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Avalanche Forecast

Archived

Jan 8th, 2020–Jan 9th, 2020

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 possible, human triggered probable.
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 possible, human triggered probable.

Regions

North Rockies.

A mix of sun and cloud on Thursday will switch to snow just in time for the weekend. Concern for a buried weak layer continues to warrant careful terrain choices.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

WEDNESDAY Night: Mainly cloudy, light wind from the northwest, alpine high temperatures around -20 C.

THURSDAY: Mix of sun and cloud with some light flurries of snow, light wind from the west, alpine high temperatures around -17 C.

FRIDAY: Flurries, accumulation 5-10 cm, moderate wind from the west, alpine high temperatures around -19 C.

SATURDAY: Flurries, accumulation 5-10 cm, moderate wind from the northwest, alpine high temperatures around -25 C.

Avalanche Summary

There is no new avalanche activity to report from the past few days. 

That being said, there is still concern for triggering a weak layer of buried surface hoar. Several large persistent slab avalanches were reported on this layer last week. As the layer gets buried deeper it becomes less likely to trigger, but the consequences of doing so are high.

Snowpack Summary

5-20 cm of new snow falling Tuesday into Wednesday now sits above previously heavily wind affected snow. 

The main layer of concern is a feathery surface hoar layer that was buried around Christmas and is now resting about 90-110 cm below the surface. While this layer was very reactive across the region last week, recent reports suggest that it's reactivity is localized to certain parts of the region. 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

  • Be careful as you transition into wind affected terrain.
  • Surface hoar distribution is highly variable. Avoid generalizing your observations.
  • Approach steep open slopes at and below treeline cautiously, buried surface hoar may exist.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Uncertainty is best managed through conservative terrain choices at this time.

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.