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Avalanche Forecast

Archived

Feb 26th, 2023–Feb 27th, 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 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, St. Mary.

Dial back your terrain choices if you are finding more than 20 cm of new snow. Use caution as you transition into wind affected areas.

The lower snowpack remains weak, small avalanches may easily step down to deeper layers. Keep a large margin of error by avoiding thin start zones and large slopes.

Confidence

Moderate

Avalanche Summary

On Saturday, a small size 1.5 machine accidental persistent slab avalanche was reported from a shallow alpine feature that had previously been explosives controlled.

Explosive control on Thursday and Friday, produced slab avalanches to size 3, including one deep persistent slab avalanche. Evidence of a natural cycle to size 3 continues to be observed.

Deep persistent slab avalanches continue to be periodically reported throughout the region. Including a very large skier-triggered, fatal avalanche last Thursday. This MIN post from Thursday observed a size 3 avalanche near Golden over 1 m deep. Likely a naturally triggered slab, the exact failure plane is unknown. The continued activity indicates the weak and unpredictable snowpack remains concerning.

Snowpack Summary

10-15 cm of new low-density snow sits over 20-50 cm of wind affected storm snow.

A layer of surface hoar is now buried by 30 to 50 cm in sheltered terrain and a thin sun crust on steep south-facing terrain. In general, the mid snowpack is well consolidated and bonding.

The lower snowpack contains a number of buried weak layers such as surface hoar with variable distribution, as well as a widespread layer of large, weak basal facets and depth hoar in places. These weaknesses have been responsible for a number of recent very large, destructive avalanches and will continue to be a concern.

Weather Summary

Sunday Night

Cloudy with isolated flurries, 2 cm accumulation. Westerley ridgetop wind 10-25 km/h. Freezing levels drop to valley bottom. Alpine high of -10 °C.

Monday

Partly cloudy with isolated flurries, trace accumulation. Light southwesterly ridgetop wind. Alpine high of -9 °C. Freezing levels valley bottom.

Tuesday

Mainly cloudy with isolated flurries, 2 cm accumulation. Light variable ridgetop winds. Freezing levels valley bottom, alpine high of -9 °C.

Wednesday

Mix of sun and cloud. Light westerly ridgetop wind. Alpine high of -8 °C. Freezing levels 700 m.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Storm snow and wind is forming touchy slabs. Use caution in lee areas in the alpine and treeline.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Avoid rock outcroppings, convexities, and anywhere the snowpack is thin and/or variable.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.