Regions
Banff Yoho Kootenay.
http://www.pc.gc.ca/apps/scond/Cond_E.asp?oID=23022&oPark=100092We have a Special Avalanche Warning in effect because we do not think the danger rating of MODERATE adequately illustrates the complexity in the snowpack right now. Low probability/high consequence. Easter weekend is here; keep it mellow out there.
Weather Forecast
After a bit more snow overnight on Thursday (< 5 cm), it looks like a nice long weekend ahead with skies clearing off on Friday afternoon and a mix of sun and clouds for Saturday. Valley bottom temperatures could reach up to +8, while freezing levels will rise to 1800 m during the mid-afternoon. Sunday looks the same, with a mix of sun and clouds.
Snowpack Summary
5-10cm of new snow overlies crusts in most locations except shaded terrain above 2000m. Shallow snowpack areas are weak, and we are concerned about facets in the lower snowpack overlain by a stiff slab which produces easy, sudden collapses in tests. In the absence of a clear night forming a surface crust, snowpacks below treeline are isothermal
Avalanche Summary
Over the past week there have been large avalanches triggered by cornice failures, explosives and humans. Of note was a large human triggered avalanche in K-Country last Sunday involving a thick, hard slab over weak depth hoar. Similar results can be expected in the our region. On warm afternoons, wet snow avalanches are occurring below treeline.
Confidence
Due to the number of field observations
Problems
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.
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.