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Avalanche Forecast

Archived

Dec 19th, 2019–Dec 20th, 2019

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

Flurries and wind continue to build and develop slabs. The deepest and most reactive deposits will be in the alpine and treeline, around ridge features and steep rolls.

Confidence

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

Weather Forecast

Thursday Night: Cloudy and scattered flurries, trace to 5 cm. Alpine temperature -6 C. South wind 25-35 km/hr.

Friday: Cloudy and scattered flurries, 5-10 cm. Alpine temperature -5 C. Southwest wind 20-45 km/hr.

Saturday: Cloudy with isolated flurries, trace accumulation. Alpine temperature -7 C. Southwest wind 20-40 km/hr.

Sunday: Mix of sun, cloud, and isolated flurries, trace to 5 cm. Alpine temperature -7 C., Southwest wind 15-30 km/hr.

Avalanche Summary

Between Tuesday and Wednesday, and further east of the Northwest Inland region, a natural wind slab avalanche cycle to size 2.5 in alpine and treeline areas was triggered by intense wind loading.

In the neighboring Northwest Coastal region, a natural avalanche cycle to size 2.5 occurred Tuesday. This area received higher precipitation amounts (30-70 cm) this week, but similarities in the storm suggest a potential for human triggered storm/wind slabs in the Northwest Inland region as well.

Snowpack Summary

Snowfall blanketed 15-30 cm snow around the region early this week. Strong south-southwesterly winds have developed reactive wind slabs, especially around ridge features and into the alpine. Daily flurries and winds are contributing to developed wind slabs.

Reports suggest there are 2 buried layers of surface hoar in the upper snowpack - one down 15-25 cm below the new snow, and another down 25-40 cm below older, recent snow. Where wind has scoured the snowpack, a crust from November is at or near the surface. In leeward terrain, a this November crust is 40-60 cm deep, and may include surface hoar in sheltered openings up to treeline.

Older crusts can be found deeper in the snowpack and are generally breaking down and bonding. Snowpack depths range from 100-180 cm around treeline, with depths diminishing rapidly below 1500 m and where the wind has scoured. In areas with a relatively thin snowpack, the base of the snowpack likely consists of weak faceted grains.

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.