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Avalanche Forecast

Archived

Nov 26th, 2021–Nov 27th, 2021

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

Regions

South Columbia.

Cautiously approach avalanche terrain during the pause in this storm. Alpine and treeline elevations produced large natural avalanches Thursday. Careful route finding is still necessary with more reactive slabs found in wind effected terrain.

Confidence

High - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

FRIDAY NIGHT: 2-10cm of snow tapering throughout the day to flurries. Greater accumulation in the north. Freezing levels drop to valley bottom. 

SATURDAY: A calm morning with light southwest winds gives way to an active pattern. 5-10cm of snow is expected over late afternoon with freezing levels at 1000m. The storm intensifies through the night as freezing levels rise above 2000m with 10-20cm of snow expected above, and mixed precipitation or rain below. Winds increase to strong southwest winds.

SUNDAY: Moderate to heavy precipitation throughout the day with freezing levels remaining elevated near 2000m. Strong west-southwest winds persist. 

MONDAY: Winds decrease to moderate westerlies as the storm exits. Light accumulations are expected with clearing skies in the evening. 

Avalanche Summary

On Thursday November 25th, numerous size 1.5 storm slab avalanches were skier triggered, failing on a mid storm weakness. Explosives produced several triggered storm slabs to size 1.5.

Nearby Glacier National Park observed a large natural storm slab avalanche cycle to size 3 on Thursday. This activity was widespread, on all aspects above 1800m. Explosives also produced avalanches up to size 3 in the same area, running full paths to valley bottom.

The North Columbia region reported numerous size 1.5-2 wind slab triggered remotely by machines at 1800m. 

Snowpack Summary

New snow overnight will accumulate over recent wind effected storm snow. Storm totals reach 80cm in some areas and winds from the south and southwest have redistributed this into deeper pockets of denser snow over softer storm snow at alpine and treeline elevations.

The mid November crust is up to 5cm thick and found down 50-120cm on all aspects to 2350m, with faceting below the crust. The lower snowpack contains several early season crusts which appear well bonded with no recent reactivity.

Snowpack depths exceed 300cm at treeline and alpine elevations, while below 1600m depth decreases rapidly. 

Terrain and Travel

  • Expect slab conditions to change drastically as you move into wind exposed terrain.
  • Approach lee and cross-loaded slopes with caution.
  • Back off if you encounter whumpfing, hollow sounds, or shooting cracks.
  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.

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.