Regions
Banff Yoho Kootenay.
http://www.pc.gc.ca/apps/scond/Cond_E.asp?oID=23022&oPark=100092A 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
Cloud cover and below surface freezing levels are expected in the next 2 days. Very light precip amounts are expected to accompany the cloud cover (~8mm). Upper level wind will shift to come from the N-NE.
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
No avalanches observed or reported today.
Confidence
Due to the number and quality of field observations on Sunday
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.