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Avalanche Forecast

Archived

Apr 20th, 2023–Apr 21st, 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.

Dangerous avalanche conditions exist in the alpine where new snow continues to incrementally add load to buried weak layers.

Carefully evaluate terrain as you move through the backcountry and when in doubt choose conservative terrain.

Confidence

Moderate

Avalanche Summary

No new avalanches were reported in the region on Thursday.

Adjacent regions continue to see natural and explosive triggered avalanches, up to size 2, on weak layers.

A fatal avalanche occurred on April 15 in the Thunderwater Lake riding area, just west of this region. The avalanche was triggered near a rocky area and was very large (size 3). Two riders were caught, one was buried approximately 2 meters deep. Despite an immediate extrication rescue response, the rider did not survive. Any additional information we have on this accident can be found in this MIN.

If you are getting out in the backcountry, consider making a post on the MIN (Mountain Information Network).

Snowpack Summary

At treeline and above, 15 - 40 cm of recent snow is being redistributed by variable winds into soft slabs in lees. This overlies a variety of crust, surface hoar and/or facet layers buried in mid March through early April. These interfaces have been the culprit in recent avalanche activity in adjacent regions. Most professional operations in the forecast area are tracking their own local layer of concern to see if they become active with increasing temperatures, or more load from new rain or snow.

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.

Weather Summary

Thursday Night

Cloudy with isolated flurries, 1-5 cm accumulation. Light southwest ridgetop winds. Freezing levels 500 m. Treeline low around -9 °C.

Friday

Mainly cloudy with isolated flurries, 1-5 cm accumulations. Light to moderate northwest ridgetop wind. Freezing levels 1600 m. Treeline high around -1 °C.

Saturday

Mix of sun and cloud with isolated flurries, trace accumulations. Light to moderate west ridgetop wind. Freezing levels rise from valley bottom to 2000 m by mid-day. Treeline high 0 °C.

Sunday

Mostly cloudy with sunny periods and isolated flurries, 1-5 cm accumulations. Moderate west ridgetop wind. Freezing levels rise from 700 m bottom to 2100 m by mid-day. Treeline high +2 °C.

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.

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.

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.