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Avalanche Forecast

Archived

Jan 11th, 2021–Jan 12th, 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 and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Lizard-Flathead.

Fresh snow is expected to continue to accumulate, with strong southwest wind. Storm slabs will likely become deeper and more reactive throughout the day. Conservative terrain selection is recommended.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

MONDAY NIGHT - Mainly cloudy with a few flurries, 5 cm / southwest wind 20-50 km/h / alpine low temperature near -5 

TUESDAY - Snow, 15-20 cm, with another 20-30 cm possible overnight / southwest wind 40-80 km/h / alpine high temperature near -1 / freezing level 1500 m

WEDNESDAY - Mainly cloudy with a few flurries, 5 cm / southwest wind 60-80 km/h / alpine high temperature near 0 / freezing level 2000 m

THURSDAY - A mix of sun and cloud / west wind 10-20 km/h / alpine high temperature near -10

Avalanche Summary

Fresh storm slabs are expected to form throughout the day on Tuesday, likely becoming increasingly reactive as the day goes on and the new snow piles up.

On Sunday, there were a few explosives triggered persistent slab avalanches to size 2.5. These were reported to have failed on weak facets above a crust that was buried in early December.

On Saturday, explosive testing near Fernie produced several large persistent slab avalanches up to size 2.5 also failing on the early December persistent weak layer.

A couple of large (size 3) naturally triggered persistent slab avalanches were reported on large alpine features on Thursday. These avalanches were triggered by either smaller wind slab avalanches, or cornice falls. These are continued reminders of the "low probability; high consequence" scenario that persistent slab problems often create. 

Snowpack Summary

15-20 cm of snow is expected throughout the day on Tuesday, with moderate southwest winds. Fresh storm slabs will be forming, and will likely become increasingly reactive during the day.

Lingering wind slabs formed by recent snow and wind remain possible to human trigger in isolated areas. Roughly 30-50 cm of snow is settling above a crust that extends up to 1900 m. In isolated areas below treeline, this recent snow may be sitting on a weak layer of surface hoar.

The main feature we are monitoring in the snowpack is a layer of weak faceted snow over a hard melt-freeze crust found around 150 cm down. Recent sporadic, large naturally triggered avalanches have occurred on this layer on large alpine slopes and were triggered by either smaller wind slabs in motion or cornice falls.

Terrain and Travel

  • Storm slab size and sensitivity to triggering will likely increase through the day.
  • Be carefull around freshly wind loaded features.
  • Avoid exposure to slopes that have cornices overhead.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.

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.