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Avalanche Forecast

Archived

Dec 23rd, 2019–Dec 24th, 2019

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

Regions

North Rockies.

A weak layer buried 30-60 cm deep remains a concern for human triggering. This layer is most likely to be found in the central and northern parts of the region.

Confidence

No Rating - Uncertainty is due to field data and reports showing a wide variation in conditions throughout the region.

Weather Forecast

MONDAY NIGHT: Clear periods, moderate southwest wind, alpine high temperature around -9 C.

TUESDAY: Mainly cloudy with isolated flurries, light to moderate west wind, alpine high temperature around -8 C.

WEDNESDAY: Mix of sun and cloud, light west wind, alpine high temperature around -10 C.

THURSDAY: Mix of sun and cloud, moderate to strong southwest wind, alpine high temperature around -5 C.

Avalanche Summary

Most recent reports of avalanche activity date back to late last week at the tail end of a stormy period. Nonetheless human triggered avalanches remain a concern at all elevations with a buried weak layer 30-60 cm below the surface most specifically in the central and northern parts of the region. See this MIN post from Hasler on Sunday, which uses some great photos to highlight the potential reactivity of the weak layer.

Snowpack Summary

20-40 cm of snow falling late last week as been redistributed by steady wind from the southwest creating wind slabs in open terrain, while soft powder will be preserved in sheltered areas.

The storm snow sits above three weak layers buried between 30 and 60 cm below the surface. These surface hoar and facet/crust layers showed signs of being reactive and have the potential to produce large avalanches. Reports suggest these layers are widespread around Pine Pass and the McGregors, but harder to find around McBride.

The lower snowpack contains several crust layers. In higher snow areas, these are likely well bonded. However, in shallow areas along the eastern slopes, these could be of concern.

Total snowpack depths are approximately 240 cm in the south of the region, 170 cm around Pine Pass and more like 120 cm in areas to the east.

Terrain and Travel

  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.