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Avalanche Forecast

Archived

Jan 18th, 2020–Jan 19th, 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 and human triggered avalanches likely.
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.

Regions

South Rockies.

Watch for continued wind slab development in exposed terrain Sunday and avoid areas of thin snow cover where triggering deep persistent slab avalanches is more likely.

Confidence

Moderate - Uncertainty is due to rapidly fluctuating freezing levels.

Weather Forecast

After a relatively stormy period the outlook is rather dry as temperatures begin to warm though the weekend.

SATURDAY NIGHT: Freezing level at valley bottom, moderate to strong west/southwest wind, trace of snow possible.

SUNDAY: Scattered cloud cover, freezing level at valley bottom, light southwest wind at lower elevations, strong west/southwest wind at ridgetop, trace of snow possible.

MONDAY: A few clouds, freezing level rising dramatically, perhaps as high as 2500 m, light southwest wind at lower elevations, strong southwest wind at ridgetop, no significant precipitation

TUESDAY: Overcast, freezing level lowering to about 1000 m, light southwest wind, 1 to 5 cm of snow possible.

Avalanche Summary

On Friday wind slab avalanches to size 1 were sensitive to control work on a southeast facing ridge crest around 2000 m. Small natural avalanches were also observed on steep rolls.

Avalanche reports from Thursday were predominantly skier triggered wind slabs up to size 1.5. Our field team also reported a natural size 2 wind slab avalanche (details here) 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.

On Monday, 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 wind slab in alpine lees, and is settling in the mild alpine temperatures. At lower elevations it may remain unconsolidated. It sits over widespread wind slab 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 as temperatures warm on Monday. 

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.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.
  • 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.