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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.

Regions

South Columbia.

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.

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

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.