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Avalanche Forecast

Archived

Mar 7th, 2023–Mar 8th, 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, Dogtooth, East Purcell.

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.

Brief periods of sun could quickly initiate natural avalanche activity. Avoid exposure to avalanche terrain and overhead hazard when solar radiation is strong.

Confidence

Moderate

Avalanche Summary

On Monday, a cornice trigged, size 3 avalanche was observed. The cornice triggered wind slabs below on a steep rock north-facing slope.

Through the weekend, natural and explosive triggered storm slab and deep persistent slab avalanches were reported to size 3. A skier remote size 3.5 deep persistent slab avalanches were reported near Golden. These avalanches all released around 2500 m and on easterly aspects. This MIN shows some large, naturally occurring deep persistent slab avalanches.

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 for this avalanche.

The Purcells are likely to continue to see very large destructive avalanches failing on the basal facets from late November. While we are seeing less avalanche activity in the Rockies side of this region the potential for these type of avalanches remains there as well.

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 previous west through south winds. Wind slabs may be found on all aspects due to variable winds in the past few days. A melt-freeze crust covers the surface of steep solar slopes

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

Tuesday Night

Clear skies. Southeasterly winds 10-20 km/h. Treeline temperatures, low of -10.

Wednesday

Mainly sunny. Easterly winds 15-25 km/h. Treeline temperatures, high of -9.

Thursday

Mainly sunny with a few clouds. Southerly winds 10-20 km/h. Treeline temperatures, high of -14.

Friday

Partly cloudy and isolated flurries, 2-5 cm accumulation. East winds 10-25 km/h. Treeline temperatures, high of -14.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Avoid areas with a thin or variable snowpack.
  • Avoid exposure to steep, sun exposed slopes, especially when the solar radiation is strong.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.
  • Remote triggering is a concern, watch out for adjacent and overhead 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.