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Avalanche Forecast

Archived

Mar 19th, 2023–Mar 20th, 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, West Purcell.

Use caution when transitioning into wind-affected terrain.

Stick to thick snowpack areas where you are less likely to trigger deep weak layers.

Confidence

Moderate

Avalanche Summary

A size 2 cornice avalanche was reported on Saturday. It occurred naturally in the alpine on a northwest aspect. It did not trigger the slope below.

On Friday, a few solar-triggered loose dry avalanches were reported in the region to size 2. A natural size 2.5 wind slab, likely triggered by the sun, was observed on a southwest aspect at 2500 m.

Explosives control triggered a deep persistent slab avalanche, size 2.5, on an east aspect at 2350 m.

Snowpack Summary

The top layer of the snowpack is made up of wind-affected snow in exposed areas and low-density snow in sheltered areas. This top layer sits on top of a varied of surfaces that include more wind-affected snow, facetted snow, and a thin sun crust. A sun crust is found on the surface of solar slopes at all elevations.

Buried surface hoar can be found 70 to 90 cm deep in sheltered terrain features. A thin sun crust exists at the same depth on steep south-facing terrain.

The lower snowpack is made up of a widespread layer of large, weak facets and/or depth hoar crystals. This weak layer has been responsible for a number of recent very large, destructive avalanches and will continue to be a concern.

Weather Summary

Sunday Night

Clear, no accumulation, winds east 25 km/h, treeline temperatures -3 to 0 °C.

Monday

A mix of sun and cloud, no accumulation, winds northeast 20 km/h, treeline temperatures around -5 °C freezing levels to 1700 m.

Tuesday

Sunny with cloudy periods, no accumulation, winds northeast 10 to 20 km/h, freezing level reaching 1700 m.

Wednesday

Sunny, no accumulation, winds northeast 20 km/h, freezing level reaching 2000 m.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Minimize exposure to sun-exposed slopes when the solar radiation is strong.
  • Avoid areas with a thin or variable snowpack.
  • Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.