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Avalanche Forecast

Archived

Jan 6th, 2023–Jan 7th, 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.

Human-triggered avalanches remain likely on weak layers buried deep in the snowpack. Avalanches occurring on these layers will be large with high consequences.

Check out the Forecasters blog for strategies to manage the persistent and deep persistent avalanches problem found in this area.

Confidence

Moderate

Avalanche Summary

A small rider-triggered avalanche was released on the persistent weak layer and explosive control continues to produce small avalanches failing on the deep persistent weak layer.

Reports of new human-triggered avalanches have started to taper. However, with little change in the snowpack structure, triggering large persistent slab avalanches remains a concern, especially in steep, shallow, rocky areas.

Snowpack Summary

Wind slabs are forming in lee terrain features from southerly wind in wind-exposed terrain. These new wind slabs sit on variable surfaces of soft snow in sheltered areas, a layer of surface hoar and/or crust.

The mid and lower snowpack is generally weak and faceted and continues to be a concern. Avalanche activity on these layers has tapered, however, the snowpack structure remains the same.

The two layers of concern are relatively close together, the upper weak layer developed in mid-December is 30 to 50 cm deep. The lower weak layer developed in mid-November 70 to 100 cm

Snowpack depths are roughly 60 to 160 cm at treeline. This year's snowpack is weaker than usual, as described in our Forecasters' blog.

Weather Summary

Friday Night

Clearing overnight, no new snow, 10 to 20 km/h southwest wind, treeline temperature -5 °C.

Saturday

Mostly clear skies with increasing clouds throughout the day, with trace amounts of snow. 10 - 20 km/h south wind, treeline temperatures -5 °C.

Sunday

Mostly cloudy, 2 to 5 cm new snow, 10 - 20 km/h southwest winds decreasing through the day, treeline temperatures - 7 °C.

Monday

Partly cloudy, trace of new snow, 15 to 20 km/h southwest winds, 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.
  • Choose conservative terrain and watch for clues of instability.
  • Remote triggering is a concern, watch out for adjacent and overhead slopes.
  • Back off if you encounter whumpfing, hollow sounds, or shooting cracks.

Problems

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.

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.