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Avalanche Forecast

Archived

Nov 27th, 2021–Nov 28th, 2021

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

Increasing temperatures, wind and precipitation embedded within this storm will test the snowpack. Avalanches could run far and step down to deeper layers.

Confidence

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

Weather Forecast

Saturday night: A storm will hit the region tonight bringing up to 10mm of precipitation. The North part of the region will see freezing levels rise to 1500m and moderate to strong Southwest winds. The Southern part of the region will see more intense weather, freezing levels will rise to 2000m and winds will be strong from the Southwest.

Sunday: Freezing levels will rise to 2000m throughout the range. 10mm of precipitation is expected for most of the region except for the Bugaboos where up to 35mm is expected. Winds will be strong from the Southwest.

Monday: Some light flurries throughout the day. Freezing level 1700m and light to moderate Southwest winds.

Tuesday: Some light flurries with light to moderate Southwest winds. Freezing levels around 1800m.

Avalanche Summary

Several storm slabs up to size 2 were reported in the Purcells on Friday. We suspect a natural cycle will take place early Sunday morning with the incoming storm.

Snowpack Summary

The incoming storm will form new storm slabs and wind slabs in the treeline and alpine. A surface hoar layer has been observed down 20cm in some parts of the range. A mid November rain crust is down 30 to 60cm and exists below 2100m.

Several early season crusts exist throughout the lower pack. A late October facet/crust layer sits at the bottom of the snowpack. it likely only exists above 1900m.

Terrain and Travel

  • Use caution on large alpine slopes, especially around thin areas that may propagate to deeper instabilities.
  • Use increased caution at all elevations. Storm snow is forming touchy slabs.

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.