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Avalanche Forecast

Archived

Mar 4th, 2023–Mar 5th, 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 possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Purcells, South Rockies, Dogtooth, East Purcell, St. Mary, Bull, Elkford East, Elkford West.

Avoid shallow rocky terrain where the snowpack transitions from thick to thin. Terrain features that fit that description at upper treeline and in the alpine are especially concerning. This is where riders are most likely to trigger a large destructive deep persistent slab avalanche.

Be prepared to back off slopes if the surface gets moist from solar input. This time of year it only takes a short period of strong solar to increase hazard.

Confidence

Moderate

Avalanche Summary

On Wednesday a fatal avalanche incident involving a group of skiers occurred in the Purcell Mountains west of Invermere BC. The size three avalanche was triggered on a southwest facing slope at 2500 m. The weak layer of facets buried in late November That sits at the bottom of the snowpack was responsible. Very large avalanches failing on this layer are most likely to be triggered in shallow rocky terrain with variable snowpack depths at upper treeline and in the alpine.

This MIN shows some large naturally occurring deep persistent slab avalanches. These avalanches likely occurred earlier in the week.

Earlier in the week several skier triggered avalanches were reported. Two of these avalanches were remotely triggered. Surface hoar layers from January and February were initially triggered. In one case the avalanche then stepped down to the facets at the bottom of the snowpack. These avalanches occurred at treeline and above and on a variety of aspects.

Snowpack Summary

Up to 40 cm of storm snow from earlier in the week sits over wind affected surfaces. Expect deeper deposits on north and east facing slopes, from west through south winds.

Buried surface hoar sits 30-60 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

Saturday Night

Mostly cloudy with the possibility of light flurries bringing a few centimeters of new snow. Light to moderate southeast winds and a low of -13°C at 2000 m.

Sunday

A mix of sun and cloud with the possibility of light flurries bringing a few centimeters of new snow. Light to moderate southeast winds and a high of -8°C at 2000 m.

Monday

A mix of sun and cloud with the possibility of light flurries bringing trace amounts of new snow. Light variable winds and a high of -7°C at 2000 m.

Tuesday

Sunny with no new snow expected. Light southwest winds and a high of -7°C at 2000 m.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Avoid areas with a thin or variable snowpack.
  • Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Remote triggering is a concern, watch out for adjacent and overhead slopes.
  • Avoid exposure to steep sun exposed slopes.

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.

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.

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.