Regions
Banff Yoho Kootenay.
We have reached the tipping point for avalanches in most of the region, and a natural cycle occurred Thursday. We may see a decrease in natural activity for Friday, but slopes are primed for human triggering. Time to step way back.
Weather Forecast
A cooling trend for Friday with alpine highs of -16C, and valley bottom highs of -12C (-5C on the East slopes like Banff). Only 1-3cm of snow is expected with Westerly alpine winds in the 30-40kmh range.
Snowpack Summary
55-75cm of snow has fallen over the past 9-days creating a lot of loose snow on the surface. Alpine winds have distributed this into leeward areas with wind slabs up to 1m thick. Three persistent weak layers lurk in the upper half of the snowpack: Jan 16 down 20-40cm; Jan 6 down 30-50cm; Dec15 down 40-80cm giving easy-mod test results.
Avalanche Summary
We have hit the "tipping" point over the weak layers mentioned above and there is a natural avalanche cycle to size 3 occurring. Some avalanches are occurring in the new snow only, with some stepping down to the numerous weak layers below. Explosive control on Bosworth produced results to size 2 within the new snow.
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.