Regions
Banff Yoho Kootenay.
http://www.pc.gc.ca/apps/links/goto_e.asp?destination=Heads Up! A Special Avalanche Warning is in effect as we do not think the danger rating of MODERATE adequately illustrates the complexity in the snowpack right now. This is a low probability, high consequence period with a highly variable snowpack.
Weather Forecast
Sunday will see light SW winds, a decent overnight freeze and freezing levels up to 1900m. Sunday night into Monday a NE flow is forecasting 15-20 cm in the Sunshine region with half that expected in Lake Louise.
Snowpack Summary
A thin melt/freeze crust exists on solar aspects, and on shady aspects up to 2000m where it disappears. Dry snow exists mainly on N aspects only. 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.
Avalanche Summary
Over the past week there have been large avalanches triggered by cornice failures, explosives and people on the deep persistent layer. Activity has tapered off in the last few days. Some loose wet avalanches noted on solar aspects over the last 24 hours to size 2 along Hwy 93 N. One slide observed pulled out a deeper slab in steep terrain.
Confidence
Forecast snowfall amounts are uncertain on Monday
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.