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Avalanche Forecast

Archived

Dec 26th, 2019–Dec 27th, 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 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.

Regions

Kootenay Boundary.

Our persistent slab problem is evolving into a low probability/high consequence scenario where you may not get feedback from a bad decision. The formation of new wind slabs will add a layer of complexity to your terrain selection on Friday.

Confidence

Moderate -

Weather Forecast

Thursday night: Cloudy with isolated flurries bringing a trace to 5 cm of new snow. Light to moderate southwest winds increasing into Friday.

Friday: Cloudy with isolated flurries and a trace of new snow. Moderate to strong west winds shifting northwest. Alpine high temperatures around -8.

Saturday: Mainly cloudy. Light to moderate west winds. Alpine high temperatures around -7.

Sunday: Mainly sunny. Light north winds. Alpine high temperatures around -4.

Avalanche Summary

A recent large (size 2) 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.

Several more large persistent slabs were triggered with explosives control in the Nelson area on Monday. These slabs featured crown depths of 15-120 cm, giving some evidence of wind loading. These avalanches are suspected to have released on our early November facet/crust layer, which exists below two other persistent weak layers of concern.

A widespread avalanche cycle occurred on Saturday, with observations of large artificially triggered avalanches continuing into Sunday. Large avalanches were observed on all aspects, generally above 2000 m. Numerous other large to very large avalanches were also triggered by explosives on the late-November layer described in our Snowpack Summary.

Snowpack Summary

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 now buried around 70 to 110 cm.
  • an older surface hoar layer buried about 90 to 130 cm, associated with a melt-freeze crust on steep south aspects.
  • a complex layer of weak and sugary faceted snow, surface hoar, and melt-freeze crusts in the bottom half of the snowpack, buried in late November.

Although avalanche activity on these layers has been declining since the storm, they each continue to produce concerning snowpack test results and they are expected to heal slowly.  

With this in mind, it remains prudent to make terrain decisions with the understanding that one or more of these deeply buried layers are present and could produce a large and destructive avalanche with a human trigger. 

Terrain and Travel

  • Don't let the desire for deep powder pull you into high consequence terrain.
  • Make conservative terrain choices and avoid overhead hazard.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

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.