Avalanche Forecast

Archived

Dec 23rd, 2019–Dec 24th, 2019

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.

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural and human triggered avalanches likely.

Regions

Northwest Coastal.

Snowfall and wind continue to progressively load the snowpack, developing a slab over a weak surface hoar layer. Its hard to know when the load will be great enough to induce a natural avalanche cycle, for now a conservative approach to avalanche terrain is recommended.

Confidence

Moderate - Uncertainty is due to unpredictable avalanche behavior. Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Monday Night: Flurries, 5-15 cm and up to 25 cm near Kitimat. Alpine temperature -5 C. South wind, 20-30 km/hr gusting to 60 km/hr. Freezing level 600 m.

Tuesday: Cloudy with isolated flurries, 5-10 cm. Alpine temperature -5 C. Southwest wind. 25-45 km/hr. Freezing level 600 m.

Wednesday: Cloudy with isolated flurries, up to 10 cm. Alpine temperature -7 C. Southwest wind, 35-55 km/hr. Freezing level 400 m.

Thursday: Heavy snow, 30-60 cm. Alpine temperature -3 C. Southwest wind 25-30 km/hr gusting to 80 km/hr.

Avalanche Summary

Last Thursday, natural storm slab to size 2 avalanches were reported in the Shames area. Further north in the region, explosives triggered storm slab avalanches to size 2. Surface hoar, which the storm snow fell on, was the likely weak layer involved. A MIN report here from Saturday identifies and discusses reactive surface hoar buried 70 cm under the recent storm snow.

Snowpack Summary

50-60 cm of recent snow is settling into a slab over a weak layer of surface hoar, old faceted surfaces, and/or a crust on south/southwest aspects in the alpine. Continued flurries are incrementally loading the snowpack and winds are impacting loose snow at upper elevations, forming windslabs in lee features.

The layer of surface hoar, down 50-60 cm around the Shames area, is concerning as it continues to be reactive in snowpack tests providing evidence it is at a tipping point where any additional load (such as a person) could trigger an avalanche on this layer. Reports from the Shames area suggest the surface hoar layer is on all aspects, but more prevalent on southeast to southwest aspects around 800-1400 m, and more likely on leeward and sheltered alpine areas.

The lower snowpack is generally considered strong, as there has been very little to report in terms of recent avalanche activity or snowpack test results on deeper layers. Snowpack depths at treeline range from 100-200 cm and taper quickly at lower elevations.

Terrain and Travel

Uncertainty is best managed through conservative terrain choices at this time.
Approach steep open slopes at and below treeline cautiously, buried surface hoar may exist.
Be careful as you transition into wind affected terrain.
Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.