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Avalanche Forecast

Archived

Jan 3rd, 2020–Jan 4th, 2020

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

Regions

North Rockies.

Human triggered avalanches are likely this weekend as fresh snow sits above a fragile weak layer.

Confidence

High - The number, quality, or consistency of field observations is good, and supports our confidence.

Weather Forecast

FRIDAY NIGHT: Cold front crosses the region, 15 cm of new snow, strong wind from the southwest, freezing level drops to valley bottom with alpine temperatures dropping to -10 C.

SATURDAY: Flurries with accumulations of 10-15 cm in southeast parts of the region and 5 cm in northwest parts of the region, moderate to strong wind from the west, alpine high temperatures around -6 C.

SUNDAY: Mostly cloudy with scattered flurries, 5-10 cm of new snow, strong wind from the southwest, alpine high temperatures around -4 C.

MONDAY: Mostly cloudy with scattered flurries, 5-10 cm cm of new snow, moderate wind from the south, alpine high temperatures around -6 C.

Avalanche Summary

Reports continue to confirm persistent slab avalanches are easy to trigger. Prior to New Year's, several large avalanches were reported on a buried surface hoar layer (including these MIN reports from the Farm and Pine Pass). Since then this persistent slab has continued to be reactive on even small terrain features like cut banks (see MIN reports from McBride and Silver Sands). With the additional snow from Friday's storm, both storm slab and persistent slab avalanches are a concern this weekend.

Snowpack Summary

A frontal system crossing the region on Friday will deliver 20-40 cm of fresh snow by Saturday afternoon. This will leave a weak layer of feathery surface hoar buried 50-70 cm below the surface. Reports suggest this layer has been reactive in all parts of the region, as numerous avalanches have been reported over the past week. This layer remains at a burial depth where it can easily be triggered by humans.

There are a pair of potential weak layers in the middle of the snowpack, a surface hoar layer and a layer of facets above a relatively thin crust. Both these layers appear to be gaining strength. The lower snowpack contains several crust layers, which could be a concern in shallow areas along the eastern slopes of the region.

Terrain and Travel

  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Avoid freshly wind loaded terrain features.
  • Dial back your terrain choices if you are seeing more than 25cm of new snow.

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.