Regions
Banff Yoho Kootenay.
Avoid exposure to avalanche terrain: the new storm has resulted in another avalanche cycle today. Explosive Avalanche Control is planned for Mt Bourgeau, Mt Stephen and Mt Dennis. Please no climbing or skiing in these areas Sunday.
Weather Forecast
A significant storm is underway Saturday with moderate to strong SW winds, moderate precip and freezing levels in excess of 2200m. The storm will taper off Sunday morning and freezing levels are expected to drop back to valley bottom as winds shift back to westerly. A cooling trend will continue with light precip until skies start to clear Monday.
Snowpack Summary
10 to 20cm of new snow, combined with 40 to 80cm from the last 14 days has created a significant load over a fundamentally weak snowpack. Rain to 2200m, warmer overall temperatures, and snow in the alpine combined with strong SW winds are building superficial wind and storm slabs and overloading the deep basal facet layers. Poor travel below 2000m.
Avalanche Summary
Control work yesterday on the 93 S produced surprisingly large results reaching historical runnouts to sz 3.5. Extensive Natural activity has been seen on flights throughout the forecasting region to size 4 with fracture lines longer than 1km in many locations. Reports today indicate new activity to the same scale as the latest storm hits.
Confidence
Intensity of incoming weather systems is uncertain on Saturday
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.
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.
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.