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Avalanche Forecast

Archived

Apr 21st, 2023–Apr 22nd, 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, Dogtooth, East Purcell, St. Mary.

Recent snowfall has added load to more buried weak layers, steer clear of thin and rocky start zones.

Head to well-supported terrain features and manage your exposure to sun-affected slopes if skies clear.

Confidence

Moderate

Avalanche Summary

On Thursday, loose wet avalanches were observed to size 1, naturally triggered by the sun. Explosive control work produced size 1 avalanches within the recent storm snow, shallow but up to 100 m wide.

A fatal size 3 avalanche occurred on April 15th in the Thunderwater Lake area, just west of this region. The avalanche was triggered near a rocky area, involving a storm snow instability on an April crust. All information on this incident can be found here.

If you are getting out in the backcountry, consider posting on the MIN.

Snowpack Summary

At treeline and above, recent snow is being redistributed by variable winds into deeper deposits likely found on many aspects. This overlies a variety of crust, surface hoar, and/or facet layers buried from mid-March through early April. These layers have produced recent avalanche activity in nearby regions and continue to be monitored here. Below treeline, expect to find moist or refrozen surfaces, and shrinking snowpack depths.

The mid snowpack is generally settled and strong. The lower snowpack includes a widespread layer of large, weak facets and/or depth hoar crystals. This weak layer has been responsible for several very large and destructive avalanches throughout the season. This layer continues to be a concern, especially during times of rapid loading or prolonged warming.

Weather Summary

Friday Night

Mainly cloudy with isolated flurries, 1-5 cm of snow possible. Light to moderate west wind. Freezing levels drop to 500 m overnight.

Saturday

Mainly cloudy with isolated flurries, and trace accumulations of snow. Light westerly wind. Freezing levels rise from valley bottom to 2000 m.

Sunday

Mostly cloudy with sunny periods and isolated flurries, 1-5 cm possible. Moderate westerly wind. Freezing levels rise to 2100 m by mid-day.

Monday

A mix of sun and cloud with light westerly winds. Flurries possible. Freezing levels remain around 2000 m.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Brief periods of sun could quickly initiate natural avalanche activity.
  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Avoid exposure to steep, sun exposed slopes, especially when the solar radiation is strong.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

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.