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Avalanche Forecast

Archived

Apr 22nd, 2023–Apr 23rd, 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

Purcells, Dogtooth, East Purcell, St. Mary.

Although avalanche danger ratings are once again decreasing, buried weak layers remain a lower probability, but high consequence problem.

Use extra caution in areas that experience warm temperatures combined with strong sunshine. These factors will stress a weak snowpack.

Confidence

Moderate

Avalanche Summary

Observations are limited at this point in the season. If you have any information, let us know what you are seeing through the Mountain Information Network.

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

Reported activity on the persistent and deep persistent weak layers has tapered off within the last two weeks. The Western Purcells have seen more recent human-triggered activity, including explosive-controlled avalanches and a fatal size 3 on April 15th in the Thunderwater Lake area. This indicates the potential for triggering, although decreased, is still present with high consequences. Continue to avoid rocky start zones for the remainder of the season.

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

Saturday Night

Some cloud, with isolated flurries delivering trace amounts of snow. Light to moderate westerly wind. Freezing levels drop to 1000 m overnight.

Sunday

Mostly cloudy with sunny periods and isolated flurries. Moderate westerly wind. Freezing levels rise to 2200 m, and remain above 1500 m overnight.

Monday

Clouds clear in the afternoon with light westerly winds. Isolated flurries. Freezing levels reach 2200 m.

Tuesday

A mix of sun and cloud with light westerly winds. No snowfall expected. Freezing levels rise to 2200m.

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.