Avalanche Forecast

Archived

Feb 5th, 2012–Feb 6th, 2012

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.

Below Treeline

Natural avalanches unlikely.

Regions

Kootenay Boundary.

Confidence

Good - -1

Weather Forecast

Warm days, light winds and strong solar radiation is expected to continue for the forecast period. Below freezing temperatures are expected to cause good overnight re-freeze of the surface layers. Some valley cloud may continue in the Columbia and Kootenay Valleys. The inversion continues, but it is forecast to weaken by Monday.

Avalanche Summary

Fewer avalanches were reported from Saturday, mostly moist solar releases on southerly aspects. A couple of large avalanches on northerly aspects were reported from just north of the region, that stepped down to a deep weak layer. There were some large deep avalanche releases on Friday. Most of the large avalanches were on solar aspects, as a result of warm overnight temperatures combined with daytime heating. There were also a couple of large avalanches that released on northerly aspects that were caused by cornice falls releasing slabs that stepped down to the ground. Also, there were a couple of large avalanches in the southern interior that released naturally or with light triggers on northerly aspects that released down to the ground or glacial ice. I decided to leave the information from the previous bulletin, as I think it shows the trend of larger avalanches. Explosive control in the Kaslo to New Denver highway corridor on Thursday produced several size 2.5 moist releases that pushed storm snow to the middle of the runout of 2 km long paths. These slides started between 2000 and 2300 metres on S-SW aspects. A larger explosive controlled avalanche occurred at Bear Lake from a SE aspect in the alpine. The avalanche was size 3.5 dry slab with an average crown of 160 cm and ran full path, 1100 vertical metres for 2.5 km.

Snowpack Summary

Temperatures dropped below freezing again on Saturday night. Crusts have been forming due to the melt and freeze cycle for the past two days. There is still dry snow on northerly aspects at higher elevations. Temperatures dropped below freezing at higher elevations on Friday night. New sun crusts are developing on steep solar aspects. Warm air trapped in the alpine caused temperatures to remain above freezing on Thursday night above about 2100 metres. Pin-wheeling and other signs of moist snow have been observed on solar aspects at and above treeline. Continued warm temperatures in the alpine may trigger buried weak layers on solar aspects. Steep planar slopes with shallow snowpacks are the most suspect. The recent storm snow layer is about 30-50 cm thick and is quite variable across the region. There are a couple of thin crusts buried below the storm snow that have been producing moderate to hard shears in tests. Some areas are still getting sudden planar shears on the mid-december surface hoar layer.

Problems

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.

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.

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.