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Avalanche Forecast

Archived

Mar 21st, 2023–Mar 22nd, 2023

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

South Rockies, St. Mary, Bull, Elkford East, Elkford West.

Human-triggering large slab avalanches remains possible in steep, rocky, and/or wind-affected terrain.

Start and finish your day early and plan to be well away from sun exposed slopes and overhead hazard before the warmest part of the day.

Confidence

Moderate

Avalanche Summary

On Monday, a size 1 natural cornice failure occurred in the St Mary's east of Kaslo. The cornice dropped onto a steep slope and gouged down to the weak facets at the base of the snowpack.

On Sunday, natural size 1 loose wet avalanches were observed from steep rocky terrain on solar aspects.

In the northern Purcells on Sunday, several large natural deep persistent avalanches were observed and in the nearby Pedley Pass, this MIN report describes what was expected to have been a remotely triggered slab that failed down 80 cm. While activity may have started to taper off on the deeper layers in this region, nearby avalanches are a reminder that the layers are likely still reactive in some locations in the region, especially thin, rocky areas in the alpine.

On Friday afternoon, a snowmobile triggered a large persistent slab avalanche (size 2.5) in the south Purcells. The avalanche occurred in burnt forested terrain on a south aspect at 2200 m. It failed on a 50-100 cm deep layer that is suspected to be a sun crust buried in February.

Snowpack Summary

The snow surface consists of a sun crust on solar aspects, small facets and surface hoar on shaded and wind-sheltered slopes, and wind-affected surfaces in exposed terrain. The wind direction recently shifted from the southwest to the northeast so wind slabs are possible on all aspects in exposed terrain.

An interface buried around March 11 sits down 30-40 cm and typically consists of a thin sun crust on solar slopes and wind-affected snow in exposed terrain.

In the Purcells around St Mary's, several weak layers from January and February can be found down 50-120 cm. These layers appear to have generally gone dormant but could still be reactive in isolated areas and should be on your radar in the Purcells part of the region.

The lower snowpack is made up of a widespread layer of large, weak basal facets and depth hoar in some areas. This weakness has been responsible for a number of recent very large, destructive avalanches and will continue to be a concern.

Weather Summary

Tuesday night

Clear. Alpine temperatures drop to a low of -6 °C. Light variable ridge wind. Freezing level at valley bottom.

Wednesday

Sunny. Alpine temperatures reach a high of 1 °C. Light variable ridge wind. Freezing level rises to 2100 metres.

Thursday

A mix of sun and cloud with isolated flurries. Alpine temperatures reach a high of -1 °C. Ridge wind 10 to 40 km/h from the southwest. Freezing level rises to 1900 metres.

Friday

Cloudy with sunny periods. Alpine temperatures reach a high of -4 °C. Ridge wind light from the west. Freezing level rises to 1000 metres.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Be careful with wind slabs, especially in steep, unsupported and/or convex terrain features.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Avoid slopes when the solar radiation is strong, especially if they have large cornices overhead.
  • Avoid rock outcroppings, convexities, and anywhere the snowpack is thin and/or variable.

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.

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.