Avalanche Forecast

Archived

Jan 6th, 2012–Jan 7th, 2012

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 possible, human triggered probable.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

Kootenay Boundary.

Confidence

Fair - Intensity of incoming weather is uncertain

Weather Forecast

Saturday: Flurries or light precipitation with no more than 5 cm of accumulation. Freezing level around 1000 m. Moderate winds from the northwest. Sunday: light to moderate precipitation (5 to 15 mm) with freezing levels rising to 2000 m. Strong southwesterly winds. On Monday, a further 5-15 mm precipitation should fall. Freezing levels should steadily fall, eventually reaching valley bottom on Tuesday.

Avalanche Summary

A skier-triggered avalanche near Nelson was reported from Thursday, running on wind slab over a crust. The avalanche was a size 2 and occurred on a north aspect, 40deg slope at 2125 m. The crown was 20-30 cm deep and the slide ran for 300-350 m. Recent activity on the mid-December surface hoar includes a snowcat remotely triggering a 70cm deep Size 2 slab avalanche from 10m away on a log-haul landing in a cutblock. Recent storm snow overlying a crust is also highly reactive to human triggers with slope-cuts producing numerous loose snow and 10-30cm thick slab avalanches up to Size 1.5.

Snowpack Summary

Recent new snow and gusty winds are keeping wind slabs and cornices fresh and weak, and in some places buried old wind slabs are a concern. A lightly buried thin crust extends into treeline elevations. Compression tests have been producing easy to moderate sudden results on the mid-December surface hoar, down 30-80cm, and propagation tests, whumpfing, and remote triggering suggest that avalanches associated with this persistent slab have a high propensity to propagate over large areas. Other weaknesses within the slab create the potential for step-down avalanches. Basal facets and depth hoar remain a concern in shallow rocky areas.

Problems

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.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.