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Avalanche Forecast

Archived

Jan 12th, 2021–Jan 13th, 2021

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

Regions

Lizard-Flathead.

Widespread natural avalanche activity is expected. Avoid avalanche terrain, including overheard hazard.

Confidence

High - We are confident a natural avalanche cycle will begin shortly after the arrival of the incoming weather.

Weather Forecast

TUESDAY NIGHT - Snow, 20-30 cm / southwest wind, 40-80 km/h / alpine low temperature near -2 / freezing level 2000 m

WEDNESDAY - Flurries, 10-15 cm / west wind, 60-80 km/h / alpine high temperature near -1 / freezing level 1500 m

THURSDAY - Mainly sunny / light northwest wind / alpine high temperature near -8 

FRIDAY - A mix of sun and cloud / light to moderate west wind / alpine high temperature near -7

Avalanche Summary

A natural avalanche cycle is expected on Wednesday. This may include very large persistent slab avalanches.

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

Tuesday brought upwards of 30 cm of new snow to the region, and another 30-45 cm with strong to extreme southwest wind is expected between Tuesday night and Wednesday afternoon. Storm slabs are anticipated to be widespread and reactive.

Roughly 50-80 cm of snow is now sitting on 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 90-150 cm down. Recent sporadic, large, naturally triggered avalanches have occurred on this layer on large alpine slopes and were triggered by either smaller wind slab avalanches, or cornice falls.

Terrain and Travel

  • Avoid all avalanche terrain during periods of heavy loading from new snow and wind.
  • Avoid exposure to overhead avalanche terrain, avalanches may run surprisingly far.
  • 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.