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Avalanche Forecast

Archived

Dec 18th, 2021–Dec 19th, 2021

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

Regions

Purcells.

Saturday's storm created storm slabs in the alpine and treeline that will likely still be reactive to human triggering. Anticipate larger, more reactive slabs in wind loaded areas. Uncertainty exists around a buried persistent weak layer. Learn more about how to manage it here.

Confidence

Moderate - Forecast precipitation (either snow or rain) amounts are uncertain. Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Saturday Overnight: 5-25 cm of new snow, tapering into the morning. Strong southwest winds easing to moderate and shifting west. Alpine temperatures dropping to around -13 C.

Sunday: Partially cloudy with light flurries, trace to 5 cm of accumulation. Light northwesterly winds and alpine temperatures around -16 C.

Monday: Partially cloudy with a chance of light flurries. Winds increasing throughout the day, moderate from the west. Alpine temperatures around -16 C.

Tuesday: Mainly clear, moderate west winds at ridgetop. Alpine temperatures rising to around -13 C.

Avalanche Summary

On Saturday, operators reported reactive wind slabs developing throughout the day.

On Friday, operators in the eastern Purcells triggered several size 2-2.5 deep persistent avalanche with explosives on N and NW aspects in the alpine.

Snowpack Summary

Yesterday, we received 20-40cm of new snow accompanied by strong to extreme southwest winds. Storm slabs are expected to have formed in the alpine and treeline. In wind exposed areas strong southwest wind will have been the dominant feature, transported the new snow into deeper deposits in lee features. 

Below this new snow, consolidated snow from the previous week's storm sits over a substantial crust that formed in early December. This crust is 10-30 cm thick and can be found down 60-120 cm. It is present across aspects below 2300m. A thin layer of weak facets (sugary snow) can found above this crust that has demonstrated reactivity in snowpack tests and recent avalanche activity. 

A late October facet/crust layer of concern sits at the bottom of the snowpack above 1900m. Although this layer has been trending less reactive, it remains on our radar.

The snowpack depth at treeline is around 115-200 cm. The deepest snowpack can be found near the Bugaboos. 

Terrain and Travel

  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Avoid shallow snowpack areas, rock outcroppings and steep convex terrain where triggering is most likely.

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.