Regions
Banff Yoho Kootenay.
Avalanche control on Mts. Bourgeau, Whymper, Bosworth, Stephen and Wardle Friday. No outdoor activities there. Forecast clearing skies means high solar impact. Expect large avalanches in the region.
Weather Forecast
Friday looks like a crux day with freezing levels to 2500-2600m, light winds and a good chance this cloud will clear off giving a high solar input. Saturday will see increased cloud and a switch to a westerly flow with moderate W winds in the alpine and freezing levels up to 2500m.
Snowpack Summary
The snowpack is in transition between winter and spring, and is a poor example of both. There was a 2-4cm crust (thinner BTL) on solar aspects to ridgetop with moist snow underneath Thursday AM, and this crust is breaking down during the day. In many places the snowpack is sitting on a deep persistent layer of weak facets near the ground.
Avalanche Summary
A flight from the Wapta to Banff showed solar induced loose,wet avalanches to size 2 in the last 24 hours, and some deeper releases to size 2.5, mainly below steep cliffs in the alpine. Reports of many solar induced natural avalanches in Kootenay today to size 3.5 where the sun has been out all day. Many of these are stepping to ground.
Confidence
Timing or intensity of solar radiation is uncertain on Friday
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.
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.
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.