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Avalanche Forecast

Archived

Dec 28th, 2019–Dec 29th, 2019

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

Regions

Kootenay Boundary.

The persistent slab is evolving to a low probability/high consequence scenario where you may not observe any signs of unstable snow before making a dangerous decision. Managing isolated wind slabs adds further complexity to choosing safe terrain on Sunday.

Confidence

Moderate - We are confident about the possible sizes of avalanches, should one release; what is less certain is the likelihood of triggering. Uncertainty is due to the extreme variability of wind effect on the snowpack.

Weather Forecast

Saturday night: Partly cloudy, light northwest winds, alpine temperatures around -7 C. 

Sunday: Mix of sun and cloud, isolated flurries with a trace of accumulation, light north winds, alpine high temperatures around -3 C, with freezing level near 800 m.

Monday: Mostly clear, light west winds, alpine high temperatures around -3 C, with freezing level near 700 m.

Tuesday: Mostly cloudy, 10-15 cm of snow possible during the day and overnight, moderate southwest winds, alpine high temperatures near -5 C, with freezing level around 600 m.

Avalanche Summary

Several large (size 2-3) persistent slab avalanches, both human and explosive-triggered, were reported on Saturday. These avalanches released on both the December surface hoar and November crust layers across a variety aspects. A recent large skier-triggered persistent slab avalanche was observed in the Kelowna area on Wednesday. It featured a 30-70 cm crown fracture, scrubbed into the lower snowpack, and was noted for being triggered on a shallow, rocky, convex slope.

These avalanches offer a much needed reminder that large, destructive avalanches are still possible even a week out from the storm and that this persistent problem still needs time to heal.

Snowpack Summary

15-20 cm of new snow has fallen on a weak interface that may be reactive on lee features in the alpine where winds have been strong enough to form slabs. 

The stormy period that ended early this week saw around 70 to 100 cm of snow deposited in the region. This snow brought a significant load to multiple weak layers, including a feathery surface hoar layer (down 70 to 110 cm), an older surface hoar layer associated with a melt-freeze crust on steep south aspects (down 90 to 130 cm), and

a layer of sugary faceted snow, surface hoar, and melt-freeze crusts buried in late November found in the bottom half of the snowpack. Snowpack tests continue to produce sudden results on these layers (check out this MIN report from Friday).

Although signs of instability are becoming less obvious, it remains prudent to make terrain decisions with the understanding that one or more of these deeply buried layers are present and could produce large and destructive avalanches with human triggers.

Terrain and Travel

  • Uncertainty is best managed through conservative terrain choices at this time.
  • Persistent slabs have potential to pull back to lower angle terrain.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.

Problems

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.

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.