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Avalanche Forecast

Archived

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

Regions

North Columbia.

Large avalanches may still be sensitive to human triggering. Hazard may be improving but keep in mind that potential size and consequences of avalanches remain high. Deep instabilities like this are difficult to assess from the surface, so let terrain selection be your solution.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength. Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Friday night: Cloudy, alpine low -15 C, alpine wind moderate northwest.

Saturday: Scattered cloud with isolated flurries, alpine high -9 C, alpine wind moderate northwest, trace of snow possible in the evening.

Sunday: Scattered cloud, alpine high -7 C, alpine wind light northeast.

Monday: Scattered cloud, alpine high -7 C, alpine wind light northwest.

A bigger storm is on the horizon as we enter the New Year.

Avalanche Summary

The spectacularly large and destructive natural avalanche activity observed during the previous storm cycle has largely tapered. But as recently as Thursday, natural size 2.5 persistent slab avalanches were still being reported in large alpine terrain features on both north and south aspects, in neighboring Glacier National Park.

Snowpack Summary

Moderate winds over the last couple of days are likely to have blown around some of the 60 to 100 cm from last weekend's big storm, forming soft windslab in 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, there remains some uncertainty as to how quickly the snowpack is gaining strength.

A combination of surface hoar, crust & facets formed in late November is now over 170 cm deep. This layer is likely trending toward dormancy.

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.