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Avalanche Forecast

Archived

Jan 9th, 2020–Jan 10th, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
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.

Regions

South Columbia.

A couple of problems are in play for the region. Seek sheltered terrain to avoid wind slabs and use careful snowpack evaluation and low consequence terrain to assess and manage problematic persistent weak layers.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength. Forecast snowfall amounts are uncertain.

Weather Forecast

Thursday night: Partly cloudy, isolated flurries with trace accumulations, light northwest wind, alpine temperature -16 C.

Friday: Cloudy, 10-20 cm of snow, moderate south wind, alpine high temperature -9 C.

Saturday: Cloudy, 5-15 cm of snow, light south wind, alpine high temperature -6 C. 

Sunday: Cloudy, 5-15 cm of snow, moderate south wind, alpine high temperature -6 C.

Avalanche Summary

A widespread avalanche cycle occurred in the aftermath of the recent storm. Numerous large (size 2-2.5) and very large (size 3-3.5) avalanches released from natural, human, and explosive triggers in the storm snow and breaking on deeper layers.

There have been several reports of large (size 2-3) avalanches from both natural and human triggers breaking 100-150 cm deep on surface hoar layers from mid to late December. These avalanches give clear evidence that the continual loading on this fundamentally weak snowpack structure remains a concern.

Within the past week, a few notable deep persistent slab avalanches released on west, north, and northeast facing slopes in the alpine. Characteristics common to these avalanches include wide propagation and full depth avalanches scouring away the snowpack to ground. 

Freshly formed wind slabs on Friday are expected to be reactive to human triggering and will have the potential to step-down to these deeper layers, forming large and destructive avalanches.

Snowpack Summary

Low density snow will begin to accumulate tomorrow afternoon with 10-20 cm possible. Moderate to strong south winds have the potential to drift this new snow into stiffer slabs on lee terrain features at higher elevations, especially near ridge crests.

The most recent storm delivered 30-70 cm of new snow across the region burying two layers of surface hoar from mid and late December down 60-140 cm deep. These layers continue to produce large avalanches across aspects and elevations.

A facet/crust layer from late November lingers near the bottom of the snowpack. This layer has shown reactivity in isolated, shallow snowpack areas.

Terrain and Travel

  • Seek out wind sheltered terrain below treeline where you can avoid wind slabs and find great riding.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.

Problems

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.

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.