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Avalanche Forecast

Archived

Jan 6th, 2020–Jan 7th, 2020

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

Regions

North Rockies.

Wind slab avalanches remain likely at higher elevations, plus concerns about a buried weak layer warrants careful terrain choices.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

MONDAY NIGHT: Clear skies with a few clouds, light to moderate wind from the west, alpine temperatures drop to -18 C.

TUESDAY: Mix of sun and cloud in the morning with increasing clouds and isolated flurries in the afternoon, light wind in the morning then moderate gusts from the southeast in the afternoon, alpine high temperatures around -12 C.

WEDNESDAY: 5-15 cm of new snow overnight then a mix of sun and cloud in the afternoon, light wind from the north, alpine high temperatures around -15 C.

THURSDAY: Mix of sun and cloud with some light flurries of snow, light wind from the west, alpine high temperatures around -15 C.

Avalanche Summary

Windy conditions over the weekend resulted in natural avalanche activity at higher elevations (such as these MIN reports from Hasler and Bijoux). Most of the natural wind slab avalanches were reported on north and east facing terrain. Several large persistent slab avalanches were reported on buried surface hoar last week, but there are no reports of avalanches on this layer over the weekend. As this layer gets buried deeper it becomes less likely to trigger, but the consequences are high.

Snowpack Summary

A series of big storms last week ended with strong wind that heavily affected open terrain and formed unstable wind slabs in steep lee terrain. Storm totals range from 40-80 cm, meaning deep powder can still be found in sheltered terrain. The main layer of concern is a feathery surface hoar layer that was buried around Christmas and is now resting about 100 cm below the surface. While this layer was very reactive across the region last week, recent reports suggest that it is now a much more spatially variable problem. There has been more recent evidence of this problem in the southern parts of the region around McBride and the McGregors, but preserved surface hoar could potentially be found on sheltered convex slopes anywhere across the region.

Terrain and Travel

  • Avoid freshly wind loaded terrain features.
  • Approach steep open slopes at and below treeline cautiously, buried surface hoar may exist.
  • Surface hoar distribution is highly variable. Avoid generalizing your observations.

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.