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Avalanche Forecast

Archived

Feb 22nd, 2023–Feb 23rd, 2023

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
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, St. Mary.

Continue to give the recent storm snow time to settle and stabilize. New snow overlays a complex and generally weak snowpack.

Confidence

Moderate

Avalanche Summary

A natural avalanche cycle occurred on Tuesday with stormy weather. The majority of the reported avalanches were storm slab and wind slab releases on various aspects and elevations.

Deep persistent slab avalanches continue to be periodically reported throughout the region. Including a skier-triggered, fatal avalanche last Thursday.

Going forward we suspect natural avalanche activity to decrease. However, human-triggered slab avalanches will remain triggerable while the recent storm snow settles and bonds to underlying surfaces.

Snowpack Summary

Up to 50 cm of new snow since the weekend arrived with variable moderate winds, burying a layer of surface hoar in wind-sheltered terrain and a thin sun crust on steep south-facing terrain.

In general, the mid snowpack is facetted, with a number of surface hoar layers reported, especially at lower elevations.

The lower snowpack contains a widespread layer of large, weak facets and depth hoar in places. This layer sits at the base of the snowpack and has been responsible for a number of very large, destructive avalanches.

Snowpack depths at treeline are roughly between 100 and 200 cm, with the shallowest snowpacks found on the eastern edge of the Purcells.

Weather Summary

Wednesday night

Cloudy with clear periods / Moderate, northeast ridgetop winds / Treeline temperatures -20 to -25 C.

Thursday

Mostly sunny / Moderate, northeast ridgetop wind / Treeline temperatures -20 to -25 C.

Friday

Sunny / Light, northeast ridgetop wind, shifting to the southwest in the PM / Treeline temperatures -20 to -25 C.

Saturday

Sunny / Moderate, southwest ridgetop winds / Treeline temperatures -15 to -20 C.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Avoid rock outcroppings, convexities, and anywhere the snowpack is thin and/or variable.

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.

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.