a resurgence in deep persistent slab avalanche activity.

Snowpack Summary

Recent snow has been loaded into pockets of soft windslab in alpine lees, and is settling in the mild alpine temperatures. At lower, colder, calmer 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 weekend.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Seek out wind sheltered terrain below treeline where you can avoid wind slabs and find great riding.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.

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.