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Avalanche Forecast

Archived

Feb 28th, 2023–Mar 1st, 2023

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

Regions

Purcells, South Rockies, Dogtooth, East Purcell, St. Mary, Bull, Elkford East, Elkford West.

Steer clear of freshly wind loaded features. Wind slabs are expected to remain reactive to human triggers.

Avoid thin and rocky start zones, especially in the alpine where deeply buried weak layers remain concerning.

Confidence

Moderate

Avalanche Summary

The storm produced avalanches up to size 3 from natural and rider triggers, and explosive control methods. Many avalanches have occurred within the storm snow or on the old snow surface, while some failed on the buried surface hoar layer.

Deep persistent slab avalanches continue to be periodically reported throughout the Purcells.

Avalanche activity on this deeply buried weak layer is expected to continue as the snowpack structure remains weak and will be slow to gain strength. Similar activity may be occurring in the Rockies however we have less operators and observations coming from this area.

Snowpack Summary

Up to 20 cm of low density recent storm snow sits over wind affected surfaces. Expect deeper deposits on north and east facing slopes, from southwest winds.

Buried surface hoar sits 30-50 cm deep in sheltered terrain features, and a thin sun crust exists at the same depth on steep south-facing terrain.

The lower snowpack contains a number of buried weak layers such as surface hoar with variable distribution, as well as a widespread layer of large, weak basal facets and depth hoar in some areas. These weaknesses have been responsible for a number of recent very large, destructive avalanches and will continue to be a concern.

Weather Summary

Tuesday Night

Mainly cloudy with isolated flurries. Light snowfall is expected in southern areas near Cranbrook. Light and variable winds. Freezing levels valley bottom.

Wednesday

Mix of sun and cloud. Light westerly wind. Alpine high of -8 °C. Freezing levels 500 m.

Thursday

A mix of sun and cloud with freezing levels rising to 1000 m. Alpine highs reach -3°C. Moderate to strong southwest winds. Light snowfall possible.

Friday

Mix of sun and cloud with moderate westerly winds. Freezing level around 500 m. Isolated flurries are possible.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.

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.

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.