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Avalanche Forecast

Archived

Mar 12th, 2023–Mar 13th, 2023

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

Regions

Purcells, Dogtooth, East Purcell, West Purcell.

Warm temperatures, new snow and strong winds will create very dangerous avalanche conditions on Monday.

Uncertainty is best managed by conservative terrain choices at this time. Avoid shallow areas where triggering the deep persistent weak layer is more likely.

Confidence

Moderate

Avalanche Summary

On Saturday, a natural size 2.5 was observed at 2400 m on a northwest aspect. The avalanche was +100 cm deep and occurred on one of the persistent buried layers within the snowpack.

Incoming snow and rising freezing levels will increase the likelihood of avalanches throughout the day tomorrow. Large avalanche activity on the deeply buried weak layers is expected tomorrow with weather inputs.

Snowpack Summary

Storm snow accumulates over a plethora of surfaces including hard, wind-affected surfaces in open areas, surface hoar and facetted snow in sheltered areas and a sun crust on steep solar slopes.

Buried surface hoar sits 50-70 cm deep in sheltered terrain features, and a thin sun crust exists at the same depth on steep south-facing terrain. Several other layers from January can also be found in the top 100cm of the snowpack.

The lower snowpack is made up of a widespread layer of large, weak basal facets and depth hoar in some areas. this weakness has been responsible for a number of recent very large, destructive avalanches and will continue to be a concern.

Weather Summary

Sunday Night

Partly cloudy with isolated flurries. the Western Purcells could see up to 10 cm while the rest of the region sees trace accumulations. Alpine temperatures reach a low of -6 °C. Ridge wind light from the south.

Monday

Cloudy with mixed precipitation, 5-20 cm of new snow accumulation at higher elevations. Alpine temperatures reach a high of -4 °C. Ridge wind southwest 25-40 km/h. Freezing levels rise to 1700 meters.

Overnight flurries continue bringing an additional 10-15 cm of snow to the Western Purcells.

Tuesday

A mix of sun and cloud with isolated flurries. Alpine temperatures reach a high of -9 °C. Light ridge wind from the southwest. Freezing levels rise to 1200 meters.

Wednesday

A mix of sun and cloud with isolated flurries. Alpine temperatures reach a high of -7 °C. Light ridge wind from the west. Freezing levels rise to 1300 meters.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Storm snow and wind is forming touchy slabs. Use caution in lee areas in the alpine and treeline.
  • Make conservative terrain choices and avoid overhead hazard.
  • Avoid areas with a thin or variable snowpack.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

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.