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Avalanche Forecast

Archived

Apr 24th, 2023–Apr 25th, 2023

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.

Regions

Purcells, Dogtooth, East Purcell, St. Mary.

Warm temperatures and sunshine continue to stress an already fragile snowpack. Persistent and deep persistent slab avalanche activity will become more likely as temperatures climb this week.

Minimize your exposure to avalanche terrain in the heat of the day. Avoid thin and rocky areas on sun-affected slopes.

Confidence

Moderate

Avalanche Summary

Observations are very limited at this time of year, please consider sharing any information or photos you have on the Mountain Information Network to help guide our forecasts.

Loose wet avalanches were observed this week, naturally triggered by the sun and warm temperatures. Natural activity is expected to continue this week with an increased likelihood of deep avalanche releases as warming continues.

The numerous buried weak layers including the basal facets remain a concern, especially with rising freezing levels. Recent avalanche activity on these layers in adjacent forecast areas highlights the need to avoid rocky and thin snowpack areas for the remainder of the season.

  • A fatal size 3 occurred near Lake Louise on April 22nd on the basal facets, in a particularly thin and rocky area with a 50 cm deep snowpack. All information can be found here.

Snowpack Summary

Warm temperatures will weaken the surface snow as freezing levels rise on Tuesday, breaking down surface crusts and moistening any previously dry snow to around 2000 m on all aspects. Sun-affected terrain will see the most intense warming, likely creating wet and slushy snow to mountain top.

Recent wind-affected storm snow then sits over a variety of crust, surface hoar, and/or facet layers buried from mid-March through early April. Crusts form an ideal sliding layer for avalanche activity as warming affects the snowpack.

The mid snowpack is generally settled and strong. However, 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.

Continued warm temperatures this week are expected to stress these buried weak layers, making them more sensitive to triggers. We are concerned about the potential for very large avalanches on the basal facet layer as temperatures remain warm overnight, limiting recovery within the snowpack.

Weather Summary

Monday Night

Cloud begins to clear. Light to moderate westerly wind. Freezing levels drop to 1500 m. No snowfall expected.

Tuesday

Mostly sunny with freezing levels rising to 2200 m. Treeline temperatures around +5 °C. Moderate westerly winds. No snowfall.

Wednesday

A mix of sun and cloud with freezing levels rising to 2500 m. Treeline temperatures of +8 °C. Moderate westerly winds. Localised areas may see 5 cm of snow/light rain.

Thursday

Full sun with freezing levels heading towards 3000 m. Treeline temperatures above +8 °C. Light westerly winds.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.
  • The more the snowpack warms-up and weakens, the more conservative you`ll want to be with your terrain selection.
  • The likelihood of deep persistent slab avalanches will increase with each day of warm weather.
  • Cornices become weak with daytime heating or solar exposure.

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.