Dashboard Regions Weather Stations Radar Alerts Glossary
Contact About
Log In

Register for an account and never miss a forecast again!

Register

Avalanche Forecast

Archived

Dec 27th, 2019–Dec 28th, 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.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Kootenay Boundary.

The persistent slab problem is evolving into a low probability/high consequence scenario where you may not observe any indication of unstable snow before making a dangerous decision. The formation of new wind slabs will add a layer of complexity to terrain selection on Saturday.

Confidence

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

Weather Forecast

Friday night: Mostly cloudy. Moderate northwest winds. Temperature around-10 C. A trace of accumulation possible.

Saturday: Mix of sun and cloud. Light to moderate west winds. Alpine high temperatures around -7 C. No precipitation expected.

Sunday: Mostly cloudy. Light north winds. Alpine high temperatures around -4 C. Scattered flurries with a trace of accumulation possible.

Monday: Mix of sun and cloud. Light northwest winds. Alpine high temperatures around -3 C. No precipitation expected.

Avalanche Summary

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.

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 last weekend 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

10-15 cm of new snow has fallen on a weak interface with moderate west winds forming shallow, touchy wind slabs on lee features in the alpine. 

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 are expected to heal slowly. 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 snowpack areas, rock outcroppings and steep convex terrain where triggering is most likely.
  • 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.