Avalanche Forecast

Archived

Feb 23rd, 2023–Feb 24th, 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, 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.

Carefully assess for cohesive storm and wind slabs. Small surface avalanches remain likely to step down to deeper weak layers creating potentially large, destructive avalanches.

Confidence

Moderate

Avalanche Summary

A natural avalanche cycle occurred on Tuesday, toward the end of the 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.

We are not currently expecting much natural avalanche activity. However, human-triggered slab avalanches will remain possible while the recent storm snow settles and bonds to underlying surfaces.

Snowpack Summary

Up to 50 cm of recent snow is being redistributed by northerly winds in exposed alpine terrain. While surface snow remains relatively unconsolidated in areas not exposed to recent winds. 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 with variable distribution, as well as a widespread layer of large, weak basal facets and depth hoar in places. These basal weaknesses have been responsible for a number of recent very large, destructive avalanches.

Weather Summary

Thursday night

Mostly clear / Light to moderate northeast ridgetop winds / Treeline temperatures -20 to -25 C.

Friday

Sunny / Light, northeast ridgetop wind, shifting to the southwest by end of the day / Treeline temperatures -15 to -25 C.

Saturday

A mix of sun and cloud / Moderate to strong southwest ridgetop winds / Treeline temperatures -15 to -20 C.

Sunday

Light snow, 2 to 5 cm / Moderate to strong southwest ridgetop winds / Treeline temperatures -10 to -15 C.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

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.