Regions
Banff Yoho Kootenay.
Avalanche control planned for tomorrow on Mt. Bourgeau and the Vermillion Crossing (KNP) slide paths. Please no recreational activities in these areas. Extremely touchy conditions in high snow fall areas: KNP, Sunshine back-country, & Yoho.
Weather Forecast
A break in the snow tomorrow and the wind will be moderate with strong gusts (SW). Another system starting Saturday night through Monday will bring another 10-25 cm, more to the North end of the region. Winds will be strong SW with gusts over 100 kph on Sunday.
Snowpack Summary
Treeline storm snow amounts are 75 cm in Kootenay & Yoho, 60 cm in the Sunshine area, & 25 cm in the Louise/Hwy 93N areas. Winds,warm temps causing slab development in exposed areas,which overlies a complex layer of facets, surface hoar and sun crust on solar aspects. Below this a generally well settled midpack overlying the basal depth hoar layer.
Avalanche Summary
Another field trip to Kootenay, forecasters were able to remote trigger several size 2`s on any steep (>35 deg) unsupported terrain features. One was triggered from 300 m away. These were running 100-300m and were on average 30-40cm deep. One natural size 3 observed on a SE aspect, ran about 1000m. Still touchy around sunshine as well.
Confidence
Due to the quality of field observations
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.
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.