Avalanche Canada ahanna, Avalanche Canada

South Columbia Avalanche Forecast

Dec 28th, 2019 4:00PM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Persistent Slabs and Wind Slabs.

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

Summary

Confidence

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

Weather Forecast

Saturday night: Scattered cloud with isolated flurries and trace accumulation. Alpine low -9 C, alpine wind light northwest.

Sunday: Cloudy with sunny breaks, alpine high -5 C, alpine wind light northeast.

Monday: Scattered cloud, alpine high -5 C, alpine wind light west building to moderate overnight.

Tuesday: Flurries with 10-20 cm accumulation, alpine high -5 C, alpine wind light to moderate southwest.

Avalanche Summary

The spectacularly large and destructive natural avalanche cycle observed during last week's big storm seems to have slowed. Persistent slab avalanches are still sensitive to explosive triggering and have been reported up to size 3. Loose dry sloughing in the new snow has been observed to size 1.

Snowpack Summary

10-20 cm of new snow arrived just before the weekend in combination with moderate southwest wind, which has likely formed soft windslab in exposed high alpine lees. Anywhere from 100 to 160 cm of snow is now resting on a widespread layer of large, feathery, surface hoar which we're calling the "December 11th Surface Hoar." This layer was the culprit for the very large and destructive natural avalanche cycle during and after last weekend's big storm. Activity on this interface has largely tapered off, and is suspected to be trending towards dormancy.

A combination of surface hoar, crust & facets formed in late November is now over 170 cm deep and is presently inactive.

Terrain and Travel

  • Avalanche hazard may have improved, but be mindful that deep instabilities are still present.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Avoid convexities, steep unsupported terrain and rocky outcroppings.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.

Problems

Persistent Slabs

An icon showing Persistent Slabs

Likelihood

Unlikely

Expected Size

2 - 3.5

Settled snow from previous storms has formed a slab on weak surface hoar 100 to 160 cm below the surface. Natural avalanche activity has waned, but human triggering may still be possible, especially in more challenging/complex terrain that hasn't avalanched since the weak layer was buried. This isn't the kind of avalanche problem you can feel under your skis, track or feet, it's far too deep for that. The answer lies in terrain selection.

Aspects:

All aspects.

Elevations:

All elevations.

Wind Slabs

An icon showing Wind Slabs

Likelihood

Possible

Expected Size

1 - 1.5

10-20 cm of recently fallen snow coupled with moderate southwest wind has likely formed fresh wind slabs in exposed alpine terrain features.

Aspects:

North, North East, East, North West.

Elevations:

Alpine.

Valid until: Dec 29th, 2019 5:00PM