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Avalanche Forecast

Archived

Jan 20th, 2020–Jan 21st, 2020

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

Northwest Inland.

Thin, soft slabs from recent snowfall will likely continue to be reactive to human triggering, especially in wind sheltered areas at and below treeline where they sit over weak facets.

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

Monday night: Mostly cloudy with isolated flurries bringing a trace of new snow. Light southwest winds. Alpine low -7 C.

Tuesday: Mostly cloudy with isolated flurries bringing a trace of new snow. Moderate southeast winds. Alpine high -6 C.

Wednesday: Mix of sun and cloud with isolated flurries bringing a trace of new snow. Moderate south to southwest winds, strong at ridgetop. Alpine high -8 C.

Thursday: Mostly cloudy with scattered flurries bringing up to 5 cm overnight followed by 5 cm over the day. Moderate southwest winds, strong at ridgetop. Alpine high -5 C.

Avalanche Summary

Over the weekend, a natural storm cycle was observed in the Howson ranges, at treeline and below. Slabs were soft and thin but propagated widely resulting in avalanches up to size 2. They ran on the faceted interface buried January 17th.

As recently as Saturday, we have received reports of windslab avalanches size 1.5-2.5 mostly around treeline. Crown depths have been up to 1 m, running on a layer of surface hoar layer buried on January 10th.

There have been reports trickling in of natural persistent slab avalanches up to size 3 in the Bulkley Valley. These are thought to have been failing on the November crust/facet layer near the ground. The last reported activity at this interface was Monday January 13th. 

Snowpack Summary

Extensive wind effect at all elevations is well illustrated in photos from this MIN post from Saturday. Southerly winds are forming fresh windslabs from 10-20 cm of recent snow in places that were previously scoured by northeasterly arctic outflows. In wind sheltered areas at treeline and below, the recent snow sits on a layer of touchy facets.

A layer of surface hoar up to 1 m in depth may also be found at treeline. A deep crust/facet layer lurks at the base of the snowpack. A couple of large avalanches are suspected to have run on this interface in the last few weeks. These larger avalanches have been specific to lee and cross-loaded features in the alpine.

Terrain and Travel

  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Watch for areas of hard wind slab on alpine features.
  • Avoid open slopes and convex rolls at and below treeline where weak layers may be preserved.
  • Be aware of the potential for large avalanches due to the presence of a deep persistent slab.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.