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Avalanche Forecast

Archived

Feb 16th, 2021–Feb 17th, 2021

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.

A bit of new snow and an uptick in wind will feed into existing wind slab problems. Assess wind-drifted snow and lingering buried weak layers if you travel in avalanche terrain.  

Confidence

Moderate - Confidence is due to a stable weather pattern with little change expected.

Weather Forecast

TUESDAY NIGHT: Cloudy, light flurries with a trace of snow, light west wind with moderate gusts at ridgetop, alpine temperatures around -16 C.

WEDNESDAY: Mix of sun and cloud, light northwest wind with moderate gusts at ridgetop, alpine temperatures around -14 C.

THURSDAY: Sunny with a few clouds, light south wind with strong gusts in the afternoon, alpine temperatures around -11 C.

FRIDAY: Cloudy, 5-10 cm of snow, moderate southwest wind, alpine temperatures around -10 C. 

Avalanche Summary

Since Monday, there have been several reports of small to very large (1.5-3) natural and human-triggered wind slab avalanches across a wide range of aspects above 2200 m. This MIN report from near Eagle Pass offers a helpful visual of this avalanche activity. Operators have also reported numerous small dry loose avalanches (size 1-1.5).

Observations of avalanches on the late January persistent weak layer continue to trickle in. This MIN report from the Gorge on Friday is a helpful example of the lower elevation slopes where the surface hoar has been preserved. Also on Friday, operators reported a large (size 2.5) natural avalanche on a southeast aspect in the alpine, releasing on facets. Over the past week the persistent slab problem has produced fewer avalanches than in the first week of February, but is still showing signs of instability and warrants assessment.

Snowpack Summary

Light flurries have begun to accumulate over a variety of surfaces including surgary facets, feathery surface hoar, and wind-affected snow that formed as a result of last week's dry, cold weather. This trace of new snow and moderate northwest winds may feed into existing wind slab problems, especially in favored areas with up to 10 cm of accumulation. 

Winds have varied in strength and direction over the past week, loading cornices and forming wind slabs at upper elevations that may still be possible to trigger. Watch for these wind slabs in unusual places as complicated cross-loading and reverse-loading patterns are prevalent. Wind slabs may overlie weak, sugary, faceted snow, meaning that they could break wider than expected and will likely be slow to bond. 

60-100 cm of snow from February is settling over a layer of surface hoar buried in late January. Reactivity on this layer has primarily been observed at treeline and in "treeline-like" features, like cutblocks, that are below treeline. On steep south-facing terrain, this layer may consist of facets on a sun crust. 

The mid/lower snowpack is generally well settled and strong.

Terrain and Travel

  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Approach lee and cross-loaded slopes with caution.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Extra caution is needed around cornices under the current conditions.

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.