Regions
Banff Yoho Kootenay.
Today, explosive control of cornices produced surprisingly large avalanches. Essentially, these large cornices acted like a rake and entrained the loose and faceted snowpack in the avalanche paths affected. Don't forget to look up.
Weather Forecast
On Friday weather models are showing up to 16cm of precipitation, Additionally, the winds will increase to moderate from the west. This new snow and wind will increase the danger rating to considerable late Friday or early Saturday.
Snowpack Summary
The recent storm snow is starting to settle. On average the storm snow has settled to about 10cm. This storm snow sits on top of wind slabs, both soft and hard, in alpine lee areas.
Avalanche Summary
Avalanche control on Mt. Whymper and Mt. Dennis produced some large 'facet' avalanches. Explosive control was used to drop the cornices above start zones, which subsequently scrapped out the faceted snowpack. However, without a large cornice like scrub brush, avalanches were actually quite hard to trigger.
Confidence
Timing, track, or intensity of incoming weather system is uncertain on Friday
Problems
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.
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.