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Avalanche Forecast

Archived

Jan 24th, 2023–Jan 25th, 2023

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

Regions

Purcells, Dogtooth, East Purcell, West Purcell.

High consequence deep persistent avalanches have occurred in the northern part of the region over the past two days.

Adopt a conservative mindset when selecting terrain.

Confidence

Moderate

Avalanche Summary

On Tuesday there was a report of a size 3.5 natural avalanche in the dogtooth range. This was likely a deep persistent avalanche with a wide propagation triggering near ridgetop at the upper end of treeline on an easterly aspect.

On Monday there was a size 3 skier triggered avalanche deep persistent avalanche on an east aspect starting at 2300m in the Terminator area in the Dogtooth range. This MIN outlines the avalanche.

On Sunday a cornice that failed naturally is suspected to have caused three size two avalanches on a storm slab.

There was an avalanche of note on Saturday. It was a size 2 that a skier triggered accidentally. This avalanche was of note because it was from our persistent slab problem, buried surface hoar and because it happened on such a low angle. Our persistent slab problem and deep persistent slab problem are still problems.

Snowpack Summary

Surface hoar can be found to be developing on the snow surface at most elevations and aspects. Wind slabs can be found in exposed areas at higher elevations.

The mid-snowpack contains a couple of layers of major concern. The first is down 30 to 50 cm and is a layer of surface hoar from early January. The second is a layer of surface hoar, facets, and crust created in December and found 40 to 90 cm down. The surface hoar is most prevalent in sheltered areas while crusts and any associated faceted snow are more widespread.

The bottom of the snowpack is weak and faceted. The total depth of this weak and shallow snowpack ranges between 80 and 180 cm at treeline and is shallowest in the eastern part of our region.

Weather Summary

Tuesday Night

Mainly cloudy with clear periods, wind northwest 25 km/h, treeline temperatures -8 C.

Wednesday

Mainly cloudy with sunny breaks, wind northwest 20 km/h gusting to 50, treeline temperatures -5 C.

Thursday

Mix of sun and cloud, wind west 25 km/h gusting to 55, treeline temperatures at -5 to -3 C.

Friday

Mainly cloudy with scattered flurries, wind northwest 20 km/h, treeline temperatures -7 C.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Uncertainty is best managed through conservative terrain choices at this time.
  • Avoid areas with overhead hazard.
  • Be carefull around freshly wind loaded features.

Problems

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.

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.

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.